The bidirectional regulatory effects of bacteria on blood vessels in the tumor microenvironment

Abstract

The diagnosis and treatment of tumors represent significant global challenges. Understanding the tumor microenvironment (TME) is crucial for a more comprehensive understanding of the mechanisms of tumorigenesis. The TME is a dynamic and complicated multicellular system consisting of tumor cells, various types of non-tumor cells, molecular signals, and extracellular matrix. Tumor blood vessels, formed through diverse mechanisms including vasculogenesis, sprouting angiogenesis, intussusceptive growth, vessel coalescence, co-option, vasculogenic mimicry, and lymphangiogenesis, serve as pivotal regulators in the TME by controlling tissue perfusion, orchestrating immune cell trafficking, facilitating metastasis, and modulating responses to therapeutic interventions. As our understanding of tumor vessels formation deepens alongside advancements in microbial research, an increasing number of studies have reported that bacteria can affect tumorigenesis and development by acting on tumor vessels; bacteria could even become a potential anti-tumor therapy. Although the important role of bacteria in tumor tissue has been well reviewed, no studies have summarized the effects of bacteria on tumor vessels. In this review, we aim to provide a comprehensive review of the important literature describing the role of bacteria in the tumor vasculature system and summarize its mechanism.

Introduction

Over the past few decades, a profound shift has occurred in the understanding of tumor biology. People gradually realize that tumors represent not merely a simple aggregate of malignant cells, but a complex, dynamic, highly structured ecosystem. To encapsulate this concept, the term “tumor microenvironment (TME)” was introduced. The TME constitutes the local milieu in which tumor cells proliferate and survive, encompassing not only the tumor cells but also the surrounding fibroblasts and immune, inflammatory, and glial cells, as well as the cellular stroma, microvessels, and biomolecules infiltrating the nearby area [1, 2]. The significance of TME in governing cancer progression has been increasingly recognized, and numerous studies have demonstrated that the TME actively participates in reprogramming fundamental tumor processes initiation, growth, invasion, metastasis, and response to therapeutic interventions [3, 4].

Because hypoxia and necrosis within the tumor maintain a chronic immunosuppressive microenvironment that favors bacterial proliferation, intratumoral bacteria can be found in many solid tumors, and different cancer subtypes have different microbial compositions [5, 6]. Commensal bacteria in tumor tissues can affect tumorigenesis, progression, and treatment responses by modulating immune responses and inflammation by altering TME [7]. It is worth noting that bacteria in the TME have bidirectional regulatory effects: certain bacterial species drive tumor progression through multiple pathways such as the secretion pro-inflammatory factors, activation of carcinogenic signaling cascades, and induction of genomic instability; while specific commensal microbiota demonstrate marked antitumor activity, effectively enhancing clinical response rates to antitumor therapies. Dysbiosis of the gut microbiome is now recognized as a hallmark of cancer, and some bacteria, such as Fusobacterium nucleatum and Helicobacter bilis, have been shown in our previous studies to be closely related to colorectal cancer tumorigenesis [8, 9]. In addition, the role of bacteria in lung, breast, and prostate cancers has also been reported [10]. Conversely, numerous studies have highlighted the important role of the microbiota in modulating responses to cancer immunotherapy and targeted therapies [11]. In fact, probiotics are emerging as potential anti-tumor therapeutics [12]. Ultimately, a better understanding of the relevant microbiota in the TME will usher in a new era of intratumoral bacteria-based cancer therapies.

The behavior of malignant tumors depends on not only the tumor cells but also the blood and lymphatic vessels. Continued tumor cell division and proliferation require new blood vessel formation to obtain nutrients and oxygen, and this process depends on tumor angiogenesis [13, 14]. Emerging evidence indicates that pathogenic bacteria exert multifaceted effects on vascular endothelial cells, including direct cytotoxicity, disruption of the cytoskeletal architecture, compromising intercellular junctions, and modulating immune functions, endothelial cell immune function alterations, ultimately culminating in structural and functional impairment of the vascular endothelial barrier [15, 16]. In addition, bacteria can promote or inhibit tumor blood vessel growth through regulating angiogenesis and inhibitory factors. Thus far, bacterial colonization has been found in the blood vessels of neuroendocrine tumors, colorectal cancer, cholangiocarcinoma, gastric carcinoma, mucosa-associated lymphoid tissue (MALT) lymphoma, and other tumors, and tumor-targeted therapy programs have been developed based on this phenomenon. While substantial evidence delineates the pathogenic involvement of intratumoral bacteria in oncogenesis, their specific modulatory effects on tumor vessels remain systematically uncharacterized. This review comprehensively interrogates bacterial-mediated tumor angiogenesis mechanisms, and evaluates its significance as an anti-tumor therapy strategy (Fig. 1).

