A systematic review of preclinical studies on therapeutic potential of mesenchymal stem/stromal cells and their secretome in bacterial infections

Abstract

Background

Bacterial infections are a globally growing health issue, with an estimated 7.7 million deaths attributed to these infections worldwide. These life-threatening infections, primarily linked to antimicrobial resistance, are difficult to treat, and the growing reliance on last-resort antibiotics is exacerbating the problem. For this reason, numerous preclinical studies have been conducted using mesenchymal stem/stromal cells (MSCs) and their secretome as an alternative new therapeutic strategy for treating bacterial infections. However, these studies exhibit substantial disparities, often due to the lack of a consensus definition for MSCs and the broad variability in their reported characteristics. Thus, the purpose of this systematic review was to summarize studies that have used various sources of human MSCs and their secretome to treat bacterial infection in rodent models, to present an overview of evidence to proceed with clinical studies.

Methods

This systematic review was registered with PROSPERO and conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Using search terms related to “mesenchymal stem cells”, “mesenchymal stromal cells” as recommended by ISCT, “bacterial infections”, and “therapy”, candidate articles were identified through the PubMed database, and data were gathered using a narrative approach.

Results

Of the 517 articles retrieved, only thirty-seven studies met the inclusion criteria, and their analysis revealed several main findings. Human MSCs demonstrated positive effects mainly in decreasing bacterial load, reducing injuries, and improving the overall survival rate in rodents, with bone marrow-derived MSCs being the most used and effective type. All studies demonstrated that MSCs and their secretome can modify and enhance the immune response in rodents after bacterial infection.

Conclusions

This study showed that employing both stem cell-based and cell-free therapies for the treatment of bacterial infections has significant results in preclinical studies, offering promising potential as alternative treatment options. However, the findings are based solely on rodent models and the absence of donor-related investigations, which significantly limit the translation of results to clinical settings. Additionally, the persisting heterogeneity of MSCs remains a key hurdle, underscoring the necessity for further research before considering clinical implementation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-025-04570-5.

Background

Worldwide, an estimated 7.7 million deaths have been associated with bacterial infections, making them the second leading cause of death [1]. These infections are becoming increasingly challenging to manage due to the aggravation of the antimicrobial resistance (AMR) issue. The World Health Organization (WHO) has forewarned that drug-resistant infections could lead to 10 million deaths annually by 2050 if no new treatment strategies are developed [2]. As it is known, AMR is a natural process that occurs when microorganisms evolve and no longer respond to antibiotics to which they were previously susceptible and treated with [3]. This process is escalating due to several factors, including the overuse and misuse of antibiotics, inadequate hygiene, poor infection control measures in healthcare settings and communities, and the lack of access to healthcare services [4].

Without a doubt, both resistant forms of Gram-positive bacteria such as Staphylococcus aureus, and Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa, play a significant role in worsening the burden of infection, increasing morbidity and mortality rates, and complicating treatment protocols within communities and healthcare facilities. Despite advances in pharmacotherapy for bacterial infections, antibiotics remain associated with adverse side effects, necessitating an urgent need for novel therapeutic strategies that can combat bacterial infections without contributing to resistance. A wide variety of candidate therapies have been proposed, including bacteriophage therapy, immunotherapies, antimicrobial peptides, nanotechnology-based approaches, nutraceuticals, and cellular therapies [5–9].

Mesenchymal stromal cells (MSCs) are adult multipotent cells that have attracted considerable interest as a potential treatment for a wide range of diseases including bacterial infections [10]. MSCs were first isolated from the bone marrow and have since been found in a diverse range of organs, including the umbilical cord, placenta, and adipose tissue [11]. In 2006, the International Society for Cell and Gene Therapy (ISCT) set minimal criteria to define MSCs as (i) being adherent to plastic under standard culture conditions; (ii) being able to differentiate into osteocytes, adipocytes, and chondrocytes in vitro; and (iii) having a specific surface antigen profile [12]. Moreover, the ISCT supported the use of the term ‘mesenchymal stromal cells’ while ‘mesenchymal stem cells’ to be reserved for cells that demonstrate stemness by clearly stated criteria [13]. However, in response to the ongoing heated debate regarding the MSC terminology and in an attempt to reduce the existing confusion, the ISCT MSC committee issued another revised position statement in 2019. The committee recommended maintaining the use of the acronym MSC but with detailed annotation of tissue source and a robust matrix approach to demonstrate relevant functionality. Notably, they emphasized the necessity of providing stringent evidence for stem cell functionality to justify the use of the “stem cell” term [14].

MSCs are increasingly recognized not only for their regenerative and tissue repair capacities but also for their potential to fight infections which is primarily mediated by their immunomodulatory properties through direct or indirect mechanisms. Directly, through the secretion of antimicrobial peptides (AMP), also known as ‘host defense peptides’, that act as the first line of defense against a wide range of organisms including bacteria. In fact, MSCs have been shown to constitutively express four AMPs including Cathelicidins LL-37, β-defensin-2, hepcidin, and lipocalin-2. It was reported that bacterial preconditioning in MSCs leads to an increased expression of LL-37, β-defensin-2, and hepcidin [15]. Indirectly, MSCs contribute to the host immune response against pathogens, particularly by modulating the balance between pro-inflammatory and anti-inflammatory components of the immune system or enhancing the activity of phagocytes. The immunomodulatory potential of MSCs is demonstrated through their interactions with immune cells via direct cell-to-cell contact or paracrine activity by secretion of cytokines including PGE2, IL-4, IL-10, IL-12, IFN-γ, and TNF-α [16, 17]. As a result, this process comprises the involvement of T cells, B cells, natural killer (NK) cells, macrophages, monocytes, dendritic cells (DCs), and neutrophils, thereby enhancing MSCs’ capacity to combat bacterial infections.

In recent years, there has been a significant shift toward the use of MSC-based cell-free therapies. These cell-free-based therapies replicate much of the benefits associated with treatments using whole-cell MSCs, including their immunomodulatory and antibacterial effects, while avoiding the risks of immune rejection, emboli formation, and infection transmission associated with MSC transplantation [18]. In 2023, Yang et al. demonstrated that extracellular vesicles (EVs) from different MSC sources alone can significantly reduce bacterial load and improve survival rates in preclinical models of sepsis, thus offering a potential cell-free alternative to traditional stem cell therapy [19].

Currently, over 80 ongoing clinical trials are investigating the use of MSCs or their secretome against infections of diverse origins, such as sepsis, local bacterial infections, and viral ones-including COVID-19 [20]. Most of these approaches have targeted either the immunomodulatory role of MSCs or their anti-inflammatory action, usually to generally improve the course of both acute and chronic infections. Hence, this systematic review aimed to summarize findings regarding the therapeutic benefits of human MSCs from different sources including their secretome in the treatment of bacterial infections caused by various pathogens in animal models in vivo. This review seeks to establish the sufficiency of these findings as a justification for progressing toward clinical trials, with the goal of developing novel MSC-based therapies as a future application for human bacterial infections.

Methods

This systematic review was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria [21]. The research question was formulated with the PICO tool: the population (P) was that of laboratory animals, especially rodents, infected with any bacterial species, the intervention (I) was the treatment with either human MSCs or their secretome, the comparison (C) included data before and after the intervention as well as any differences observed between groups that were administered with stem cells or their secretome and those that were not such as untreated control (when such information was available), and the outcome (O) was the changes in the rates of overall survival, bacterial clearance, and disease complications, as well as any other therapeutic effects observed. This review protocol was registered with PROSPERO, number CRD42024544249 under the title “Stem cell therapy and its secretome on bacterial infections: a systematic review” https://www.crd.york.ac.uk/PROSPERO.

Inclusion and exclusion criteria

We included all studies addressing diseases caused by bacterial infections treated with either human MSCs or their secretome, provided they meet the following criteria: (i) written in English, (ii) original articles, (iii) addressed the effects of human MSCs and/or their secretome on preclinical animal models with bacterial infection. Otherwise, the following were excluded: (i) studies that do not fit the research type, including reviews, conference abstracts, case reports, editorials, letters to editors, or book chapters (ii) studies involving non-human MSCs interventions, and (iii) those involving large animal models rather than rodents.

Information source and search strategy

Two authors independently performed and validated the literature search, ensuring thoroughness and accuracy by performing the search in duplicate. A PubMed search was conducted up until 18th of May 2024 using the following MeSH keywords [22]: (((“Mesenchymal Stem Cells“[Mesh]) OR (((((((((Stem Cell*, Mesenchymal) OR (Mesenchymal Stem Cell*)) OR (Mesenchymal Stromal Cell*)) OR (Stromal Cell*, Mesenchymal)) OR (Multipotent Mesenchymal Stromal Cell*)) OR (Mesenchymal Stromal Cell*, Multipotent)) OR (Mesenchymal Progenitor Cell*)) OR (Wharton* Jelly Cell*)) OR (Wharton’s Jelly Cell*))) AND (((“Therapeutics“[Mesh]) AND “therapy” [Subheading]) OR ((therap*) OR (treatment*)))) AND ((((“Bacterial Infections“[Mesh]) AND “Gram-Positive Bacterial Infections“[Mesh]) AND “Gram-Negative Bacterial Infections“[Mesh]) OR (((((((((Bacterial Infection*) OR (Bacterial Disease)) OR (Infection*, Bacterial)) OR (Gram Positive Bacterial Infection*)) OR (Bacterial Infection*, Gram-Positive)) OR (Infection*, Gram-Positive Bacterial)) OR (Bacterial Infection*, Gram-Negative)) OR (Gram Negative Bacterial Infection*)) OR (Infection*, Gram-Negative Bacterial))). For the sake of this review, publication date was not considered as an exclusion aspect.

