Novel therapeutic strategies and recent advances in gut microbiota synergy with nanotechnology for colorectal cancer treatment

Abstract

Over the last two decades, molecular biology advances have revealed the gut microbiota's active and crucial role in colorectal cancer (CRC) pathogenesis and its dysregulation during tumorigenesis. This understanding has underscored the urgency of exploring novel therapeutic approaches to address the challenges posed by CRC. Among these approaches, nanosynergistic therapies, with their potential to modulate the gut microbiota, increase drug bioavailability and stability, and reduce side effects, have emerged as a promising avenue. Furthermore, the efficacy of nanotechnology-based approaches can be enhanced by combining them with different therapeutic methods, including chemotherapy, radiotherapy, immunotherapy, photothermal therapy, and sonodynamic therapy. Herein, recent progress in nano synergistic therapy has been reviewed, focusing on the synergy between gut microbiota and nanotechnology in CRC treatment. Additionally, the diverse applications of nanoparticles have been analyzed to provide innovative ideas and effective solutions for developing highly effective CRC treatment strategies.

Graphical abstract

Highlights

• •The impact of gut microbiota on the development and progression of colorectal cancer.
• •The promoting role of gut microbiota in various treatment strategies for colorectal cancer.
• •The therapeutic effects of nanomaterials on colorectal cancer through synergistic interactions with gut microbiota.
• •Prospects for the application of different nanomaterials in combination with gut microbiota for the treatment of colorectal cancer.

1. Introduction

The human gut microbiota can consist of a population of 10¹⁴ microorganisms, and owing to its crucial roles in digestion, metabolism, and immune function, it is regarded as a “hidden organ.” [1] Interactions between the normal gut microbiota and a healthy host maintain a dynamic equilibrium that promotes mutual dependence and regulation. However, disruption of this balance may adversely affect various physiological functions, including nutrient absorption, the immune response, and metabolic regulation [2]. Consequently, the gut becomes vulnerable to harmful bacteria, leading to health issues such as inflammatory bowel disease, obesity, diabetes, and colorectal cancer (CRC) [[3], [4], [5], [6], [7]].

Currently, conventional therapeutic methods such as chemotherapy and immunotherapy are widely used to eliminate CRC cells and improve the survival rates of patients. Notably, the efficacy of these methods is impacted by diverse gut microbiota, which are considered to affect the treatment outcomes of patients with CRC. Davar et al. investigated 30 chemotherapy drugs, among which 10 drugs were inhibited by Escherichia coli or Weissella viridescens, whereas six drugs exhibited enhanced antitumor activity [8]. Notably, oral administration of intestinal commensal bacteria, such as Bacteroidales, Bifidobacterium, and Akkermansia muciniphila, has been shown to promote the efficacy of programmed death-ligand 1‐mediated immunotherapies for CRC [9].

Many approaches have been proposed to decrease the risk of CRC by regulating the gut microbiota, including the use of antibiotics, making dietary changes, incorporating probiotics, and performing fecal microbiota transplants (FMTs) [10]. However, these approaches often present limitations that can negatively impact patient prognosis. For example, the lack of precise targeting capabilities in these strategies can lead to the disruption of beneficial bacteria, which may inadvertently worsen the patient's condition and contribute to suboptimal treatment outcomes. This disruption can hinder immune system function, alter drug metabolism, and increase the risk of infections, all of which negatively affect the clinical prognosis.

In recent years, significant strides have been made in leveraging nanoparticles (Nps) for CRC treatment. Nps can be tailored to bind various molecules, such as antibodies, aptamers, and specific polysaccharides, to increase the direct concentrations of drugs at tumor sites [11]. They can also enhance the bioavailability of drugs with low water solubility and can be designed for controlled release, facilitating precise drug delivery timings and locations [12]. Furthermore, the use of Nps can extend the effectiveness of drugs while reducing their administration frequency. Nps have been shown to target specific gut pathogens to inhibit harmful microorganisms while promoting the growth of beneficial probiotics. Their use can enable real-time monitoring of changes in the gut microbiota, allowing for adjustments to personalized treatment regimens [13]. Overall, nanotechnology-based approaches offer a multifaceted strategy for CRC treatment, enhancing specificity and effectiveness while minimizing side effects and thereby improving overall survival for patients with CRC.

While several recent reviews have discussed the relationship between the gut microbiota and nanotechnology in CRC treatment, they have focused primarily on isolated aspects, such as how Nps target pathogenic bacteria and modulate the microbiome, the challenges of nanotechnology in cancer therapy, and the types of Nps used in CRC treatment [[14], [15], [16]]. In contrast, our review emphasized the synergistic effects of combining nanotechnology with various therapeutic strategies, including immunotherapy, radiotherapy, and chemotherapy. We also analyzed the bidirectional interaction between Nps and the microbiome, highlighting that Nps not only modulate the microbiome but also that the microbiome can enhance therapeutic efficacy. Through the comprehensive, multimodal regulation of Nps and the microbiome, we offer a novel framework to improve CRC treatment outcomes. The synergistic regulation of the gut microbiota with various treatment modalities and Nps for effective enhancement of the therapeutic effects on CRC is illustrated in Scheme 1.