Fig. 1.

Concept diagram of the mechanism of bacterial action on tumor vessels and bacteria-specific antiangiogenic tumor-targeting therapy

Different mechanisms of tumor angiogenesis

As we mentioned earlier, the growth of solid tumors requires blood vessels to obtain oxygen and nutrients. Although anti-angiogenic therapy, which targets vascular endothelial cells (ECs) to inhibit tumor vascularization, has shown efficacy in numerous preclinical models, its clinical benefits have been suboptimal. This limitation may stem from the existence of alternative mechanisms for vascular regeneration [17]. With technological advances, the mechanisms underlying the tumor angiogenesis process have now been categorized into seven primary types: (1) sprouting angiogenesis, (2) vasculogenesis, (3) vasculogenic mimicry (VM), (4) vessel intussusception, (5) vascular co-option of existing vessels, (6) angiogenesis mediated by cancer stem cells, and (7) bone marrow-derived angiogenesis [18, 19]. As an alternative vascularization mechanism, VM within the TME is gaining significant prominence. Besides traditional sprouting angiogenesis, mounting evidence indicates that diverse microbial species can modulate tumor angiogenesis by influencing VM pathways.

In 1999, based on the study of highly aggressive and metastatic uveal and cutaneous melanoma tumors, Maniotis et al. first described an endothelial-independent vascularization mechanism called “vasculogenic mimicry” [20]. And then, Frenkel et al. in 2008 demonstrated blood circulation in VM tube in a choroidal melanoma patient using lasers canning confocal angiography [21]. Unlike angiogenesis, VM is a non-endothelium-dependent channel-like structure lined with tumor cells; the tumor cells assume the behavior of endothelial cells, mimicking the tubulogenesis process, in search of oxygen and other nutrients to sustain tumor growth. Two types of VM have been described, namely, the tubular type and the patterned matrix type [22]. In recent years, VM has been observed in more than 15 solid tumor types, including melanoma, glioblastoma, osteosarcoma, hepatocellular carcinoma, breast, lung, gastric, colorectal, and prostate cancers [23]. Moreover, VM is associated with high tumor grades, invasion, metastasis, and poor prognosis in patients with malignant tumors [24].

The molecular mechanisms underlying vasculogenic mimicry (VM) formation remain an active area of investigation. Key mechanisms identified to date encompass epithelial-mesenchymal transition (EMT), extracellular matrix (ECM) remodelling, the closely associated epithelial-endothelial transition (EET), specific tumor microenvironmental factors (notably hypoxia), and the involvement of cancer stem cells (CSCs) or dedifferentiated stem-like cells [25, 26]. EMT is commonly described as a phenomenon in which epithelial cells undergo a morphological shift to acquire fibroblast-like or mesenchymal phenotypes, characterized by the loss of cell polarity, reorganization of the cytoskeleton, and enhanced migratory and invasive capabilities. Notably, EMT regulators and EMT-related transcription factors are highly up-regulated in VM-forming tumor cells, which demonstrated that EMT-acquired cells are more prone to form VM-like vascular channels [27]. EET, frequently considered a specific subtype of EMT, describes the direct transformation of epithelial cells into cells exhibiting an endothelial phenotype [28]. Among microenvironmental factors, hypoxia emerges as the most potent inducer of VM. Compelling evidence implicates hypoxia in VM formation through multiple signaling pathways [29]. Crucially, hypoxia drives both EET and the generation/enrichment of CSCs–a tumor cell population endowed with self-renewal capacity and multipotent differentiation potential–within the context of VM development. Collectively, CSCs and EET facilitate VM by promoting tumor cell plasticity, mediating ECM remodeling, and enabling the integration of VM channels with the host vasculature.