Study selection, risk of bias, and quality assessment

Separately, two authors evaluated the selected papers to ensure that their methods and publication suitability were met. To add to that, the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) risk-of-bias tool [23] was used to evaluate the bias risk in addition to the quality of the study. A study meeting the applicable criterion was given a “yes” judgment indicating a low risk of bias; however, those that did not were labeled with a “no” judgment indicating a high risk of bias. Studies containing insufficient information to judge were labeled with “unclear” indicating insufficient details to assess the risk of bias properly. All the SYRCLE assessment results are presented in Supplementary Table 1. In case of discrepancy, an agreement was made through discussion by the authors; otherwise, a third reviewer acted as an arbiter.

Data collection process

Data were extracted by two independent authors reading through the titles and abstracts for relevance of each publication. Further, they considered whether the language of the paper was suitable, whether the topic was conveyed correctly within the content, and whether the methodology met the necessary standards for inclusion. This process ensured that only studies of the highest standard were considered. Specific details of each selected study, including the first author’s name, year of publication, the type of animal used, the source and characterization of the hMSCs or the secretome, and the route of administration were presented in Table 1. Additionally, the table provides information on the type of bacteria studied, the main outcomes, any adverse effects observed, and the specific disease being treated. This approach helps ensure that the review is founded on a thorough and impartial selection of relevant studies.

Table 1.