Scheme 1.

The synergistic regulation of gut microbiota by various treatment modalities and nanoparticles effectively enhances the therapeutic effects for colorectal cancer. The figure was created using BioRender.com.

2. Probiotics: inhibitors of CRC development

Bacterial growth is mostly associated with negative effects on health. However, not all bacteria exert harmful effects and can be beneficial to health; these beneficial bacteria are referred to as probiotics [17]. Probiotics, beneficial bacteria that can improve the balance of the gut microbiota, have shown promising potential in preventing CRC progression. This finding offers hope for the development of effective preventive measures against CRC [18,19]. Reportedly, the specific mechanisms primarily include anti-inflammatory activities, the biological effects of metabolites, immune system activation and gut barrier function regulation (Table 1).

Table 1.

Types of probiotics/metabolites and their mechanisms in CRC development.

Probiotics/metabolites	Mechanisms	Details	Refs
Bifidobacterium strain B breve Iw01 and its metabolite indole-3-lactic acid	Anti-inflammatory effects	Decreases inflammation, reduces macrophage infiltration, promotes differentiation of immature colonic macrophages, and activates the aryl hydrocarbon receptor in macrophages.	[22]
Short-chain fatty acids	Biological effects of metabolites	Inhibits HDACs to regulate cell cycle and apoptosis genes; promotes CRC cell differentiation; attenuates Wnt signaling.	[25]
Bile Acids	Biological effects of metabolites	A high-fat diet increases secondary bile acids, which enhance CRC pathogen colonization by activating TGR5 signaling, influencing tumor growth.	[31]
Lactobacillus casei and Bifidobacterium longum	Immune responses activation	Boosts phagocytic activity of antigen-presenting cells and enhances NK cell killing.	[34]
Streptococcus thermophiles and Lactobacillus acidophilus	Regulation of intestinal barrier function	Enhances phosphorylation of tight junction and cytoskeletal proteins in mucosal cells to strengthen tight junctions between intestinal epithelial cells.	[37]

2.1. Anti-inflammatory effects

In CRC pathogenesis, the inflammation-to-carcinogenesis transition is considered critical, with chronic inflammation acting as a “promoter.” Probiotics have been shown to exert anti-inflammatory effects, inhibiting the inflammation-mediated onset of CRC [20]. Li et al. reported that the oral administration of Bifidobacterium breve Iw01 could inhibit or delay CRC progression by reducing colonic inflammation, decreasing macrophage infiltration, and promoting the differentiation of immature colonic macrophages [21]. Notably, the antitumor effect of B. breve Iw01 was attributed to the metabolite indole-3-lactic acid, which can activate the aryl hydrocarbon receptor in macrophages to regulate their differentiation [22]. Overall, probiotic-mediated enhancement of intestinal inflammatory responses can be effective in preventing inflammation associated with CRC onset.

2.2. Biological effects of probiotic metabolites

Microorganism-derived metabolic products are referred to as microbial metabolites. For example, short-chain fatty acids (SCFAs), the byproducts of probiotics, can exert immunomodulatory and anticancer effects by inhibiting histone deacetylases (HDACs) or binding to SCFA receptors (namely, G protein-coupled receptors [GPRs]) [23,24]. Furthermore, SCFA-mediated inhibition of HDACs has been associated with cell cycle arrest, leading to antiproliferative and proapoptotic effects and, thus increased cell differentiation in CRC tissues [25,26]. In addition to HDAC inhibition, GPR43, and GPR109A activation has been associated with a notable decrease in CRC growth and increased survival rates in mouse models [27].

Furthermore, increased secretion of bile acids induced by a high-fat diet may promote the conversion of primary bile acids into secondary bile acids by microbes, potentially contributing to colon tumor formation [28]. Studies in germ-free mice indicate that reducing secondary bile acid levels supports the hypothesis that the gut microbiota influences colon tumor formation [29]. Mutations in the Apc gene reduce the expression of bile acid transporters, increasing secondary bile acid availability, which in turn promotes the intestinal colonization of CRCpathogens such as Streptococcus [30]. Additionally, bile acids regulate the gut microbiota and metabolic mechanisms by activating TGR5 signaling in intestinal L cells, which increases the production of glucagon-like peptide 1, a metabolic hormone that enhances insulin secretion and regulates glucose [31]. Interestingly, GLP-1 receptor agonists have been shown to promote intestinal growth and colon tumor formation in mouse models, highlighting the potential role of bile acids in CRC development [32]. Overall, these findings suggest that microbial metabolites, including SCFAs and bile acids, play crucial roles in modulating CRC development and progression.