Promoting effects of bacteria on tumor vessels (Table 1 ([30–38]

Table 1.

An overview of related studies on the promoting effect of bacteria on tumor angiogenesis

Time	Type of angiogenesis	Event	Refs.
2005	Sprouting angiogenesis	Helicobacter pylori infection promotes gastric cancer cells invasion through activation of VEGF expression via the NF-kB pathway.	[30]
2010		Helicobacter bilis infection activates NF-κB in bile duct carcinoma cells, thereby increasing expression of the angiogenic factor VEGF from the cells and lead to enhancement of angiogenesis.	[31]
2011		Bartonella are associated with various vascular tumors including verruga peruana and bacillary angiomatosis, with their pathogenesis closely linked to VEGFR-2 activation.	[32]
2013		Bartonella can invade pericytes and inhibit pericyte proliferation, thereby reducing pericyte coverage and ultimately enhancing vascular proliferation.	[33]
2020		The Bartonella autotransporter BafA activates the host VEGF pathway to drive angiogenesis.	[35]
2022		The oral bacterium Streptococcus mutans promotes tumor metastasis by inducing vascular inflammation.	[37]
2022	Vasculogenic mimicry	Intestinal microbial metabolite deoxycholic acid promoted VM formation and EMT by activating the VEGFR2 signaling pathway.	[36]
2022	NA	Bacterial colonization near the blood vessels of the neuroendocrine neoplasms	[38]
2017		Bacterial presence in the blood vessels of MALT lymphoma.	[34]

VEGF vascular endothelial growth factor, VEGFR-2 vascular endothelial growth factor receptor-2, MALT mucosa-associated lymphoid tissue, VM vasculogenic mimicry, EMT epithelial-mesenchymal transition, NA not available

Sprouting angiogenesis is a complex process of coordinated pro-angiogenic and inhibitory factors. Members of the vascular endothelial growth factor (VEGF) family are major pro-angiogenic factors and include five VEGF glycoproteins (VEGF-A, VEGF-B, VEGF-C, VEGF-D, and VEGF-E) [39]. VEGF binds to three high-affinity tyrosine kinase VEGF receptors on vascular endothelial cells: Flt-1/VEGF receptor-1 (VEGFR-1), KDR/Flk-1/VEGF receptor-2 (VEGFR-2), and Flt-4/ VEGF receptor-3 (VEGFR-3) [40]. Takayama et al.. demonstrated that compared with control conditions, a human bile duct cancer cell line infected with Helicobacter bilis had a nearly 3-fold increase in NF-κB activity and a nearly 2.4-fold increase in VEGF production [31]. NF-κB is a key factor involved in the upregulation of VEGF mRNA. The result means that Helicobacter bilis infection activates NF-κB in bile duct carcinoma cells, thus increasing expression of the angiogenic factor VEGF in the cells and enhancing angiogenesis. Similarly, Helicobacter pylori infection promotes gastric cancer cell invasion through activating VEGF expression via the NF-κB pathway [30]. Bartonella are fastidious, gram-negative, facultative intracellular bacteria capable of triggering pathological angiogenesis [41, 42]. These pathogens are associated with various vascular tumors including verruga peruana and bacillary angiomatosis, with their pathogenesis closely linked to VEGFR-2 activation [32]. Recent mechanistic studies by Tsukamoto et al. have significantly advanced our understanding of Bartonella-induced vascular proliferation. Employing functional transposon mutagenesis screening, the researchers identified a bacterial mitogenic factor termed BafA, which directly interacts with VEGFR-2 to activate its downstream signaling pathways [35]. Furthermore, Bartonella can invade pericytes and inhibit pericyte proliferation, thereby reducing pericyte coverage, promoting increased VEGF production, and ultimately promoting vascular proliferation [33].

Furthermore, the oral bacteria Streptococcus mutans can promote tumor metastasis by inducing vascular inflammation [37]. Streptococcus mutans is a Gram-positive, facultative anaerobic, catalase-negative bacterium that produces lactic acid and has been implicated as the key pathogen in the development of dental caries [43]. Yu et al. showed that Streptococcus mutans primarily induces vascular inflammation by directly invading endothelial cells and activating NF-κB. Streptococcus mutans exposure potentially disrupts endothelial integrity by decreasing vascular endothelial-cadherin expression [37]. This ultimately leads to blood vessel hyperpermeability, thus increasing the transendothelial migration of tumor cells.