Studies included in the systematic review

#	First author’s name - Publication Year	Animal Model	MSCs Characterization	MSCs Source	Culturing Method	Dose	Route of Administration	Bacteria Studied	Control Group Administration	Outcomes	Adverse Effects	Disease treated
1	Ahn et al., 2018 [48]	Newborn Sprague Dawley rats	CD73+, CD105+, CD14-, CD34- and CD45-, passage 6	human umbilical cord blood-derived mesenchymal stem cells	Cultured in vitro	1 × 10^5 of UCB-MSCs in 10 µl saline	Intraventricularly	Escherichia coli	Saline	- hUCB-MSCs treatment improved survival rates of meningitis-induced rats. - hUCB-MSCs treatment reduced bacterial count in CSF, and decreased levels of inflammatory cytokines and active brain macrophages.	No adverse effects mentioned	Neonatal bacterial meningitis
2	Curley et al., 2017 [35]	Male Sprague Dawley rats	CD13+, CD105+, CD73+, MHC-class I+, CD34-, CD133-, CD45-, and MHC-II-, passage 3	human umbilical cord and bone marrow mesenchymal stem/stromal cells	Cultured in vitro	1 × 10^7 of each hBM & hUC-MSCs in 300µL PBS	Intravenously	Escherichia coli	PBS	- UC-MSCs and BM-MSCs reduced wet-to-dry ratios, neutrophil infiltration, bacterial counts in BAL fluid, enhanced oxygenation, and respiratory compliance in E. coli pneumonia models. - UC-MSC treatment improved survival rates in E. coli-induced acute respiratory distress syndrome models compared to PBS controls. - UC-MSC treatment reduced alveolar infiltration of white blood cells and neutrophils, BAL protein, and pro-inflammatory cytokines (TNF-α, IL-6), while increasing anti-inflammatory proteins (IL-10, TNF-inducible gene 6). - UC-MSC treatment showed increased airspace levels in histologic analysis. - UC-MSC treatment reduced ROS levels and the expression of NOX-2 and iNOS in lung tissue.	No adverse effects mentioned	Acute Respiratory Distress Syndrome
3	Devaney et al., 2015 [38]	Male Sprague Dawley rats	passage 4	Human bone marrow mesenchymal stem/stromal cells & their conditioned medium (CM)	Cultured in vitro	1 × 10^7, 2 × 10^7, 5 × 10^6, 2 × 10^6 of hBM-MSCs in 300µL PBS	Intravenously + intratracheally	Escherichia coli	PBS	- hBM-MSCs increased survival rates in rats with E. coli-induced acute lung injury. - hBM-MSCs decreased lung injury severity, improving oxygenation, and lung compliance, and reducing lung bacterial load. - hBM-MSCs-CM improved animal survival but did not significantly reduce the severity of E. coli-induced lung injury in surviving animals.	No adverse effects mentioned	Acute Lung Injury
4	Gonzalez et al., 2023 [44]	Male Sprague Dawley rats	CD91+, CD61+, CD9+, CD146+, and CD29+, passage 3	Extracellular vesicles (EVs) of the human umbilical cord and bone marrow mesenchymal stem cells	Cultured in vitro	1 × 10^9 of Evs in 300µL PBS	Intravenously or through a Nebulizer	Escherichia coli	PBS	- MSC-EVs reduced the severity of lung injury and pneumonia caused by E. coli in rat models. - MSC-EVs improved oxygenation and reduced bacterial load.	No adverse effects mentioned	Pneumonia
5	Han et al., 2022 [47]	Male C57BL/6 mice	short hairpin scrambled RNA shSCR+, passages 5–6	human bone marrow-derived mesenchymal stromal cells	Cultured in vitro	5 × 10^5 of hBM-MSCs in 200µL PBS	Intravenously	Escherichia coli	PBS	- The shSCR MSC group led to a 70% bacterial reduction. - The shSCR MSC-treated group had a survival rate of 67%, indicating a 44% absolute improvement in survival compared to the control group. - Syndecan-2 expression in MSCs played a critical role in enhancing bacterial clearance and promoting the resolution of inflammation. - MSC treatment led to a reduction in inflammatory markers promoted better immune regulation, and showed a significant decrease in tissue damage	No adverse effects mentioned	Sepsis
6	Hao et al., 2019 [40]	Male C57BL/6 mice	CD105+, CD90+, CD73+, CD44+, CD166+, passages 3–8	Extracellular vesicles (EVs) of human bone marrow mesenchymal stem cells	Cultured in vitro	1 × 10^10 of Evs in 90µL PBS	Intravenously	Escherichia coli	PBS	- hBM-MSCs, especially EVs, reduced the severity of lung injury in mice induced by E. coli bacteria. - EVs increased bacterial clearance from mice lungs by increasing the levels of leukotriene B4 in the alveolus.	No adverse effects mentioned	Acute Lung Injury
7	Horie et al., 2020 [45]	Male Sprague Dawley rats	CD362+, CD73+, CD105+, CD90+, and MHCI+, CD34-, CD45-, CD80-, CD86-, passages 3,5,7,10	Human bone marrow and umbilical cord mesenchymal stromal cells	Cultured in vitro	1 × 10^7 hBM or hUC-MSCs in 300µL PBS	Intravenously	Escherichia coli	PBS	- All hMSCs utilized enhanced arterial oxygenation and lung compliance. - All hMSCs lowered the bacterial load, cell infiltration, and inflammatory cytokines within the BAL fluid of E. coli-infected mice. - CD362 + hUC-hMSCs, especially when used in association with antibiotics, enhanced therapeutic effects in infected lungs. - Cryopreserved CD362 + UC-MSCs were as efficient as those just harvested. - Dual-dose manner of CD362 + hUC-MSCs administration was more potent than a single dose.	No adverse effects mentioned	Acute Lung Injury
8	Jerkic et al., 2019 [41]	Male Sprague Dawley rats	CD90+, CD73+, CD105+, CD140b+, CD166+, CD31-,	Human umbilical cord and IL-10 umbilical cord mesenchymal stromal cells	Cultured in vitro	1 × 10^7 hUC-MSCs in 300µL PBS	Intravenously	Escherichia coli	PBS	- Both IL-10 UC-MSCs and UC-MSCs increased the survival rate in rodents - Both improved static lung compliance and decreased alveolar fluid protein concentrations. - IL-10 UC-MSCs were more effective in improving blood oxygenation and decreasing the alveolar arterial gradient. - Both reduced alveolar E. coli counts and alveolar concentrations of TNFα and IL-6.	No adverse effects mentioned	Acute Respiratory Distress Syndrome
9	Kim et al., 2011 [33]	Male ICR mice	CD105+, CD73+, CD34-, CD45-, CD14-, HLA-AB+, MHC-II-, passage 5	human umbilical cord blood-derived mesenchymal stem cells	Cultured in vitro	1 × 10^5 hUC-MSCs in 300µL PBS	Intratracheally	Escherichia coli	PBS	- hUCB-MSCs increased the survival rates in mice with E. coli-induced acute lung injury. - hUCB-MSCs decreased the lung injury scores and reduced inflammation in mice. - hUCB-MSCs lowered the bacterial counts in blood and BAL fluid.	No adverse effects mentioned	Acute Respiratory Distress Syndrome
10	Kim et al., 2022 [49]	Newborn Sprague Dawley rats	CD9+, CD64+, CD81+, passage 6	Extracellular vesicles (EVs) of Wharton’s jelly of human umbilical cord mesenchymal stem cells	Cultured in vitro	1 × 10^5 EVs in 10µL PBS	Intra-cerebroventricular transplantation	Escherichia coli	Saline	- EVs of Wharton’s jelly-derived MSCs reduced brain injury through anti-apoptotic, anti-gliosis, and anti-inflammatory effects. - MSC-EVs significantly reduced the levels of inflammatory cytokines. - There was no significant reduction in bacterial growth in the cerebrospinal fluid of newborn rats with meningitis caused by E. coli.	No adverse effects mentioned	Neonatal bacterial meningitis
11	Kim et al., 2023 [37]	Male ICR mice	CD73+, CD105+, CD14-, CD34-, CD45-, passages 5–6	Human umbilical cord blood mesenchymal stem cells	Cultured in vitro	1 × 10^5 hUC-MSCs in 0.05 cc PBS	Endotracheally	Escherichia coli	PBS	- hUCB-MSCs attenuated injury scores for alveolar congestion, alveolar wall thickness, alveolar hemorrhage, and neutrophil infiltration in the E. coli-induced ALI model. - hUCB-MSCs decreased high levels of pro-inflammatory cytokines (IL-1α, IL-1β, TNF-α, IL-6) in the lung tissue of E. coli-induced ALI mice. - hUCB-MSCs significantly reduced the M1 macrophage marker CD86 while significantly increasing the M2 macrophage marker and reducing M1 markers.	No adverse effects mentioned	Acute Lung Injury
12	Krasnodembskaya et al., 2010 [32]	Male C57BL/6 mice	CD45-, CD19-, passages 5–10	Human bone marrow mesenchymal stem cells	Cultured in vitro	1 × 10^6 hBM-MSCs in 30µL PBS	Intratracheally	Escherichia coli	PBS	- hBM-MSCs reduced bacterial counts and improved lung function. - The antimicrobial effect was linked to the secretion of the peptide LL-37 by MSCs.	No adverse effects mentioned	Acute Lung Injury
13	Masterson et al., 2018 [46]	Male Sprague Dawley rats	CD73+, CD105+, CD90+, MHCI+, CD34-, CD45-, CD80-, CD86-, passages 3–4	CD362+/- human bone marrow mesenchymal stromal cells	Cultured in vitro	1 × 10^7 hBM-MSCs in 300µL PBS	Intravenously	Escherichia coli	PBS	- CD362+, CD362-, and heterogeneous hBM-MSCs reduced the severity of E. coli-induced lung injury in rats. - All three types improved arterial oxygenation and lung compliance by decreasing the bacterial count. - CD362 + and heterogeneous hBM-MSCs reduced lung microvascular permeability, BAL tumor necrosis factor α concentrations, and significantly reduced the absolute number of neutrophils. - CD362 + hBM-MSCs therapy decreased alveolar thickening and increased recovery of airspace volume. - CD362 + hBM-MSCs reduced the overall alveolar inflammatory cell infiltration.	No adverse effects mentioned	Acute Lung Injury
14	Monsel et al., 2015 [39]	Male C57BL/6 mice	CD44+	Micro-vesicles (MVs) of human bone marrow mesenchymal stem cells	Cultured in vitro	90µL of MVs derived from 9 × 10^6 hBM-MSCs	Intratracheally or intravenously	Escherichia coli	PBS	- MVs of hBM-MSCs improved survival rates in mice with bacterial pneumonia, partly due to keratinocyte growth factor (KGF) secretion. - hBM-MSCs MVs treatment decreased inflammatory cells, cytokines, protein, and bacteria in the lungs.	No adverse effects mentioned	Pneumonia
15	Sung et al., 2016 [34]	Male ICR mice	CD105+, CD73+, HLA-AB+, CD34-, CD45-, CD14-, MHC-class II-, passage 5	Human umbilical cord blood mesenchymal stem cells	Cultured in vitro	1 × 10^5 hUC-MSCs in 0.05 ml PBS	Intratracheally	Escherichia coli	Saline	- hUC-MSCs improved the histopathology of the lungs by reducing alveolar congestion, hemorrhage, neutrophil infiltration, and wall thickening. - hUC-MSCs decreased the bacterial load in the colony-forming units (CFU) of BAL fluid. - hUC-MSCs increased the levels of human beta-defensin 2 (an antibacterial agent) via toll-like receptor 4 signaling. - hUC-MSCs decreased the levels of pro-inflammatory cytokines (IL-1α, IL-1β, IL-6, TNF-α).	No adverse effects mentioned	Pneumonia
16	Varkouhi et al., 2019 [43]	Male Sprague Dawley rats	CD29+, CD73+, CD44+, CD105+	Extracellular vesicles of interferon–primed or naïve human umbilical cord mesenchymal stromal cells	Cultured in vitro	1 × 10^8 of Evs in 100µL PBS	Intravenously	Escherichia coli	PBS	- Treatment with interferon-γ–primed and naïve hUC-MSCs EVs improved survival rates in rats with E. coli-induced pneumonia. - γ–primed hUC-MSCs EVs significantly reduced the alveolar-arterial oxygen gradient and alveolar protein leak, indicating less lung injury. - EVs from γ–primed hUC-MSCs increased mononuclear phagocyte numbers and reduced tumor necrosis factor-alpha concentrations within the alveolar fluid. - The γ–priming hUC-MSC-derived EVs increased mononuclear phagocyte numbers and decreased tumor necrosis factor-alpha in the alveolar fluid.	No adverse effects mentioned	Acute Lung Injury
17	Zhu et al., 2017 [36]	Neonatal mixed-gender pups	CD73+, CD90+, CD105+, CD11b-, CD19-, CD34-, CD45-, HLA-DR, passage 2	Human umbilical cord mesenchymal stromal cells & their conditioned medium (CM)	Cultured in vitro	25µL of CM derived from 1 × 10^7 hUC-MSCs	Intravenously	Escherichia coli	Saline	- hUC-MSCs increased the survival rates in neonatal rats with E. coli-induced sepsis. - Both MSCs and their conditioned medium (CM) significantly decreased bacterial counts in blood, lung, spleen, and brain. - Both increased the number of CD206 + cells in the spleen, enhancing the phagocytic capacity of spleen macrophages. - Both hUC-MSCs and their CM increased plasma levels of the antimicrobial peptide LL-37 in E. coli-infected animals compared to saline-treated animals.	No adverse effects mentioned	Sepsis
18	Park et al., 2019 [42]	C57BL/6 mice	CD81+, flotillin-1+, beta-actin+, passage 5	Nanovesicles (NVs) of human bone marrow mesenchymal stromal cells	Cultured in vitro	100 µL of NVs derived from 2 × 10^9 hBM-MSCs	Intraperitoneally	Escherichia coli-derived outer membrane vesicles	Mice injected with Escherichia coli-derived OMVs without treatment with NVs	- NVs of hBM-MSCs reduced inflammatory symptoms in mice infected with E. coli OMVs. - NVs of hBM-MSCs decreased the number of inflammatory cells and pro-inflammatory cytokines in the peritoneal cavity. - IL-10 was an essential key for the immunomodulatory effects of NVs.	No adverse effects mentioned	Sepsis
19	Park et al., 2021 [70]	Male C57BL/6 mice	-	Human placenta-derived mesenchymal stem cells & their conditioned medium (CM)	Cultured in vitro	100 µL of CM derived from 1 × 10^7 hPD-MSCs	Orally	Helicobacter pylori	Vehicle	- CM of hPD-MSCs reduced gastric inflammation following H. pylori infection. - CM of hPD-MSCs prevented stomach tissue damage. - CM of hPD-MSCs promoted tissue regeneration by increasing the expression of markers associated with tissue regeneration, such as Lgr 5+, Ki-67, and Musashi-1. - CM of hPD-MSCs restored gut microbiome balance.	No adverse effects mentioned	Chronic Atrophic Gastritis
20	Wang et al., 2020 [59]	Male C57BL/6 mice	CD73+, CD90+, CD105+, HLA-ABC+, CD34-, CD45-, MHC-class II-, passages 15–20	Human placental-derived mesenchymal stem cells	Cultured in vitro	5 × 10^3 hPD-MSCs in 100µL PBS	Intraperitoneally	Hypervirulent Klebsiella pneumoniae	PBS	- hPD-MSCs enhanced polymorphonuclear leukocyte functions (PMN) recruitment and activation, leading to improved PMN functions such as phagocytosis and reactive oxygen species (ROS) production, primarily due to IL-1β production. - hPD-MSCs suppressed T cell and NK cell activity, reducing their counts in the peritoneal cavity. - hPD-MSCs promoted bacterial clearance in the liver and abdominal cavity, resulting in improved overall survival rates.	No adverse effects mentioned	Pneumonia
21	Byrnes et al., 2023 [57]	Male Sprague Dawley rats	passages 3–4	Human naïve or cytomix-preactivated umbilical cord mesenchymal stromal cells	Cultured in vitro	5 × 10^7 hUC-MSCs in 1mL PBS	Intravenously	Klebsiella pneumonia	PBS	- Two doses of hUC-MSCs reduced bacterial counts and improved lung function. - Cytomix hUC- MSCs restored arterial oxygenation with both single and multiple doses. - Cytomix hUC- MSCs reduced inflammatory cytokines in BAL fluid. - hUC- MSCs increase CD4+/CD8 + ratio T cells.	No adverse effects mentioned	Pneumonia
22	Perlee et al., 2019 [55]	Female C57BL/6 mice	-	Freshly cultured and cryopreserved human adipose-derived mesenchymal stem cells	Cultured in vitro	1 × 10^6 hAD-MSCs in 200 µl Ringer’s lactate	Intravenously	Klebsiella pneumonia	Ringer’s lactate	- hAD- MSCs reduced bacterial growth and dissemination in both lungs and distant organs. - hAD- MSCs decreased the levels of pro-inflammatory cytokines TNF-α, IL-6, and MCP-1, thus attenuating the release of systemic cytokines.	Transient formation of multiple microthrombi in the lungs	Pneumosepsis
23	Perlee et al., 2019 [56]	Female C57BL/6 mice	-	Freshly cultured and cryopreserved human adipose-derived mesenchymal stem cells	Cultured in vitro	1 × 10^6 hAD-MSCs in 200 µl Ringer’s lactate	Intravenously	Klebsiella pneumonia	Ringer’s lactate	- hAD-MSCs reduced Klebsiella load from mice lungs, blood, and liver. - hAD-MSCs induce systemic coagulation through tissue factor (TF) expression, leading to transient microthrombi formation in the lungs.	plasma thrombin-antithrombin complexes had increased, eliciting an inflammatory response in the lungs	Pneumosepsis
24	Byrnes, Masterson, Brady, et al., 2023 [58]	Male Sprague Dawley rats	passages 1–3	Human naïve or cytomix-preactivated umbilical cord, bone marrow, & adipose tissue mesenchymal stromal cells	Cultured in vitro	1 × 10^7 hAD, hBM, & hUC-MSCs in PBS	Intravenously	Klebsiella pneumoniae	PBS	- Naïve BM- and UC-MSCs increased lung airspace fraction, while pre-activated AD-MSCs restored it. - Naïve BM-MSCs reduced lung inflammation. - Naïve MSCs prevented the increase in neutrophils and monocytes in BAL fluid, while pre-activated MSCs increased them. - Naïve UC- and AD-MSCs increased Treg counts. - Naïve and pre-activated BM-MSCs decreased NK cell counts.	No adverse effects mentioned	Pneumosepsis
25	Wang et al., 2023 [60]	C57BL/6 mice	-	Human placental mesenchymal stem cells	Cultured in vitro	3 × 10^5 hPD-MSCs in PBS	Intravenously	Klebsiella pneumoniae	PBS	- hPD-MSCs boosted M2 alveolar macrophages over M1 bone marrow macrophages. - IL-1β secretion from hPD-MSCs improved pathogen clearance and survival rates. - hPD-MSCs decreased bacterial load, tissue injury, and inflammation.	No adverse effects mentioned	Acute Respiratory Distress Syndrome
26	Shaw et al., 2024 [84]	Female balb/c mice	passages 2–6	Human bone marrow mesenchymal stromal cells	Cultured in vitro	1 × 10^6 hBM-MSCs in PBS	Intravenously	Mycobacterium avium	Saline	- hBM-MSCs decreased pulmonary CFU, indicating a reduction in bacterial burden in mice lungs. - Bacterial load decrease was not observed in the spleen and liver of the mice.	No adverse effects mentioned	Mycobacterium avium complex pulmonary disease
27	Bonfield et al., 2021 [83]	C57BL/6J wild-type (WT) mice & cystic fibrosis (CF) mouse model	passages 2–3	Human bone marrow mesenchymal stem cells	Cultured in vitro	1 × 10^6 hBM-MSCs in 100 µL	Intranasally	Mycobacterium avium & Mycobacterium intracellulare	Placebo	- hBM-MSCs significantly reduced the CFUs of Mycobacterium intracellulare and M. avium in infected mice compared to untreated mice.	No adverse effects mentioned	Cystic Fibrosis
28	Krasnodembskaya et al., 2012 [64]	Male C57BL/6J mice	CD45-, CD19-, passages 3–10	Human bone marrow mesenchymal stem cells	Cultured in vitro	1 × 10^6 hBM-MSCs in 150 µL PBS	Intravenously	Pseudomonas aeruginosa	PBS	- hBM-MSCs improved the survival rates in mice with Pseudomonas aeruginosa sepsis. - MSC-treated mice had a marked reduction in bacterial colony-forming units of Pseudomonas aeruginosa in their blood. - hBM-MSCs enhanced the phagocytic activity of blood monocytes, leading to better bacterial clearance. - MSC treatment was associated with lower levels of plasma plasminogen activator inhibitor 1 (PAI-1) and higher platelet counts, indicating an improvement in sepsis severity.	No adverse effects mentioned	Sepsis
29	Shi et al., 2021 [67]	C57BL/6 & BALB/c mice	CD9+, CD63+, CD81+, TSG101+, passages 4	Extracellular vesicles (EVs) of human adipose tissue mesenchymal stromal cells	Cultured in vitro	2 × 10^6 hAD-MSCs EVs	Inhaled via Nebulizer	Pseudomonas aeruginosa	Saline	- Nebulized EVs of hAD-MSCs increased the survival rate to 80% at 96 h in a mouse lung injury model caused by Pseudomonas aeruginosa. - Nebulized EVs of hAD-MSCs significantly decreased lung inflammation and histological severity.	No adverse effects mentioned	Acute respiratory distress syndrome
30	Wang et al., 2022 [66]	Male Sprague Dawley rats	passages 4–5	Human chorion and bone marrow mesenchymal stem cells	Cultured in vitro	5 × 10^5 hBM & hC-MSCs in 0.2 ml PBS	Intravenously	Pseudomonas aeruginosa	PBS	- hC-MSCs and hBM-MSCs reduced inflammatory cell infiltration and interstitial thickening caused by Pseudomonas aeruginosa. - Both decreased the lung injury score, reduced inflammatory cell influx and decreased the secretion of inflammatory cytokines (IL-1β, IL-6, TNF-α) in the injured alveolus. - Both significantly reduced the lung wet-to-dry ratio and total protein concentration in BAL fluid. - Both increased Tregs and IL-10 levels while reducing Th17 cells, levels of IL-17 and IL-22, and increasing the Tregs to Th17 cells ratio. - Both restored KGF-2 and SPC mRNA levels and decreased the levels of Caspase 3 and PCNA mRNA. - hC-MSCs showed the best therapeutic effect.	No adverse effects mentioned	Acute respiratory distress syndrome
31	Sutton et al., 2016 [65]	C57BL/6J mice & Cftrtm1Kth knockout mice	passages 2–3	Human adipose tissue & bone marrow mesenchymal stem cells	Cultured in vitro	1 × 10^6 hBM & hAD-MSCs	Intravenously	Pseudomonas aeruginosa & Staphylococcus	Wild-type mice infected with either PA or S. aureus	- Reduced bacterial load of PA and S. aureus after 24 h of infection. - Enhanced the reduction of bacterial load when combined with antibiotics. - hAD-MSCs combined with antibiotics showed more potent effects than hBM-MSCs.	No adverse effects mentioned	Cystic Fibrosis
32	Kim et al., 2014 [75]	HLA-DR4 transgenic C57Bl/6 mice	passages 3,6	Human mesenchymal stem cells	Cultured in vitro	2.5 × 10^5 h-MSCs	Intravenously	Staphylococcal enterotoxin B	PBS	- hMSCs suppressed pro-inflammatory cytokine production, such as IL-2, IL-6, and TNF, induced by SEB in a mouse model. - hMSCs failed to improve overall survival in HLA-DR4 transgenic mice with toxic shock syndrome.	No adverse effects mentioned	Toxic shock syndrome induced by Staphylococcal enterotoxin B (SEB)
33	Li et al., 2020 [76]	Male C57BL/6J mice	passages 4–6	BPI21/LL-37-engineered human umbilical cord mesenchymal stem cells	Cultured in vitro	2 × 10^5 hUC-MSCs	Intravenously	Staphylococcus aureus	PBS & Wild type human umbilical cord mesenchymal stem cells	- Engineered hMSCs significantly reduced the bacterial load and serum LPS (lipopolysaccharide) levels in septic mice. - Engineered hMSCs improved survival rates more than using WT-hUC-MSCs.	No adverse effects mentioned	Sepsis
34	Chow et al., 2020 [79]	Nu/nu mice	passage 4	Human bone marrow mesenchymal stem cells & their conditioned medium (CM)	Cultured in vitro	1 × 10^6 hBM-MSCs in 200µL DPBS	Intravenously	Staphylococcus aureus	PBS or antibiotics alone (Amoxiclav)	- hBM-MSCs, especially its conditioned medium (CM), combined with amoxicillin-clavulanic acid antibiotic, reduced bacterial growth by over 2 log CFU/mL in mice infected with S. aureus.	No adverse effects mentioned	S. aureus biofilm infection
	Fridoni et al., 2019 [78]	Wistar male adult rats	CD73+, CD90+, CD105+, CD45-, CD34- passage 4	Human bone marrow mesenchymal stem cells & their conditioned medium (CM)	Cultured in vitro	500 µL CM of hBM-MSCs	Intraperitoneally	Methicillin-resistant Staphylococcus aureus (MRSA)	Placebo	- CM of hBM-MSCs in combination with photo biomodulation therapy (PBMT) significantly enhanced wound healing in diabetic rats infected with MRSA. - The combined therapy decreased inflammation by significantly reducing neutrophil and macrophage counts. - Both therapies increased fibroblast numbers, especially when used together. - Enhancement in angiogenesis was observed, with the most substantial effects in the combined group.	No adverse effects mentioned	Wound healing
36	Huang et al., 2021 [77]	Male C57BL/6J mice	passages 3–4	Human umbilical cord mesenchymal stem cells	Cultured in vitro	1 × 10^7 hUC-MSCs in PBS	Intradermally	methicillin-resistant Staphylococcus aureus (MRSA)	Untreated group	- Improved wound healing in diabetic mice with MRSA-infected wounds, especially when combined with 5-aminolevulinic acid photodynamic therapy (ALA-PDT). - Reduced the bacterial burden in a diabetic mouse model infected with MRSA when combined with ALA-PDT therapy (nearly 100% reduction in bacterial colonies).	No adverse effects mentioned	Wound healing
37	Kouhkheil et al., 2018 [74]	Wistar male adult rats	CD105+, CD90+, CD73+, CD34-, CD45- passage 4	Human bone marrow mesenchymal stem cells & their conditioned medium (CM)	Cultured in vitro	500 µL CM of hBM-MSCs	Intraperitoneally	Methicillin-resistant Staphylococcus aureus (MRSA)	Placebo	- hBM-MSC-CM combined with the application of Pulsed Wave Low-Level Laser Therapy (PW LLLT) significantly accelerated wound healing in an MRSA-infected diabetic rat model. - The combined therapy decreased the colony-forming units (CFUs) of MRSA from the rat wounds. - Both PW LLLT and hBM-MSC-CM exerted a synergistic effect in promoting wound healing and reducing bacterial load in infected wounds of diabetic rats.	No adverse effects mentioned	Wound healing