2.3. Activation of immune responses and regulation of intestinal barrier function

The gut mucosal immune system consists of various immune cells, including macrophages, dendritic cells, T cells, and B cells. Notably, probiotics such as Lactobacillus casei and Bifidobacterium longum can regulate innate and adaptive immune responses [33]. The phagocytic activity of antigen-presenting cells such as phagocytes and dendritic cells has been shown to be enhanced by innate immune functions, along with the strengthening of the CRC cell-eliminating ability of natural killer cells [34]. In contrast, adaptive immunity is involved in lymphocyte stimulation in the intestinal mucosa, leading to immunoglobulin A secretion [35,36]. Taken together, these processes improve the ability of the host to respond to CRC cells.

A complete intestinal barrier is crucial for blocking the entry of harmful substances and pathogens into the body. For example, Streptococcus thermophiles and Lactobacillus acidophilus have been shown to promote the phosphorylation of tight junction and cytoskeletal proteins in mucosal cells, thereby enhancing tight junctions between intestinal epithelial cells [37]. Probiotics can prevent the attachment of invading microbes to mucosal cells, preventing bacterial entry into deeper tissues [38]. In mouse models, the administration of prebiotics has been reported to inhibit intestinal mucosal cell lysis and apoptosis, reducing the risk of CRC [39]. Taken together, these studies show that probiotics and prebiotics support host health by maintaining intestinal permeability and strengthening the mucosal barrier.

3. Pathogenic bacteria: enhancers of CRC development

While probiotics have protective effects, the balance of the gut microbiota can be disrupted by certain pathogenic bacteria. This disruption not only negates the benefits provided by probiotics but also actively contributes to CRC development [[40], [41], [42]]. Although not fully understood, the main mechanisms include inflammation induction, the oxidative stress response, and immune evasion effects (Table 2).

Table 2.

Types of pathogenic bacteria and their mechanisms in CRC development.

Pathogenic bacteria	Mechanisms	Details	Refs
Fusobacterium nucleatum	Inflammation	Secretes enzymes and toxins (e.g., FadA) to activate NF-κB signaling pathway.	[44,45]
enterotoxigenic Bacteroides fragilis	Inflammation	Engages signaling pathways like IL-17 receptor and STAT3, leading to NF-κB activation and stimulation of T helper 17 cells.	[46]
Escherichia coli (pks+)	Inflammation	Sustains expression of pks-related genes, enhancing inflammatory responses through IL-17 and NF-κB activation.	[48]
gram-positive/negative bacteria	Inflammation	Secretes toxins to bind Toll-like and Nod-like receptors, triggering the NF-κB signaling pathway.	[[50], [51], [52]]
Enterococcus faecalis	Oxidative stress	Damages healthy cells through the bystander effect, while hydroxyl radicals induce greater genomic instability and cell mutations.	[60]
Parvimonas anaerobius	Oxidative stress	Enhances ROS production by activating TLR2 and TLR4; promotes cholesterol synthesis and cell proliferation.	[62]
Parvimonas micra	Oxidative stress	Activates key enzymes responsible for ROS production (NADPH oxidase) through miR-218-5p/Ras/ERK/c-Fos pathway.	[65]
Escherichia coli and enterotoxigenic Bacteroides fragilis	Oxidative stress	Regulates inflammatory responses and cell growth by influencing the nuclear translocation of NF-κB.	[68,69]
Fusobacterium nucleatum and Parvimonas anaerobius	Immune evasion	Secretes CXCL1 to attract MDSCs to the tumor and releases IL-10 and TGF-β to suppress effector T-cell activity.	[[77], [78], [79]]
Klebsiella pneumoniae	Immune evasion	Influences TAMs with M2 characteristics, leading to the secretion of factors such as VEGF and MMPs.	[82]
Helicobacter pylori	Immune evasion	Causes excessive upregulation of immune checkpoints in tumor cells, inhibiting T-cell activation and cytokine production.	[86,87]
Intestinal streptococci	Immune evasion	Interferes with MHC I expression and reduces CD8+ T-cell recognition of tumor cells.	[[90], [91], [92]]

3.1. Proinflammatory effects of pathogenic bacteria

In the context of gut microbiota dysbiosis, chronic inflammation initially occurs, which may subsequently lead to inflammatory bowel disease and subsequent CRC development [43]. For example, Fusobacterium nucleatum can secrete specific enzymes (such as Fusobacterium adhesin A) and toxins to disrupt host cell signaling by activating inflammatory pathways, such as nuclear factor (NF)-κB, thereby upregulating inflammatory and chemotactic factors, along with inflammatory cell migration [44,45]. Enterotoxigenic Bacteroides fragilis (ETBF) activates pathways such as interleukin (IL)-17 receptor, signal transducer and activator of transcription, and NF-κB, leading to the activation of T helper (TH)17 cells, which can produce inflammatory factors, including IL-17, and are critical for the inflammatory response [46,47]. Notably, polyketide synthase (pks) gene-carrying E. coli strains can produce colibactin, a toxin directly associated with damage to host cell DNA and genetic mutations [48,49]. Although the inflammatory potential of such E. coli strains is not as significant as their carcinogenic effects, they may contribute to cancer development by maintaining pks gene expression and activating inflammatory pathways such as the IL-17 and NF-κB pathways in the intestine.