Additionally, bacteria stimulate tumor vascularization by modulating alternative angiogenic pathways, such as VM. Song et al. demonstrated that the intestinal microbial metabolite deoxycholic acid promotes activation of EMT and higher expression of EMT-associated transcription factors by activating the VEGFR-2 signaling pathway. Crucially, this EMT process drives VM formation in tumors. Deoxycholic acid-induced upregulation of EMT-related proteins thereby facilitates VM, further exacerbating the malignant progression of intestinal carcinogenesis [36]. Intriguingly, in addition to bacteria, the Epstein-Barr virus (EBV) can also activate the NF-κB signaling pathway through upregulating CXCL 8 expression, which in turn significantly promotes VM formation in gastric cancer cells [44].

Moreover, a close relationship between bacteria and blood vessels has been observed in a growing variety of tumors in recent years. For example, Massironi et al. observed bacterial colonization near the blood vessels of neuroendocrine neoplasms [38]. Persistent bacterial infection of the gastrointestinal mucosa is causally associated with gastric cancer and mucosa-associated lymphoid tissue (MALT) lymphoma, and Hoehne et al. observed bacteria in MALT lymphoma blood vessels [34]. The specific mechanisms, however, are not explained in these articles. Based on the above, the promoting effect of bacteria in the TME improves the pathological mechanisms of tumor development and provides new potential targets for antitumor therapy.

Probiotics can inhibit tumor angiogenesis (Table 2 ([45–48]

Table 2.

An overview of related studies on probiotics affecting tumor angiogenesis

Time	Type of angiogenesis	Event	Refs.
2012	Sprouting angiogenesis	The Bifidobacterium infantis-mediated prokaryotic expression system of sKDR significantly suppressed VEGF-induced angiogenesis.	[45]
2013		Saccharomyces boulardii could reduce basal VEGFR-2 phosphorylation, VEGFR-2 phosphorylation in response to VEGF and activation of the downstream kinases PLCγ and Erk1/2, finally inhibiting angiogenesis.	[46]
2014		Lactobacillus coryniformis could reduce the vascular pro-oxidative and pro-inflammatory status.	[47]
2023	Vasculogenic mimicry	Clostridium butyricum could inhibit epithelial-mesenchymal transition and VM formation in intestinal carcinogenesis though downregulation METTL3.	[48]

sKDR soluble kinase insert domain receptor, VEGFR vascular endothelial growth factor receptor, VM vasculogenic mimicry, METTL3 Methyltransferase-like 3

Probiotics are live non-pathogenic microbes that can potentially exert positive effects on the body by maintaining microbiota balance [49]. Traditional probiotics are mainly Bifidobacterium spp., Lactobacillus spp., and other lactic-acid-producing bacteria, including Streptococcus, Enterococcus, and Lactococcus species, as well as yeasts of the genus Saccharomyces [50]. In addition, with the development of genome sequencing and other technologies, new probiotics have been discovered; most of these are called next-generation probiotics and include Akkermansia muciniphila, Faecalibacterium prausnitzii, Bacteroides fragilis, Eubacterium hallii, Roseburia spp., and others [51]. The major characteristics of probiotics are acid resistance, bile tolerance, mucosal or epithelial cell adhesion, antimicrobial resistance, bile salt hydrolase potential, immunostimulatory and antimutagenic properties, and antagonistic activity against pathogens [52]. Based on these features, probiotics have shown promise in the treatment of cancer and cancer-related complications [50]. Current anti-cancer research on probiotics has focused mainly on colorectal cancer (CRC); some studies have also confirmed the role of probiotics in the treatment of breast cancer, lung cancer, and other tumors [53–55]. Probiotics have many targets in cancer treatment, and this review introduces the effects of probiotics on tumor blood vessels.