The table provides a comprehensive overview of the included studies investigating the therapeutic potential of mesenchymal stem cells (MSCs) in bacterial infections. h= human; AD−MSCs=human adipose−derived mesenchymal stem cells, BM−MSCs = bone marrow−derived mesenchymal stem cells, UCB−MSCs= umbilical cord blood−derived mesenchymal stem cells, PD−MSCs= placental− derived mesenchymal stem cells, C−MSCs = chorion−derived MSCs, UC−MSCS = umbilical cord−derived mesenchymal stem cells, EVs = extracellular vesicles, PBS= phosphate buffered saline, MRSA= Methicillin−resistant Staphylococcus aureus

Data synthesis

Because of the heterogeneity among studies in terms of MSC sources, intervention protocols, and outcome measures, a meta-analysis could not be conducted. Therefore, all studies that met the inclusion criteria were synthesized narratively. This approach is discursive offering an overview of the current knowledge in a specific area by considering a wide variety of sources and generating conclusions via reasoning and argumentation [24]. To further analyze the effectiveness of the study results, bacterial reduction percentages were determined from data of figures using the Web-plot Digitizer software program (version 5) [25] or from the descriptive text in the article results. The bacterial reduction percentage was calculated according to the formula: [(control CFU – test CFU)/ control CFU] × 100 [26].