Bacterial toxins bind to pattern recognition receptors, such as Toll-like receptors (TLRs) and Nod-like receptors (NLRs), to activate downstream signaling pathways [[50], [51], [52]]. For example, TLR4 and TLR2 can recognize lipopolysaccharides (LPSs) and lipoteichoic acid from gram-negative and gram-positive bacteria, respectively [53,54]. Reportedly, activation of the associated signaling pathways can lead to the activation of NF-κB, thereby promoting the inflammatory response [55]. NLRs have been shown to interact with TLR signaling pathways to regulate the inflammatory response and suppress excessive inflammation, thereby counteracting TLR-mediated proinflammatory signals to prevent CRC development [56]. Overall, gut dysbiosis-associated inflammation can contribute to CRC pathogenesis.

3.2. Imbalance of oxidizing molecules

An imbalance between the production of reactive molecules (such as reactive oxygen species [ROS] and reactive nitrogen species [RNS]) and antioxidant defenses can result in oxidative stress [57]. Inflammation-mediated ROS and RNS production by inflammatory cells can result in DNA damage, leading to the activation of oncogenes or the inactivation of tumor suppressor genes, which may increase CRC risk [58]. For example, Enterococcus faecalis can induce DNA damage by generating superoxide molecules via the bystander effect, where infected cell-derived ROS affect nearby healthy cells, causing DNA damage and mutations [59,60].

Furthermore, E. faecalis-derived hydroxyl radicals can exacerbate genomic instability, promote cell mutations, and thus promote CRC development [61]. Notably, Parvimonas anaerobius enhances ROS production in intestinal epithelial cells by activating TLR2 and TLR4 receptors, leading to increased cholesterol synthesis and cell proliferation, thereby triggering hyperplasia and carcinogenesis in intestinal cells [62,63]. Cholesterol synthesis is related to the stability and synthesis of cell membranes, and excessive cholesterol production can lead to abnormal cell proliferation [64]. Additionally, Parvimonas micra has been reported to facilitate CRC progression via the miR-218-5p/Ras/extracellular signal-regulated kinase/c-Fos pathway, which activates key cellular enzymes for ROS production (namely, nicotinamide adenine dinucleotide phosphate hydrogen oxidase) [[65], [66], [67]]. Reportedly, both E. coli and ETBF can stimulate oxidative reactions in intestinal epithelial cells [[68], [69], [70], [71], [72]]. Overall, these studies highlight that ROS can enhance NF-κB migration from the cytoplasm to the nucleus, thereby activating genes associated with cell proliferation and tumorigenesis.

3.3. Mechanisms of tumor cell escape recognition

The mechanism by which tumor cells avoid detection and elimination by the immune system of the host is referred to as immune evasion, which involves various strategies to increase the growth and dissemination of tumor cells [73]. Some of the main mechanisms of immune evasion are as follows: (1) proliferation of immunosuppressive cells, (2) upregulation of immune checkpoints, and (3) interference with major histocompatibility complexes (MHCs).

In CRC, the population of immunosuppressive cells, including myeloid-derived suppressor cells, regulatory T cells (Tregs), and tumor-associated macrophages (TAMs), is often increased in the tumor microenvironment [[74], [75], [76]]. In particular, F. nucleatum and P. anaerobius can induce tumor cells to secrete the chemokine C-X-C motif chemokine ligand (CXCL)1, thereby attracting myeloid-derived suppressor cells to the tumor site and facilitating tumor progression [[77], [78], [79]]. Additionally, Tregs can secrete various cytokines, such as IL-10 and transforming growth factor-β, to inhibit the function of effector T cells, thereby weakening the immune response against tumors [80,81]. Klebsiella pneumoniae has been reported to affect the functional regulation of TAMs, which are susceptible to reprogramming by the tumor microenvironment and typically exhibit characteristics of M2-type macrophages [[82], [83], [84], [85]]. TAMs release factors such as vascular endothelial growth factor (VEGF) and matrix metalloproteinases that can promote tumor growth, angiogenesis, and tissue remodeling, thereby facilitating tumor survival and progression.