Saccharomyces boulardii, a non-pathogenic yeast with documented trophic and antimicrobial effects on human small intestinal mucosa, was emerged as a promising antiangiogenic agent [56]. Chen et al. demonstrated that Saccharomyces boulardii could reduce basal VEGFR-2 phosphorylation in response to VEGF and activation of the downstream kinases PLCγ and Erk1/2, ultimately inhibiting angiogenesis [46]. This pioneering work established the first evidence of probiotics possessing intrinsic antiangiogenic functions, highlighting their therapeutic potential for inflammation-associated malignancies. Further building on this paradigm, Li et al. engineered a Bifidobacterium Bifidobacterium infantis-mediated prokaryotic expression system of soluble kinase insert domain receptor (sKDR). This innovative approach achieved significant suppression of VEGF-induced angiogenesis and demonstrated potent antitumor efficacy in lung cancer models [45]. In addition, probiotics such as Lactobacillus coryniformis can reduce the vascular prooxidative and pro-inflammatory status by reducing NADPH oxidase activity, suggesting the therapeutic potential of this gut microbiota manipulation in preventing vasculopathy [47].

As was described previously, tumor vessel growth depends on VM generation and EMT, and methyltransferase-like 3 (METTL3) can accelerate EMT and induce VM formation. Recent evidence has revealed that METTL3 plays key roles in a variety of cancer types. It can not only affect tumor prognosis but also antagonize the therapeutic effects of chemotherapeutic drugs such as 5-fluorouracil, and high levels of expression in tumors are associated with poor clinical prognosis [57, 58]. Kexin et al. demonstrated that Clostridium butyricum downregulates METTL3 expression in colorectal cancer (CRC) cells through activation of G-protein coupled receptor 43. This inhibition counteracts the oncogenic effects of METTL3 overexpression, include: (1) upregulation of VEGFR2 on the nuclear membrane surface, and (2) activation of EMT-related genes, altering E-cadherin and vimentin expression to promote vasculogenic mimicry (VM). By suppressing METTL3, Clostridium butyricum reduces these events, thereby inhibiting the EMT process, VM formation, and overall tumor progression [48].

Unfortunately, the long-term therapeutic effects and safety profile of probiotics in cancer treatment remain inadequately established through rigorous clinical validation. Significant challenges persist regarding the standardized mass production, stability control, and scalable commercialization of probiotics intended for clinical oncology applications. Current published studies have not proven that next-generation probiotics could provide tumor vascular inhibition. In contrast, a more empirically supported and actively researched strategy involves the engineering of bacteria (including certain attenuated strains or genetically modified bacteria) as targeted drug delivery vehicles.

Bacteria-specific antiangiogenic tumor-targeting therapy (Table 3 ([59–66]

Table 3.

An overview of related studies on bacteria-specific antiangiogenic tumor-targeting therapy

Time	Event	Refs.
2009	Listeria monocytogenes -based cancer vaccine could suppresses angiogenesis in primary and metastatic breast cancer tissues in mice through VEGFR-2 inhibition.	[59]
2010	A tumor-targeting double-auxotrophic mutant of Salmonella typhimurium termed A1-R was developed and demonstrated that it can disrupt tumor blood vessels.	[60]
2013	Primer doses of Salmonella typhimurium A1-R could enhance tumor targeting and efficacy in immunocompetent mice.	[61]
2018	Tumor-targeting Salmonella typhimurium A1-R Inhibits osteosarcoma angiogenesis.	[62]
2020	On basis of traditional photothermal therapy, through intravenous injection attenuated Salmonella, make its colonization in the tumor, could destroy tumor blood vessels and trigger tumor specific thrombosis.	[63]
2022	Salmonella-mediated tumor-targeted delivery of tumstatin (54–132) significantly suppresses tumor growth in mouse model by inhibiting angiogenesis and promoting apoptosis	[64]
2023	Hyaluronate-based hydrogel loaded with Thiobacillus denitrificans could be used to normalize tumor blood vessels and improve chemotherapy.	[65]
2023	Listeria-based vaccination safely induced potent anti-tumor effects that consisted of recruiting functional Type-1-associated T cells into the tumor microenvironment and reducing tumor blood vessel content.	[66]