Results

Throughout the initially conducted search run, 517 documents were retrieved. Seventeen articles were eliminated before screening because they were either not written in English or lacked an abstract or full text. In the first round of screening, 371 articles out of the remaining 500 articles were excluded for various reasons, including those that were not original research, were unrelated to the intended topic, did not address either stem cell therapy or bacterial infection, focused on bacterial infection but not stem cell therapy, or stem cell therapy but not bacterial infection. In the second round of screening, 92 out of the remaining 129 articles were excluded because researchers were using non-human mesenchymal stem cells, did not specify the bacteria used, used an animal model other than rodents, or conducted in vitro experiments. At the end of the screening process, only 37 articles were retained for the systematic review and narrative analysis (Fig. 1).

Fig. 1.

Flow chart summarizing the study selection procedure

Due to the diversity across the studies, results were presented according to pathogen type. Out of the 37 articles retrieved, 29 focused on Gram-negative bacteria and 8 on Gram-positive bacteria. Specifically, Escherichia coli (E. coli), was used in 18 studies; while both Klebsiella pneumoniae (K. pneumoniae) and Staphylococcus aureus (S. aureus) were featured in 6 studies each. Four studies examined Pseudomonas aeruginosa (P. aeruginosa), while Helicobacter pylori (H. pylori) and Mycobacterium species (M. spp.) were investigated in 1 and 2 studies, respectively. The results of each study were discussed individually, focusing on bacterial clearance, survival rates, immune modulation, and treatment methods.

The articles reviewed manifest various MSC sources, with human bone marrow-derived MSCs (hBM-MSCs) being the most frequently used (43%), followed by human umbilical cord blood-derived MSCs (hUC-MSCs) (34%), human adipose-derived MSCs (hAD-MSCs) (12%), and other MSC sources (11%), including placental (hP-MSCs) and chorion-derived MSCs. Out of these studies, thirteen articles used MSC derivatives such as conditioned media (CM), extracellular vesicles (EVs), micro-vesicles (MVs), and nano-vesicles (NVs). Figure 2 illustrates the distribution of MSC sources and the routes of administration across the studies with some utilizing either more than one MSC source or more than one route of administration or a combination of both. The most commonly used route of administration was systemic, mainly via intravenous delivery, with 30 out of 37 studies employing this method.

Fig. 2.

Distribution of MSC sources and routes of administration across studies. This figure illustrates the distribution of MSC sources and their corresponding routes of administration in the included studies. The inner pie chart shows the percentage distribution of MSCs derived from different sources: hBM-MSCs (43%), hUC-MSCs (34%), hAD-MSCs (12%), and Other MSCs (11%), including placental and Chorion derived MSCs. The outer pie chart displays the routes of administration used across the studies, grouped into broader categories: Systemic Route (intravenous administration), Respiratory Route (intratracheal, endotracheal, and nebulized delivery), CNS Route (intracerebroventricular or intraventricular administration), Abdominal Route (intraperitoneal delivery), Local Route (intradermal), Nasal Route (intranasal), and Oral Route

For hBM-MSCs, 14 studies used the systemic route, 4 studies applied the respiratory route (via intratracheal, endotracheal, or nebulized delivery), 3 studies utilized the abdominal route (intraperitoneal administration), and 1 study used the nasal route. Similarly, studies using hUC-MSCs predominantly relied on systemic administration (9 studies), followed by the respiratory route and the central nervous system (CNS) route, which included intracerebroventricular and intraventricular administration. Only one study employed intradermal administration (local route). The majority of studies involving hAD-MSCs also used systemic administration (4 studies), with 1 study opting for the respiratory route. For other MSC types, 3 studies used the systemic route, while the remaining 2 studies employed either the abdominal or the oral routes (Fig. 2). These data underscore the predominance of both the BM-MSCs usage, and the systemic administration across different MSC sources, with less frequent reliance on routes such as oral and abdominal delivery.

Mesenchymal stem cells in gram-negative bacterial infections

hMSCs and Escherichia coli

Escherichia coli (E. coli), associated with more than 2 million deaths annually, is a member of the Enterobacteriaceae family. It is a Gram-negative, rod-shaped bacilli that is known to be part of the commensal intestinal human microflora; however, it can become pathogenic and cause both intestinal and extraintestinal illnesses [27, 28]. There are growing concerns regarding multidrug resistance in E. coli. The latest statistics published by the CDC in 2021 showed that 34.7% of E. coli isolates were resistant to fluoroquinolone [29], 23.9% resistant to cephalosporins [30], and 0.8% resistant to carbapenems [31], categorizing them as urgent threats. Thus, resistant isolates of E. coli have become increasingly difficult to treat, urging the need for alternative therapies like stem cell treatment. Herein, E. coli studies utilized both BM-MSCs and UC-MSCs, as well as cell-free therapies. Among the 11 studies involving UC-MSCs, 4 employed MSC secretome. Similarly, out of the 10 studies using BM-MSCs, 4 utilized MSC secretome.

In 2010, Krasnodembskaya and coauthors were the first to suggest a possible alternative therapy using stem cells given their potential antimicrobial effect on a mouse model of E. coli pneumonia. Intratracheal administration of hBM-MSCs significantly reduced bacterial counts and neutrophil levels, as compared to the PBS group, in the bronchoalveolar lavage (BAL) samples of mice with E. coli pneumonia through the antimicrobial peptide LL-37 secreted by MSCs [32]. Another study demonstrated that administration of hUCB-MSCs intratracheally in a mouse model of E. coli-induced acute lung injury (ALI) decreased the bacterial counts in blood and BAL and attenuated lung injury and inflammation by decreasing the levels of inflammatory cytokines and subsequently improved survival [33]. It was indicated that β-defensin 2 secreted by the MSCs was the key paracrine factor mediating the antibacterial effects against E. coli [34].

hUC-MSCs were also effective in reducing lung injury severity and bacterial load in E. coli-induced Acute Respiratory Distress Syndrome (ARDS) [35] as well as in improving survival and bacterial clearance in neonatal sepsis with E. coli-induced sepsis after administrating the MSCs intravenously [36]. A similar research work suggested that the anti-inflammatory effects of hUCB-MSCs were mediated by the suppressor of cytokine signaling (SOCS) proteins in E. coli-induced ALI mouse model [37].

In a study by Devaney and colleagues, both intravenous and intratracheal administration of hBM-MSCs significantly improved survival rates, enhanced arterial oxygenation, and increased lung compliance in rat models of E. coli-induced acute lung injury (ALI). Additionally, the treatment decreased alveolar fluid protein levels and reduced lung bacterial load. Notably, while the hBM-MSCs secretome improved animal survival, it did not attenuate the E. coli-induced lung damage [38]. It was hypothesized that using a higher concentration of hMSCs for secretome production or repeated dosing might have been more effective. On the other hand, other investigators demonstrated for the first time that micro-vesicles (MV) derived from hBM-MSCs were as effective as their parent stem cells in improving survival, decreasing the influx of inflammatory cells, cytokines, protein, and lowering the total bacterial load in E. coli–induced ALI in mice [39]. It was shown that the antimicrobial activity of hMSC-MVs was associated with increased production of leukotriene (LT) B4 in the injured alveoli and subsequently increased phagocytosis of E. coli through LTB4/BLT1 signaling [40]. Similarly, Jerkic and co-authors reported that IL-10 overexpression enhanced the efficacy of hUC-MSCs in bacterial E. coli-induced ARDS in rodent models [41].

Likewise, the therapeutic effect of intraperitoneally administered MSC-derived nanovesicles (NVs) was also shown to be via IL-10 release in a mouse model of sepsis provoked by E. coli [42]. Additionally, studies have shown that extracellular vesicles derived from interferon-γ–primed hUC-MSCs were more effective at reducing E. coli–induced ALI in rats than vesicles derived from naïve MSCs [43]. Recently, Gonzalez et al. studied the therapeutic efficacy of directly nebulized BM or UC-MSCs-EVs in attenuating E. coli-induced pneumonia in rat models [44]. The results showed that nebulized EVs improved blood oxygenation, reduced bacterial load, lowered inflammatory cytokine levels, and decreased lung injury severity in the E. coli-induced pneumonia models.

A research group from Ireland demonstrated that purified BM or UC-derived CD362 + hMSCs were as effective as their heterogenous cell populations in reducing E. coli-induced lung injury in rats [45, 46]. Another research group explored the important role of endogenous CD362 in experimental polymicrobial sepsis. Data demonstrated that CD362 plays a crucial role in resolving inflammation, leading to reduced tissue damage and enhanced survival during sepsis [47]. Using a single marker that defines a specific pure population may lead to a more characterized therapeutic and less batch-to-batch variability, thus easier licensing in the future. These findings suggest that MSCs from different sources are promising for various infectious disorders such as E. coli-induced pneumonia and sepsis. However, the therapeutic efficacy of MSCs in other infectious diseases such as neonatal bacterial meningitis has not been tested yet.