Reportedly, Helicobacter pylori infection can excessively upregulate immune checkpoints (including cytotoxic T lymphocyte-associated protein [CTLA]-4, programmed cell death 1 [PD-1], and programmed death-ligand 1 [PD-L1]) in tumor cells [[86], [87], [88]]. These checkpoints bind to T-cell receptors, inhibiting their activation and cytokine production and thus weakening their ability to target tumor cells.

MHC I is a class of proteins found on the surface of all nucleated cells [89]. Intestinal streptococci can inhibit the expression of MHC I, inhibiting the ability of cluster of differentiation (CD)8⁺ T cells to recognize tumor cells and affecting immune surveillance and antitumor responses [[90], [91], [92]].

Nevertheless, despite the abundant literature, significant gaps remain regarding the unexplored mechanisms of action of the gut microbiota in CRC development and progression. Some of these gaps are associated with the conduct of different types of studies, as follows: conducting personalized studies that consider host-specific factors; undertaking long-term dynamic observational studies; implementing targeted interventional studies; and identifying microbial biomarkers for early diagnosis or prognosis of CRC.

4. Impact of the gut microbiota on CRC treatment efficacy

In recent years, many studies have been conducted on microbiome-based cancer therapies, along with various clinical trials. These studies provide evidence of the role of the gut microbiota in modulating the response to chemotherapy, radiotherapy, targeted therapy, and immunotherapy. Additionally, they highlight key microbial species, their mechanisms, and the potential of targeting the microbiota to improve anticancer efficacy and reduce toxicity.

4.1. Modulation of the effectiveness of chemotherapy

Presently, the main classes of chemotherapy drugs for CRC include fluoropyrimidines (such as 5-fluorouracil and capecitabine (Cap)), platinum-based drugs (such as oxaliplatin), and topoisomerase inhibitors (such as irinotecan). The effects of the gut microbiota on CRC chemotherapy can either be positive or negative.

In terms of positive effects, the gut microbiota can increase the production of beneficial metabolic products, promoting immune cell activation and optimizing drug metabolism. For example, bacteria-derived butyrate has been shown to increase the efficacy of oxaliplatin by promoting the proliferation and stimulation of CD8⁺ T cells [93]. In contrast, many studies have reported that in germ-free or antibiotic-treated mice, the anticancer efficacy of cyclophosphamide is significantly reduced, which is restored following the administration of Enterococcus hirae and Barnesiella intestinihominis in CRC mice [94,95]. These bacteria increase the production of Th1 and Th17 cells, thereby promoting the activation of tumor-specific T cells (CD4⁺ and CD8⁺ T cells) [95]. After activation, immune cells can more effectively recognize and attack tumor cells, restoring the effects of cyclophosphamide. In another study, Bifidobacteria and Lactobacillus secreted β-glucuronidase, which can activate the liver metabolite SN38 (the active metabolite of irinotecan) by hydrolyzing its glucuronic acid conjugate, thereby enhancing the effectiveness of chemotherapy against CRC [96,97].

In terms of negative effects, the gut microbiota can interfere with protective autophagy, self-renewal of cancer stem cells, and metabolic reprogramming, affecting treatment outcomes. For example, F. nucleatum can activate the TLR4 and myeloid differentiation primary response 88 signaling pathways, thereby promoting the expression of autophagy-related genes such as unc-51-like autophagy activating kinase 1 and autophagy-related 7 (ATG7) [[98], [99], [100]]. Following activation, these genes increase the autophagic capability of cells, thereby reducing the effectiveness of chemotherapeutic drugs, as the autophagic process protects CRC cells from drug-induced damage [101,102]. Moreover, F. nucleatum has been shown to stimulate fatty acid oxidation and the neurogenic locus notch homolog protein signaling pathway to promote the self-renewal of CRC stem cells [103]. Cancer stem cells often exhibit strong drug resistance and metastatic potential. Additionally, F. nucleatum-derived metabolites, such as formate, have been shown to increase tumor invasiveness and drug resistance by promoting the aryl hydrocarbon receptor signaling pathway [[104], [105], [106]]. Reportedly, autophagy inhibitors (such as chloroquine and 3-methyladenine) or ATG7 gene knockout/silencing have been explored to counteract such protective mechanisms and weaken the protective effect of F. nucleatum on cancer cells.

In addition to known mechanisms, various metabolic pathways and microbial enzymes remain unexplored, and they may have the potential to affect the activity, metabolism, and excretion of chemotherapy drugs. These findings underscore the need for further investigations to better understand their roles in drug efficacy and safety.