VEGFR-2 vascular endothelial growth factor receptor-2

Since the early 19th century, reports have indicated that patients with cancer who had bacterial infections sometimes had spontaneous tumor regression [67]. Then, there are gradually studies demonstrating that motile bacteria can penetrate the distal regions of blood vessels and accumulate in tumor tissues, which makes bacteria potential anticancer drug transport carriers [68]. Furthermore, Shirai et al.. showed that bacterial proteolytic activity can degrade the extracellular matrix of collagen, increase the water conductivity of the stroma, and reduce interstitial fluid pressure, thereby increasing the transport of antitumor drugs through the vessel wall [69]. In contrast to conventional chemotherapy and radiotherapy, live bacterial therapeutics can colonize tumor tissues without causing notable host toxicity and gradually become an excellent choice for targeted cancer therapy [70]. Moreover, the combination of therapeutic bacteria and conventional chemotherapy results in large synergistic effects [71].

Given the important role of tumor blood vessels in TME, angiogenesis has become an increasingly attractive target in the development of anti-cancer therapies, and anti-angiogenic factor-targeting agents have been approved as first-line agents for a variety of cancers [72, 73]. Nearly 20 years of clinical experience with anti-angiogenic drugs have demonstrated the importance of this therapeutic modality for cancers [74]. Combining the advantages of bacteria in tumor therapy, the use of motile bacteria as drug delivery carriers and therapeutic agents targeting tumor vessels has been supported by many studies. In this article, we will focus on reviewing these studies related to bacteria-specific antiangiogenic tumor-targeting therapy.

Salmonella is a bacterial pathogen with a remarkably diverse host range and pathogenicity; it is involved in infection processes in many mammalian, avian, amphibian, and reptile hosts [75]. Salmonella is considered the most promising bacterium for treating cancer because of its intrinsic capabilities, such as killing tumor cells and targeting, penetrating, and proliferating tumors [76]. In addition, Salmonella would induce inflammation to trigger thrombosis in infected tumors by destroying tumor blood vessels [63]. However, Salmonella treatment alone cannot usually effectively eliminate a tumor. To solve the above problems, Yi et al. utilized conventional photothermal therapy in combination with an intravenous injection of attenuated Salmonella, which colonized tumor tissue, destroyed tumor blood vessels, and triggered tumor specific thrombosis; these effects increased tumor-specific absorbance to achieve more effective tumor photothermal ablation. In addition, this bacteria-based photothermal therapy could induce an anti-tumor immune response and create a robust immune memory effect to reject rechallenged tumors, thus inhibiting tumor metastasis and recurrence [63]. Furthermore, Liu et al. developed a tumor-targeting double-auxotrophic mutant of Salmonella typhimurium termed A1-R and demonstrated that it can disrupt tumor blood vessels. The researchers observed that in nude mice, following A1-R treatment, massive amounts of blood flowed into the tumor due to vascular destruction and eventually caused tumor tissue necrosis [60]. Moreover, tumor vascularity was positively correlated with the tumor efficacy of A1-R. However, in immunocompetent mice, the dosage of A1-R must be adjusted to avoid toxicity. To address this, Tome et al. developed an administration strategy: initially, a low priming dose of A1-R was administered (1 × 10⁶ colony-forming units [CFU] via intravenous injection), followed by a high dose (1 × 10⁷ CFU via intravenous injection) four hours later. This approach achieves maximal efficacy with minimal toxicity for A1-R-based tumor-targeted therapy [61]. A1-R efficacy was enhanced in the treatment of orthotopic mouse models of human breast cancer, prostate cancer, osteosarcoma, pancreatic cancer, fibrosarcoma, lung cancer, and brain cancer, as well as spinal cord glioma [62]. Furthermore, Bao et al. engineered an innovative Salmonella-mediated targeted expression system of tumstatin (VNP-Tum5) under the control of the hypoxia-induced J23100 promoter. Mechanistically, VNP-Tum5 demonstrated significant downregulation of key angiogenic regulators including VEGF-A, platelet endothelial cell adhesion molecule-1, and phosphorylated components of the PI3K/AKT signaling pathway, and upregulated the expression of cleaved-caspase 3 in melanoma tissues. These molecular alterations collectively inhibited the proliferation and migration of mouse umbilical vascular endothelial cells to impede angiogenesis [64]. Unfortunately, the above studies are all limited to animal models and cannot yet be applied to clinical practice.