In 2018, Ahn et al. aimed to determine the effect of hUCB-MSCs on neonatal bacterial meningitis which is associated with high mortality and morbidity rates, despite appropriate antimicrobial therapy. Intraventricular transplantation of hUCB-MSCs attenuated brain injury and significantly reduced the cerebrospinal fluid (CSF) bacterial burden in neonatal E. coli meningitis, which led to a significant improvement in survival rates [48]. To further explore the protective mechanisms of MSC-EVs against brain injury via paracrine manner, they were intracerebroventricularly transplanted. MSC-EV treatment significantly attenuated brain cell death, reactive gliosis (a reaction to brain injury), and inflammation in an E. coli-induced neonatal meningitis model [49].

To summarize, out of the 18 articles examining E. coli infections, 16 reported increased survival rates in the animal models, while 17 demonstrated improved bacterial clearance at the infection site, with some studies also highlighting reductions in bacterial burden at secondary sites. Interestingly, all studies described various immune modulation mechanisms that significantly reduced inflammation, from increased macrophage activity to decreased inflammatory cytokine production. Thus, these results highlight MSCs’ important immunomodulatory function in the fight against bacterial infections.

hMSCs and Klebsiella pneumoniae

Klebsiella pneumoniae (K. pneumoniae) is a Gram-negative, nonmotile, rod-shaped bacterium commonly found on human mucosal surfaces including the gastrointestinal tract and the oropharynx [50]. It is the second leading cause of bloodstream infections, following E. coli [51]. K. pneumoniae accounts for a variety of infectious diseases. However, up till now, it remains unclear why Klebsiella species cause more infections than other gram-negative opportunistic pathogens. Many studies have addressed the issue and concluded that this may be due to Klebsiella’s antimicrobial-resistant genes [52], its ability to withstand starvation [53], and its exchange of DNA with other members of the human microbiome [54]. As the emergence of multidrug-resistant (MDR) K. pneumoniae increases, thereby limiting the available medications, the development of a new antimicrobial agent is an urgent issue [52].

In 2019, Perlee et al. were the first to address the effect of intravenously administrated hAD-MSCs on rats with pneumosepsis caused by K. pneumoniae. They observed that hAD-MSCs reduced bacterial growth and dissemination in the lungs as well as attenuated the release of systemic cytokines [55, 56]. Another study reported that multiple dosing of preactivated hUC-MSCs enhanced lung function by improving arterial oxygenation and reduced bacterial counts in BAL and blood of rat models of established MDR K. pneumoniae-induced pneumosepsis [57]. The same research group went on to determine the efficacy of naïve and cytokine pre-activated MSCs from different sources in antibiotic-resistant Klebsiella pneumosepsis at 48 h post-infection [58]. hUC- or hBM-MSCs were more effective than hAD-MSCs in treating rats with K. pneumoniae. In addition, the naïve MSCs demonstrated a more favorable profile of efficacy when compared to pre-activated MSCs [58]. Similarly, a Taiwanese research group reported that human placental-derived MSCs (hP-MSCs) promoted bacterial clearance, decreased local inflammation, and improved lung injury and overall survival in Hypervirulent Klebsiella-infected mice primarily through secretion of IL-1β [59, 60]. Taken together, these studies utilized a variety of MSC sources, with AD-MSCs being the most commonly used in 3 studies. Two studies focused on UC-MSCs, and another two on hP-MSCs, while only one study utilized BM-MSCs. It is worth noting that all treatments against K. pneumoniae were cell-based therapies. Herein, findings clearly demonstrate the effectiveness of MSCs from various sources in treating Klebsiella infections in preclinical animal models with reductions in bacterial load and enhancements in survival rates.

hMSCs and Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) is a widely distributed motile, Gram-negative rod-shaped bacterium that can cause a variety of serious bacterial infections, especially in critically ill and immunocompromised patients [61]. Treatment of P. aeruginosa has become increasingly difficult due to its ability to grow in diverse environmental conditions and its capacity to develop antibiotic resistance [62]. The emergence of multidrug-resistant strains has rendered standard antibiotic treatments ineffective [63]. Consequently, it is crucial to find novel treatment methods to combat P. aeruginosa infections.

An early study by Krasnodembskaya et al. investigated the therapeutic effect of hBM-MSCs in a mouse model of peritoneal sepsis induced by P. aeruginosa [64]. The intravenous administration of hBM-MSCs significantly improved the survival rate of mice and reduced sepsis severity by lowering plasma plasminogen activator inhibitor 1 (PAI-1) levels and increasing platelet counts [64]. As mentioned earlier, human hAD- & hBM-MSCs reduced not only S. aureus bacterial burden but also P. aeruginosa from the Cystic fibrosis (CF) mice models due to the secretion of LL-37 antimicrobial peptide [65]. Another research team studied the effects of MSCs on ALI caused by lipopolysaccharide (LPS) derived from P. aeruginosa [66]. In this LPS-induced ALI model, both intravenously administered human chorion-derived MSCs and hBM-MSCs significantly reduced lung injury, inflammatory cytokine levels, and the recruitment of inflammatory cells; thereby demonstrating the therapeutic effects of MSCs in LPS-induced lung injury [66]. In a similar model, Shi et al. focused on assessing the preclinical efficacy of cell-free therapy using hAD-MSC-EVs [67]. MSC-EVs showed similar therapeutic effects to MSCs in the P. aeruginosa-induced murine lung injury model by decreasing lung inflammation and histological severity, thus improving the survival rate of mice [67].

For the selected P. aeruginosa studies, three used BM-MSCs, two employed AD-MSCs, and a single study utilized chorion-derived MSCs. Consistent with previous findings, all studies highlighted the immunomodulatory effects of MSCs. Additionally, three out of four studies reported bacterial reduction, while half of the studies demonstrated an increase in survival rates in the in vivo models.

hMSCs and Helicobacter pylori

Helicobacter pylori (H. pylori) is a spiral-shaped, Gram-negative, microaerophilic pathogen that affects the stomach and accounts for high morbidity and mortality rates due to the serious gastrointestinal conditions it causes [68, 69]. H. pylori infections can also induce chronic atrophic gastritis (CAG) that can ultimately lead to gastric cancer [70]. Therefore, repairing gastric atrophy could be an effective approach to prevent H. pylori-associated gastric carcinogenesis. Given that, a Korean research group hypothesized that hP-MSCs and their secretome might be an attractive therapeutic option to treat H. pylori-associated CAG and thus prevent carcinogenesis [70]. In their study, they established the H. pylori-induced CAG mouse and administered ten oral doses of hP-MSCs and their secretome [70]. Results clearly showed that both treatments had remarkably reduced gastric inflammation, dysplastic changes, and atrophy. In addition, the community compositions of the microbiome in the treated mice were improved reflecting the rejuvenation of the CAG tissues in response to MSC intervention [70]. These findings present MSCs and their secretome as a novel rejuvenating agent that can prevent the progression of H. pylori-associated CAG to gastric cancer.

Mesenchymal stem cells in gram-positive bacterial infections

hMSCs and Staphylococcus species

Staphylococcus species, which are part of the normal human microbiota, are Gram-positive, non-motile, non-spore-forming cocci that can cause opportunistic infections in both community and hospital settings [71, 72]. Among these pathogens, Staphylococcus aureus (S. aureus) stands out as the most predominant coagulase-positive Staphylococci, causing a wide range of clinical infections in humans such as bacteremia, endocarditis, and other conditions associated with invasive medical devices [73]. In contrast, coagulase-negative Staphylococci are generally considered less pathogenic. Staphylococci are characterized by their rapidly increasing antibiotic resistance, making them one of the most critical challenges in the management of Staphylococcal infections worldwide [72]. It has been reported that nearly 50% of all S. aureus isolates are methicillin-resistant Staphylococcus aureus (MRSA) [74], highlighting the urgent need for alternative antimicrobial agents to combat S. aureus and its resistant strains. Numerous researchers have stressed the significance of MSCs and their secretome, as a potential antibacterial agent against S. aureus.

In 2014, Kim et al. explored the therapeutic potential of hMSCs in an experimental model of fatal Staphylococcal toxic shock syndrome (TSS) [75]. It is worth noting that no drug or vaccine has been approved by the FDA for TSS, which is characterized by systemic capillary leak, septic shock, multiple organ dysfunction, and death. In assessing the effects of MSCs, results showed that hMSCs suppressed proinflammatory cytokine production such as IL-2, IL-6, and TNF induced by Staphylococcal enterotoxin B (SEB) in the mice model. However, they did not improve survival rates suggesting that their immunomodulatory properties were insufficient to counteract the lethal effects of SEB-induced TSS [75].

Another study explored the antimicrobial efficacy of human adipose-derived MSCs (hAD-MSCs) and hBM-MSCs in murine CF models infected with S. aureus. CF is a fatal genetic disease, characterized by chronic endobronchial bacterial infection, which is the main cause of morbidity and mortality in CF [65]. Results showed that hMSCs, regardless of their origin, could slow the bacterial growth of S. aureus through the secretion of antimicrobial peptide LL-37. Further, it was shown that hMSCs enhanced antibiotic sensitivity, thereby improving their ability to kill bacteria and subsequently the ability of the CF lung to resolve the infection in murine cystic fibrosis [65].