4.2. Modulating radiosensitivity and reducing toxicity

Radiotherapy has been used as a therapeutic approach in CRC treatment for more than a century. However, to the best of our knowledge, the anticancer effects of the gut microbiome on radiotherapy were first confirmed in 2021 [107]. The gut microbiome is closely associated with radiosensitivity and susceptibility to toxic side effects. For example, Yang et al. revealed that the proportions of Gammaproteobacteria, Enterobacteriaceae, and Fusobacteriales increased in the feces of radiotherapy-resistant CRC mice [108]. These changes may be associated with gut microbiome dysregulation, leading to resistance to radiotherapy. Furthermore, in vivo studies have shown that forkhead box Q1 knockout can modulate the silent information regulator sirtuin 1/β-catenin axis, reducing the proportions of CRC-associated pathological gut bacteria and thus inhibiting the xenograft formation of radioresistant CRC cells [108]. Notably, Benej et al. positively associated a hypoxic state with an increase in the abundance of F. nucleatum, which, in turn, promotes the proliferation of immune cells in inactive states, such as M0 macrophages, resting natural killer cells, and memory B cells. These changes may enable CRC cells to become radioresistant, reducing the efficacy of radiotherapy and affecting overall patient survival [109].

In contrast, Zhou et al. reported a gut microbiome metabolite called methylglyoxal, which could reverse the radioresistance of CRC by upregulating ROS and reducing tumor hypoxia. Specifically, methylglyoxal enhances radiotherapy-mediated tumor immune microenvironment regulation by activating the cyclic guanosine monophosphate–adenosine monophosphate synthase–stimulator of interferon genes (STING) pathway and inducing immunogenic cell death. Additionally, Dong et al. reported that oral administration of F. nucleatum exacerbated radiotherapy-induced colitis, epithelial integrity, and intestinal structure [110]. Metronidazole (MTZ) administration has been reported to ameliorate F. nucleatum-induced colonic damage, markedly reduce the number and volume of CRCs, and downregulate liver metastasis-associated protein levels. Nevertheless, the present understanding of the mechanisms of action of the gut microbiome on the response of the host to radiotherapy remains limited, warranting more research for further exploration.

4.3. Modulating the efficacy of targeted therapy and reducing toxicity

Presently, targeted therapy is among the widely employed approaches for treating various malignant tumors. For CRC, the primary drugs for targeted therapy include the epidermal growth factor receptor inhibitor cetuximab and the VEGF inhibitor bevacizumab. Many recent studies have shown a significant correlation between the gut microbiota and the efficacy of targeted drugs. For example, Zhao et al. reported that administering Lactobacillus reuteri via gavage to cetuximab-treated mice alleviated intestinal inflammatory responses and regulated the ratio of TH cells, thereby mitigating cetuximab-induced gastrointestinal toxicity [111].

In another study in which VEGF inhibitors were administered to patients with diarrhea, the composition of the gut microbiota revealed a relatively high abundance of Bacteroides, whereas that of Prevotella and Bifidobacterium was relatively low [112]. Chen et al. reported that in 110 patients with metastatic CRC who underwent chemotherapy combined with cetuximab or bevacizumab, adverse outcomes were associated with increased abundances of K. pneumoniae, Lactobacillus, Bifidobacterium, and F. nucleatum in the gut [113]. Furthermore, the α diversity of the gut microbiota was greater in the partial response subgroup receiving bevacizumab, whereas a notable difference in β diversity was detected between the partial response and disease progression groups [113]. Many targeted therapies have been proposed for CRC treatment, such as regorafenib, apatinib, panitumumab, and dabrafenib; however, further studies are needed to fully elucidate the mechanisms of action of these drugs and their effects on the gut microbiome.

4.4. Leveraging the microbiota to improve immune checkpoint inhibitor (ICI) therapy

Many studies have investigated the effects of the gut microbiota on the anticancer immune responses of the host [114,115]. Montalban-Arques et al. reported that administering Clostridium strains (CC4) to a CRC mouse model increased CD8⁺ T-cell infiltration into tumor tissues, with CC4 alone nearly eradicating all tumor cells [116]. The immunotherapy approach referred to for CRC is focused on ICIs. In 2017, the U.S. Food and Drug Administration approved the use of ICIs for treating CRC; however, only 14 % of CRC subtypes, particularly those with mismatch repair deficiencies or microsatellite instability, achieved significant progression-free survival rates [[117], [118], [119]]. Mager et al. showed that Bifidobacterium pseudolongum, Lactobacillus johnsonii, and Eubacterium hallii enhanced the efficacy of ICIs in a CRC mouse model [120]. Specifically, B. pseudolongum enhances the immune response by producing inosine, which promotes TH1 cell activation via the adenosine A2A receptor signaling pathway. Notably, F. nucleatum has been shown to increase the efficacy of anti-PD-L1 antibodies and prolong survival time in mice with CRC allografts [121]. The colonization of F. nucleatum activated the STING–NF-κB signaling pathway and increased PD-L1 expression in CRC cells [121]. Interestingly, inhibiting PD-L1 promoted CD8⁺ T-cell activation [121].