Hypertonic tumor blood vessels promote non-specific extravasation, resulting in high interstitial flow. This creates a tumor microenvironment that is hypoxic, acidic, and characterized by elevated interstitial pressure, hindering the delivery and efficacy of chemotherapeutic agents [77]. Li et al. suggested that restoring the structural and functional integrity of tumor vessels creates a favorable tumor microenvironment that reduces tumor metastasis and promotes drug delivery by promoting blood return. The key to achieving these effects is Thiobacillus denitrificans, a bacterium that can oxidize sulfide under anaerobic conditions to continuously and effectively scavenge H2S. In that study, a thiol hyaluronate-based hydrogel (HA-SH) was synthesized via amide reaction, and the colon was targeted by loading Thiobacillus denitrificans into the hydrogel to form a bacterial hydrogel. The bacteria loaded in the HA-SH hydrogel removed the excess H2S from colon cancer, which in turn promoted tumor blood vessel normalization and improved chemotherapeutic drug delivery, thereby inhibiting tumor progression [65].

Therapeutic cancer vaccines have experienced a resurgence in the past decade due to new technologies for antigen delivery and improved vaccine design [78]. As a safe and promising treatment, cancer vaccines could cause tumor regression and induce protective and durable immune responses against components of the tumor microenvironment [79, 80]. Some studies have used attenuated live bacteria to create anti-tumor vaccines that target tumor blood vessels. For example, Anderson et al. demonstrated that Listeria-based anti-tumor vaccination can protect against vascular and colon cancer [66]. Listeria-based vaccination safely induced potent anti-tumor effects that included functional Type-1-associated T cell recruitment into the tumor microenvironment and reduced tumor blood vessel content. Moreover, Seavey et al. demonstrated that the Listeria monocytogenes -based cancer vaccine not only suppresses angiogenesis in primary and metastatic breast cancer tissues in mice through vascular endothelial growth factor receptor-2 (VEGFR-2) inhibition but also overcomes host vascular tolerance by driving epitope spreading to endogenous tumor proteins, thereby inducing active tumor regression. In summary, these approaches leverage the unique physiological properties of bacteria to develop them into anti-tumor vaccines or drug carriers, which can selectively and directly deliver chemotherapeutic drugs or other therapeutic payloads to the tumor microenvironment. By synergizing with conventional chemotherapeutic drugs, they aim to achieve targeted effects on the tumor vasculature, holding the potential to enhance therapeutic efficacy while reducing systemic toxicity. These results provide insights into the development of effective clinical strategies for bacteria-based cancer therapy in the future.

Conclusions

Abnormal angiogenesis is a hallmark feature of the TME that drives cancer progression through multiple mechanisms [81]. Abnormal blood vessels can provide fuel for cancer cell growth, and the more abundant the tumor vessels are, the denser the tumor cells are. More critically, abnormal blood vessels can promote tumor cell infiltration and lead to tumor metastasis, which may be related to endothelial junction damage of the tumor blood vessels and insufficient pericyte coverage [82]. This hematogenous metastasis pathway accounts for approximately 90% of tumor-related mortality in clinical settings [83]. The role of bacteria in tumors and crosstalk with the TME has been extensively studied and characterized [84]. Many bacteria were found to grow and aggregate around tumor blood vessels, suggesting that these bacteria may play a role in promoting or inhibiting tumor development through tumor blood vessels. Therefore, elucidating the effects of bacteria on tumor blood vessels is key to understanding tumor pathogenesis. In the treatment of tumors, the safety and stability of conventional chemotherapeutic drugs and radiotherapy regimens are questionable [85]. To overcome this difficulty, scientists have been working to find effective strategies for anticancer drug delivery. Fortunately, the application of bacteria has promising potential to make a great impact on the development of controllable targeted drug delivery for combatting cancer [86]. Current studies have shown that combining bacterial products with radiotherapy and chemotherapy can target tumor cells more precisely, with selective toxicity toward tumor cells and fewer side effects for normal cells; these advantages make up for the limitations of traditional anti-tumor therapy [87]. Furthermore, most anti-angiogenic drugs currently used in clinical practice and trials target endothelium-dependent vessels by blocking VEGF, VEGFRs, epidermal growth factor receptor, or platelet-derived growth factor receptors [88]. However, as the mechanism of VM formation differs from that of endothelium-dependent vessels, this may account for the poor efficacy of conventional anti-angiogenic drugs in VM-positive cases. Thus, the role of bacteria in VM is highly promising. Although its mechanism of action remains unclear and clinical data on the use of bacteria-targeted drugs as anti-tumor therapies are still limited, it has fully demonstrated the value of bacteria as anti-tumor vascular targeting agents, particularly in VM-positive cases.