Sepsis is another life-threatening condition and a leading cause of death in intensive care units, with no drugs specifically approved for its treatment [76]. For treating sepsis caused by S. aureus, researchers engineered hUC-MSCs to express antibacterial peptides containing BPI21 and LL-37, creating what is known as BPI21/LL-37-engineered hUC-MSCs [76]. These engineered hUC-MSCs significantly improved the overall survival in S. aureus-induced septic mice suggesting that the BPI21/LL-37-engineered MSCs could offer a novel therapeutic approach for treating sepsis in the future [76]. hUC-MSCs were also shown to promote wound healing in methicillin-resistant S. aureus (MRSA)-infected wounds in a diabetic mouse model [77]. In similar studies, it was reported that hBM-MSCs secretome also played a significant role in reducing bacterial counts of MRSA at the wound sites and in accelerating the wound healing process in MRSA-infected cutaneous wounds in diabetic rats [74, 78]. Results suggested that the secretome improved wound healing by promoting anti-inflammatory responses and stimulating angiogenic activities [78].

To date, very few studies have investigated whether MSCs can be used in the treatment of chronic bacterial infections. To address that, a research team from the US studied the activity of MSCs in a mouse S. aureus biofilm infection model that was established by implanting S. aureus-coated surgical mesh subcutaneously in mice [79]. It is worth noting that bacteria in biofilms are well-protected as they reside in an environment that helps them survive and avoid being attacked by the body’s defenses as well as antibiotics [79]. Intravenous injections of activated MSC significantly reduced the bacterial burden on and around the implanted mesh material and improved wound healing when combined with antibiotic therapy as compared with animals treated with antibiotics only [79]. This indicates that MSCs can be used as adjunctive therapy along with antibiotics to treat chronic bacterial infections that involve bacterial biofilm formation. All these findings lead to the same conclusion that MSCs and their secretome are effective against S. aureus infections in preclinical studies. Specifically, five out of six studies demonstrated bacterial reduction, which was closely linked to the immunomodulatory properties of MSCs. However, only one study highlighted the ability of MSCs to improve survival, with most studies focusing on their role in wound healing. BM-MSCs were used in three studies, UC-MSCs in two, and one study employed hMSCs without specifying the source.

hMSCs and Mycobacterium species

Mycobacteria are aerobic, rod-shaped bacteria, composed of mycolic acids in their cell walls [80]. The Mycobacterium avium complex (MAC), a subgroup in this family, mainly includes Mycobacterium avium (M. avium) and Mycobacterium intracellulare (M. intracellulare) which are considered as multiple Nontuberculous Mycobacterium (NTM) species [81]. MAC has developed multiple virulence mechanisms to elude immune responses and maintain infection within the host. Its treatment is challenging and requires long-term use of several antibiotics [82]. Hence, alternative therapies to shorten the duration of combination antibiotic treatment, while reducing side effects and other complications, are urgently needed. To explore the therapeutic potential of hBM-MSCs against M. avium and M. intracellulare, Bonfield et al. performed a study on CF mice models infected intravenously with both types of bacteria. hBM-MSCs significantly reduced lung colony-forming units (CFUs) and attenuated lung inflammation, indicating that hMSCs have the capacity to treat NTM infections in animal models [83]. In agreement with these findings, a recent study demonstrated that hBM-MSCs can inhibit bacterial replication and reduce inflammation in the murine model of Mycobacterium avium complex pulmonary disease (MAC-PD) [84].

As demonstrated throughout the studies, the antibacterial effects of MSCs were attributed to a variety of molecules. The secretion of the LL-37 peptide by MSCs was closely associated with their antibacterial function [32, 65, 76]. LL-37 has shown a significant role in reducing bacterial load and enhancing the immune response, and in some cases, working synergistically with antibiotics to enhance their effectiveness. Another molecule, as important as LL-37, is β-defensin-2 which acts directly against pathogens increasing their clearance and modulating the immune response [34]. Additionally, leukotriene B4 (LTB4), produced by MSCs, was identified as another key molecule in promoting phagocytosis and facilitating bacterial clearance [40].

To further investigate the effectiveness of hMSC therapy, we analyzed each article to calculate the percentage of bacterial reduction where applicable. Out of the 37 articles, only 21 provided data suitable for these calculations. The majority of studies demonstrated a significant reduction in bacterial counts as shown in Fig. 3, with two studies reporting as much as 97% bacterial reduction. One of these studies employed BM-MSC secretome against the Gram-negative E. coli, while the other used UC-MSCs against the Gram-positive MRSA. In addition to that, studies utilized three different MSC dose ranges, Dose 1 (< 10⁶ MSCs/kg), Dose 2 (≥ 10⁶ - < 10⁸ MSCs/kg), and Dose 3 (≥ 10⁸ MSCs/kg), with Dose 2 being the most commonly used (Fig. 3). Among the Gram-negative bacteria, hBM-MSCs emerged as the most effective MSC source; whereas, hUC-MSCs showed greater efficacy in Gram-positive bacteria. However, these findings require further investigation, particularly due to the small number of articles available for some bacterial species, limiting the ability to draw absolute conclusions.

Fig. 3.

Efficacy of MSC source and dose on bacterial reduction for Gram-negative and Gram-positive bacteria. This bar graph presents the percentage reduction in bacterial counts for various strains of Gram-negative (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus and Mycobacterium avium) following treatment with distinct mesenchymal stem cell (MSC) sources. These sources include human umbilical cord (hUC-MSCs), bone marrow (hBM-MSCs), adipose tissue (hAD-MSCs), placental (hP-MSCs), and secretome-based treatments. The bars are categorized based on three dosage ranges: Dose 1 (< 10⁶ MSCs/kg), Dose 2 (≥ 10⁶ - < 10⁸ MSCs/kg), and Dose 3 (≥ 10⁸ MSCs/kg). Each bar represents the bacterial reduction achieved in the studies included (totaling 21 articles), with some articles contributing multiple bars if multiple approaches were tested

Discussions

MSCs, “a riddle wrapped in a mystery inside an enigma”, as described by Pittenger et al. [85], have been shown to be a wellspring of surprises. This is due to their remarkable capacity to differentiate into multiple cell types, modulate immune responses, and promote tissue regeneration. More importantly, they have the ability to exhibit low immunogenicity, home to the site of injury, and release bioactive molecules that aid in tissue repair and inflammation reduction [86]. This systematic review aims to provide benchmark data on preclinical evidence regarding hMSCs and their secretome for treating bacterial infections in rodents and alleviating disease symptoms, with the goal of assessing the readiness for proceeding into clinical trials. It was observed that the majority of the articles included in this review had positive outcomes concerning bacterial reduction and immune modulation.

All the articles reviewed have used human MSCs rather than animal cells in rodent disease models which is considered one of the strengths of our study as it better mimics the human milieu, facilitating the translation of MSC therapy into clinical practices. A previous review highlighted the success of human MSCs in 88 out of 94 cross-species experimental studies, demonstrating a high effectiveness rate of 93.6% [87]. Another recent study in 2022 showed that human chorion-derived MSCs were more effective than murine MSCs in LPS-induced lung injury in mice [66]. In our search strategy, we have used both “mesenchymal stem cells” and “mesenchymal stromal cells”. However, it is essential to distinguish the differences between these two terms which are often used interchangeably but refer to distinct populations of cells [6]. Mesenchymal stem cells refer to a progenitor/ stem cell population with self-renewal and differentiation properties [7, 8], while Mesenchymal stromal cells are indicative of a heterogeneous population of cells with a small proportion of stem/progenitor cells. In most of the studies included in this review, they were used interchangeably and referred to as MSCs.

Out of 517 articles retrieved using specific search terms, only thirty-seven preclinical studies met the inclusion criteria. Almost all selected studies have shown promising results in reducing bacterial infections after hMSC administration. This bacterial reduction was reported in studies that used both Gram-negative and Gram-positive bacteria as well as in articles involving antibiotic-resistant strains. Among the articles assessed, it was demonstrated that the use of hBM-MSCs-derived MVs resulted in an almost full bacterial reduction (97%) against the Gram-negative bacteria, E.coli, with no adverse effects reported after treatment [39]. Likewise, hUC-MSCs proved effective in treating antibiotic-resistant Gram-positive bacteria (MRSA), with almost total bacterial clearance of 97.36% [77]. This emphasizes the ability of MSCs or their secretome to combat a broad bacterial coverage, through their antimicrobial activity (Fig. 4). Although MSCs exhibit diverse mechanisms of action, two primary pathways have been identified for combatting bacterial infections mediated by their immunomodulatory properties. The first pathway is all about MSCs helping the body keep its immune response in check. MSCs work by increasing the expression of proteins such as SOCS [37], which aid in mitigating excessive inflammation, while simultaneously enhancing anti-inflammatory molecules like IL-10 [44, 45] to foster a more balanced response. Moreover, MSCs modify additional signals, including LTB4 [43] and PAI-1 [67], to prevent the immune system from exhibiting an exaggerated response that may lead to harm.

Fig. 4.

Therapeutic potential of MSC and their secretome as antibacterial agents against common pathogens. This figure represents MSC sources and their secretome as antibacterial agents. MSCs derived from various tissues including the placenta, Wharton’s jelly, umbilical cord, adipose tissue, and bone marrow are cultured until they secrete their secretome, which comprises exosomes, growth factors (KGF (Keratinocyte Growth Factor)), and cytokines (IL-1β (Interleukin-1 beta), IL-6 (Interleukin-6), IL-10 (Interleukin-10), TNF (Tumor Necrosis Factor), SOCS (Suppressor of Cytokine Signaling), Leukotriene B4 (LTB4), & PAI-1 (Plasminogen Activator Inhibitor 1). These secreted products are shown to have antibacterial effects against several bacterial pathogens, including Gram-negative bacteria such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Helicobacter pylori, and Gram-positive bacteria like Staphylococcus aureus, Mycobacterium intracellulare, and Mycobacterium avium

The second pathway focuses on the direct antimicrobial properties exhibited by MSCs. These cells were observed to release antimicrobial peptides, including LL-37 and β-defensin 2, which actively target pathogens by destabilizing microbial membranes and bolstering host defense systems. LL-37 directly disrupts microbial membranes; neutralizes endotoxins, and modulates the immune response [88], playing a crucial role in clearing pathogens in conditions like pneumonia and sepsis. Similarly, β-defensin 2 enhances the killing of the microbes via pathogen membrane disruption and chemotaxis of the immune cells to the site of infection, thereby reinforcing the body’s innate defenses [89].