Recently, Zhang et al. decreased the effectiveness of PD-1 inhibitors against CRC by depleting the gut microbiota in mice via the use of broad-spectrum antibiotics [122]. Furthermore, the introduction of a Lactobacillus strain controlled tumor growth, suggesting that the strain induced tumor cells to express the chemokine CXCL10, which facilitated CD8⁺ T-cell migration to the tumor site [122]. Presently, studies on the relationships between these immune checkpoints (such as CTLA-4, lymphocyte activation gene 3, T-cell immunoglobulin and mucin domain 3, and T-cell immunoreceptor with immunoglobulin and tyrosine-based inhibitory motif domains) and the gut microbiome are limited, underscoring the need for future exploration of such mechanisms.

5. Recap and future directions

The abovementioned studies reveal the essential role of the gut microbiota, particularly probiotics and pathogenic bacteria, in the development and advancement of CRC and in affecting treatment responses. Probiotics exhibit beneficial effects in supporting treatment by modulating the immune system, maintaining intestinal barrier integrity, and producing beneficial metabolites. Furthermore, they enhance the efficacies of chemotherapy, radiotherapy, targeted therapies, and immunotherapy, reduce the side effects of medications and increase the response of tumors to treatment. In contrast, pathogenic bacteria can undermine treatment efficacy through various mechanisms, including the induction of chronic inflammation, oxidative stress, and immune evasion, making elimination of cancer cells more challenging. Alteration of the gut microecosystem can be an effective approach to address the negative effects of diminished therapeutic efficacy of drugs and increased toxicity.

Therefore, in clinical settings, developing personalized treatment strategies on the basis of the characteristics of the gut microbiota of individuals is essential. In this dynamic relationship, a comprehensive understanding of the supportive role of probiotics and the negative impact of pathogenic bacteria is crucial and may provide new avenues and methods for optimizing treatment regimens.

6. Applications of nanotechnology in CRC treatment

Presently, antibiotics, FMT, and probiotic supplementation are the main methods used to regulate the gut microbiota [123,124]. However, these methods present certain limitations, such as the broad-spectrum action of antibiotics, which may kill beneficial bacteria, and FMT may introduce unwanted harmful bacteria, suggesting that both methods lack precision. Furthermore, the types of probiotics available commercially are limited, and there is a lack of sufficient scientific evidence of the explicit benefits of all probiotics [124]. To address such challenges, it is important to explore other avenues, among which nanotechnology appears to be a promising direction. Specifically, nanotechnology-based approaches can leverage their targeting capabilities to eliminate specific harmful bacteria precisely [125]. Furthermore, nanotechnology can be combined with other therapeutic methods to enhance targeted drug delivery to CRC cells, reduce the effects on healthy cells, and minimize side effects [126]. Herein, various nanosystem-based therapies are discussed, with a focus on the types, effects, and mechanisms involved in antibacterial applications, along with Nps with different therapies and properties.

6.1. Direct regulation of the gut microbiota by Nps

F. nucleatum plays a significant role in the occurrence, metastasis, drug resistance, and immune microenvironment regulation of CRC, making Np-mediated elimination of F. nucleatum an important strategy for treating CRC. Abbas et al. reported that silver Nps (diameter: 10 nm) inhibited the proliferation of F. nucleatum in a dose-dependent manner [127]. Similarly, Haidari et al. designed highly monodispersed, ultrasmall-sized (<3 nm) polycationic silver Np clusters, which reduced the minimum inhibitory concentration for F. nucleatum from 25 μg/mL to 3.375 μg/mL and the minimum bactericidal concentration from 50 μg/mL to 6.75 μg/mL [128]. Additionally, Liu et al. developed a type of AZO and PAMAM-AZO-based nanosphere with CP [5]A attached to the surface, forming the composite PAMAM-AZO@CP [5]A [129], which uses quaternary ammonium salts to target and kill F. nucleatum, reducing oxaliplatin resistance [129]. Compared with oxaliplatin, the nanocomposite significantly reduced the tumor volume, indicating superior anticancer efficacy [129]. Dong et al. utilized phage display technology to screen for M13 bacteriophage strains that specifically bind to F. nucleatum in vitro [130]. Notably, bacteriophages carrying silver Nps (M13@Ag) were assembled via electrostatic interactions to bind silver ions to phages [130], and M13@Ag selectively eliminated F. nucleatum and reversed the F. nucleatum-associated immunosuppressive microenvironment. After 24 h of F. nucleatum colonization in the mouse gut, the CRC volume in the M13@Ag treatment group was significantly reduced.