The effects of bacteria on tumors are very complex and have different roles in tumor therapy; our review focused on the influence on tumor blood vessels. We reviewed the promoting effects of pathogens on tumor blood vessels, the inhibitory effects of probiotics on tumor blood vessels, and indicated that the mechanism is mainly related to the regulation of two pathways, vasculogenesis and VM. We also highlight the great potential of bacteria in anti-tumor vascular targeting therapy in terms of drug-targeting vectors and therapeutic cancer vaccines. Many of the recent findings discussed in this review have shed light on microbiome-cancer interplay, some of which could pave the way for promising novel cancer therapies and provide insights into the basic biology of microbiota-associated cancer initiation and progression.

Nevertheless, the majority of research concerning tumor progression and vascular dynamics has centered on bacteria, with the role of other microorganisms receiving insufficient attention. For instance, viruses, being vital components of the TME, play a key role in angiogenesis. EBV, a ubiquitous human oncovirus infecting >90% of the population, was the first linked to human cancer [89]. Recent research by Xiang et al. revealed that EBV drives VM in epithelial cancers. This occurs through LMP2A-mediated activation of the PI3K/AKT/mTOR/HIF-1α signaling pathway and involves a critical secretory cross-talk between cancer cells and the immune microenvironment [90]. Furthermore, human cytomegalovirus (HCMV) has been demonstrated to induce angiogenesis [91]. HCMV is a ubiquitous beta-herpesvirus and typically infects of the cell types involved in angiogenesis and vascular disease including ECs, smooth muscle cells, pericytes, fibroblasts, and macrophages and promotes angiogenesis by both direct and indirect mechanisms. In addition, infection proximal to blood vessels may stimulate angiogenesis via the release of angiogenic factors or induction of local inflammation. Specifically, HCMV infection of fibroblasts and endothelial cells enhances the synthesis and release of multiple angiogenic factors, thereby stimulating various stages of the angiogenic process [92]. Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as Human herpesvirus 8, is a member of the lymphotropic gammaherpesvirus subfamily and a human oncogenic virus [93]. Recently, studies indicate that KSHV infection directly promotes angiogenesis and inflammation through an autocrine and paracrine mechanism by inducing pro-angiogenic and pro-inflammatory cytokines [94]. In addition, viruses such as hepatitis C virus and Porcine reproductive and respiratory syndrome virus 2 have also been confirmed to be associated with angiogenesis [95, 96].Therefore, conceptualizing bacteria as discrete biological units risks oversimplifying their intricate crosstalk within the microbiome ecosystem.

The role of bacteria in tumor angiogenesis remains in the preliminary stage of mechanistic exploration; most studies are preliminary mechanistic investigations or small-sample experiments (e.g., animal models or in vitro assays), with large-scale independent replication studies or clinical validation being scarce—one of the core challenges in this field. While current clinical research data remain limited and further research is needed to establish the safety profile of bacteria-based antitumor therapeutic approaches, this strategy nonetheless holds significant research potential. As investigations into tumor microecosystems continue to evolve at an unprecedented pace, researchers must therefore adopt a dual focus: elucidating the multidimensional microbiota-cancer interplay while advancing precision intervention strategies through therapeutic optimization.

Author contributions

Wei Peng and Xiaomei Huang wrote the main manuscript text. Yinquan Pu and Yi Zhang prepared figure 1. Yujun Luo and Maosen Feng prepared Tables 1, 2 and 3. Xiaoan Li reviewed the manuscript.

Funding

This study was funded by Sichuan Science and Technology Program (No. MZGC20230031); Self-funded projects of the Department of Gastroenterology (No. XHZDZK014).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.