Herein, it is important to note that MSCs, comprised of a heterogenous population of cells with different levels of stemness, vary widely under inflammatory or anti-inflammatory microenvironments, resulting in varied effects of MSC-mediated immunomodulatory functions. Besides, MSCs derived from different tissue sources also differ greatly in their immunomodulatory capacity. Li et al. (2023) reported that MSCs from umbilical cord and adipose tissue are superior to MSCs from bone marrow in terms of immunomodulation, while MSCs from placenta origins exhibit the least immunomodulatory capacity [90]. In our review, various MSC sources were utilized with hBM-MSCs being the most frequently used, followed by hUC-MSCs and hAD-MSCs. Thus, it remains crucial to carefully choose the most suitable source of MSCs before applying them for clinical use.

Although both MSC-based therapy and MSC-derived secretome-based therapy are shown to be effective in fighting bacterial infections, they operate differently. MSCs exert their functions via paracrine action and direct cell-cell contact; whereas, secretome-based therapies rely solely on paracrine effects. Both approaches have been shown to have direct antimicrobial action and reduction of pro-inflammatory cytokines thus reducing inflammation and subsequent tissue damage. In several studies, MSC-based therapy influenced macrophage polarization toward an anti-inflammatory M2 phenotype, thus enhancing the phagocytosis of pathogens. Moreover, MSCs activated regulatory T cells (Tregs), which help modulate immune responses and prevent excessive inflammation as shown in Supplementary Table 2. On the other hand, Secretome-based therapies depend on paracrine factors, such as LL-37, and LTB4, and growth factors like KGF, which is known to exert roles in killing bacteria and modulating the immune response. Furthermore, Secretome therapies have been associated with increased fibroblast activity in wound healing, attributed to the growth factors present in the conditioned medium.

Earlier reports have indicated that factors such as MSC dosage play a crucial role in shaping therapeutic outcomes [91]. However, the reviewed studies lacked a standardized dosing regimen, with significant variability in the number of cells administered and no consensus on the optimal dose for achieving effective bacterial clearance and immune modulation. Most studies administered a dose of (≥ 10⁶ - < 10⁸ MSCs/kg), though the rationale for this dose range was not consistently addressed. This variability underscores the need for dose optimization in future research to determine the most effective and safe dosing range, which is essential for advancing MSC therapies toward clinical applications. Besides, pre-conditioning or priming MSCs with specific stimuli before administration has emerged as a therapeutic strategy to enhance the antibacterial effect and immunomodulatory capacity of MSCs [92]. In our review, it has been shown that using repeated doses of Cytomix-preactivated umbilical cord MSCs attenuated K. pneumoniae-induced pneumosepsis, improved lung compliance and oxygenation, while reducing bacteria and injury in the lungs of rat models of established K. pneumoniae-induced pneumonia [51]. Similarly, in a rat model of E. coli–induced pneumonia, EVs derived from IFNγ–activated UC-MSCs attenuated E. coli–induced lung injury than did extracellular vesicles from naïve MSCs possibly by enhanced macrophage phagocytosis and killing of E. coli bacteria [37]. However, there are still many obstacles to determining the optimal methods for preconditioning in MSC and secretome-based therapies.

This review possesses several strengths. First, we rigorously followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, which enhanced the credibility of our conclusions. Additionally, the consistency of the results, despite the small differences, points to MSC therapy advantages in improving bacterial clearance and survival rates, and decreasing disease complications in preclinical studies, therefore paving the way for clinical studies. However, before taking this important step, findings should be interpreted with caution, bearing in mind some limitations of the current studies. First and foremost, the studies included in our review were all conducted on rodents, thus increasing the limitation possibility because they do not predict with sufficient certainty what will happen in humans. A previous systematic review of an animal study demonstrated that less than 30% of preclinical studies are translated into human randomized trials, with only 10% of these trials reaching approval to be used in humans [93]. Secondly, the selected research has not thoroughly investigated the MSC’s donor characteristics that could affect its antibacterial properties such as sex and age. Selle et al., [94] have indicated that MSC functions, specifically those derived from the bone marrow, decrease proportionally as the age increases, and more accurately in female donors. These issues necessities more discussions to study whether other functions such as the production of soluble derivatives as antimicrobial peptides, may alter MSC efficacy. Furthermore, using the SYRCLE tool to assess bias and quality in animal studies, we discovered that the research included in our review showed a moderate risk of bias, averaging 32.43% (Supplementary Table 1). Finally, we could not perform a meta-analysis due to the lack of homogeneity in outcome assessments because of the differences in methodology between the studies included in our systematic review (variety of mice strains, variety of MSCs sources, and methods of MSCs injection). Another important issue to note is that, despite the ISCT committee endorsing the use of the term “Mesenchymal stromal cells” and recommending the inclusion of additional criteria such as tissue of origin and functional assays to better characterize MSCs, our review shows that the adoption of the ISCT definition is inconsistent across most articles [14]. Many studies continued to use the term “Mesenchymal stem cells” even in the absence of evidence supporting the stemness of the cells. To better address clinical translation, reproducibility, and transparency within the field of MSC research, the scientific community must agree on a definition of MSCs and must support its widespread adoption. Without a consensus definition, there will continue to be challenges in comparing findings across studies, extrapolating results, and potentially influencing their clinical significance.

Conclusion

Despite the progress that has been made over the past decade in the development of MSC-based products for the treatment of different inflammatory, immune-mediated, and degenerative diseases, very few studies have been conducted regarding the use of MSCs in treating bacterial infections. This systematic review highlights the promising benefits of MSCs and their cell-free derivatives in rodent models, demonstrating significant potential for bacterial clearance and immune modulation. Nonetheless, due to the significant variability in sources of MSC tissue, donor characteristics, and culture environments in different studies, it is challenging to directly compare results, and any generalization necessitates cautious interpretation. Furthermore, the microenvironment in which MSCs reside plays a crucial role in influencing their behavior, function, and therapeutic potential. Based on this, it is necessary to acknowledge that the variability in the spectrum of antibacterial outcomes could be attributed to the MSC heterogeneity, which remains one of the major challenges in translating the therapeutic efficacy of MSCs to clinical settings.

Thus, to enhance future studies and clinical relevance, several recommendations are proposed. Firstly, establishing a clear, consistent definition of MSCs based on their biological activities and biomarkers is crucial. Secondly, comparative studies investigating MSCs from diverse tissue sources and donor backgrounds can elucidate the most effective subtypes for antibacterial uses. Thirdly, optimizing dosing strategies, administration routes, and cell-preconditioning protocols is vital to minimize variability and standardize treatment outcomes. Addressing these aspects is vital for validating the efficacy of MSCs and secretome-based therapies as treatments for bacterial infections in future human clinical trials.

Supplementary Information

Below is the link to the electronic supplementary material.

Abbreviations

ALA-PDT

Aminolaevulinic acid photodynamic therapy

ALI

Acute lung injury

AMR

Antimicrobial resistance

ARDS

Acute respiratory distress syndrome

BAL

Bronchoalveolar lavage

CAG

Chronic atrophic gastritis

CF

Cystic fibrosis

CFUs

Colony forming units

CM

Conditioned medium

CSF

Cerebrospinal fluid

E.coli

Escherichia coli

EVs

Extracellular vesicles

H. pylori

Helicobacter pylori

hAD-MSCs

Human adipose-derived mesenchymal stem cells

hBM-MSCs

Human bone marrow-derived mesenchymal stem cells

hMSCs

Human mesenchymal stem cells

hPD-MSCs

Human placental- derived mesenchymal stem cells, hC-MSCs: Human chorion-derived MSCs

hUC-MSCS

Human umbilical cord-derived mesenchymal stem cells

hUCB-MSCs

Human umbilical cord blood-derived mesenchymal stem cells

IL-1β

Interleukin-1 beta

IL-10

Interleukin-10

IL-2

Interleukin-2

IL-6

Interleukin-6

K. pneumoniae

Klebsiella pneumoniae

KGF

Keratinocyte growth factor

LL-37

Cathelicidin LL-37

LPS

Lipopolysaccharides

LTB4

Leukotriene B4

M. avium

Mycobacterium avium

M. intracellulare

Mycobacterium intracellulare

MAC-PD

Mycobacterium avium complex pulmonary disease

MAC

Mycobacterium avium complex

MDR

Multidrug resistant

MRSA

Methicillin-resistant Staphylococcus aureus

MV

Micro-vesicles

NTM

Nontuberculous mycobacteria

NVs

Nanovesicles

OMVs

Outer membrane vesicles

P. aeruginosa

Pseudomonas aeruginosa

PAI-1

Plasminogen activator inhibitor 1

PBMT

Photo biomodulation therapy

PBS

Phosphate buffered saline

PGE2

Prostaglandin E2

PMN

Polymorphonuclear leukocyte functions

PRISMA

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

PW LLLT

Pulsed wave low-level laser therapy

ROS

Reactive oxygen species

S. aureus

Staphylococcus aureus

SEB

Staphylococcal enterotoxin b

SOCS

Suppressor of cytokine signaling

SYRCLE

Systematic Review Centre for Laboratory Animal Experimentation

TNF

Tumor necrosis factor

TSS

Toxic shock syndrome

Author contributions

FAS contributed to the conception and the main idea of the work. LA and FS drafted the main text. LA drafted the figures and tables. ESS reviewed and revised the manuscript. FAS registered the systemic review in PROSPERO, supervised the work, and provided comments and additional scientific information. All authors read and approved the final version of the work to be published.

Funding

Not applicable.

Data availability

Not applicable.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declared that they have no competing interests.