Many other bacteria also demonstrate high sensitivity to Nps. For example, metal Nps have been shown to inhibit growth and kill common bacteria such as E. coli and Staphylococcus aureus [131]. Titanium dioxide (TiO₂) and zinc oxide Nps are toxic to both gram-negative and gram-positive bacteria and fungi [131]. Even under light-protected conditions, TiO₂ Nps can maintain their ability to eliminate E. coli. However, in the context of CRC research, the effects of other bacteria on the efficacy of Np-based therapies remain insufficiently explored, warranting further research.

6.2. Synergistic effects of different treatment modalities

Herein, the synergistic application of Nps with other treatment methods, including chemotherapy, radiotherapy, immunotherapy, photothermal therapy (PTT), sonodynamic therapy (SDT), and multimodal therapy, is discussed (Table 3). The integration of these diverse treatment approaches can provide insights into the comprehensive regulation of the gut microbiome and the enhancement of the overall efficacy and safety of CRC treatments.

Table 3.

Role of nanoparticles in enhancing various CRC therapies.

Therapies	Nanoparticles	Bacteria	Characteristics	Refs
Chemotherapy	Supramolecular nanomedicine (PG-Pt-LA/CB [7])	Kill F. nucleatum	High glutathione response mechanism	[132]
Chemotherapy	Dextran Nps loaded with Irinotecan	Decreases F. nucleatum levels, increases Clostridium butyricum levels	Phage-guided nanomedicine	[133]
Chemotherapy	Xylan-stearic acid conjugate loaded with Capecitabine	Enhance probiotics growth and SCFA production.	Oral administration	[134]
Radiotherapy	Antigen-adsorbing cationic polymer Nps	Genetically attenuated Salmonella	Enhances the crosstalk between antigens and dendritic cells	[138]
Radiotherapy	PLGA/PVA Nps encapsulating astaxanthin	Microbial protection	Protects small intestine from radiation-induced damage	[139]
Immunotherapy	P3C-Asp combined an aspirin prodrug and the prebiotic dextran	Increases anticancer immune response microbiota abundance	Enhances the accumulation of salicylic acid and clearing ROS	[140]
Immunotherapy	CoAmuc_2172 Nps	Akkermansia muciniphila	Enhances tumor-killing function of CD8+ cytotoxic T lymphocytes	[142]
Immunotherapy	Magnetic mesoporous silica Nps loaded with 6-gingerol	Increases beneficial bacteria; reduces harmful bacteria	Induces apoptosis and ferroptosis through Gin/ferrous ion-induced oxidative stress and magnetic hyperthermia	[143]
Immunotherapy	F. nucleatum biomimetic nanomedicine	Tumor-associated F. nucleatum	Targets tumors with overexpressed Gal-GalNAc	[144]
Immunotherapy	Lipid-protamine-DNA Nps	LPS from gram-negative bacteria	Blocks LPS-mediated signaling pathways	[146]
Photothermal therapy	CTPB oral nanoparticle platform (copper sulfide and titanium dioxide)	Pathogenic bacteria	Improves the intestinal mucus penetration efficiency	[154]
Photothermal therapy	E. coli@Cu2O nanocomposite	Escherichia coli	Exhibits photothermal conversion in the NIR II biological window	[155]
Photothermal therapy	DCY51T-AuCKNps	Lactobacillus kimchicus DCY51T	One-pot biosynthesis	[156]
Sonodynamic therapy	Au@BSA-CuPpIX	suppressing F. nucleatum	Reduces the phototoxicity caused by metal porphyrins accumulated in the skin	[163]
Sonodynamic therapy	E. coli pE@PCN	Escherichia coli BL21	Elicites an immune memory response to help prevent tumor recurrence	[164]
Multimodal therapy	VA-SAM@BTO Micro-Robots	Staphylococcus aureus	Biomimetic technology	[165]
Multimodal therapy	CBS-CS@Lipo/Oxp/MTZ Complex	Clostridium butyricum spores	Targets and accumulation in the colon	[166]
Multimodal therapy	Amphiphilic inulin-deoxycholic acid conjugate UDIN	Increased beneficial short-chain fatty acids	Self-assembly	[167]

6.2.1. Chemotherapy synergy with Nps

Compared with conventional interventions, the use of prebiotics to enter the gut offers a more controllable method for regulating the gut microbiota. Cap is a first-line chemotherapy drug for CRC but lacks a carrier capable of extending its half-life. In this study, Lang et al. constructed Cap-loaded Nps via a prebiotic xylan-stearic acid conjugate (SCXN) (Fig. 1a I) [132]. SCXN promoted the proliferation of probiotics and the production of short-chain fatty acids (Fig. 1a II). In a CRC mouse model, the oral administration of SCXN delayed drug clearance from the bloodstream and increased the Cap concentration within tumors (Fig. 1b and c). This prebiotic-based Np, which combines gut microbiota regulation with chemotherapy, provides a promising approach for CRC treatment.

Fig. 1.

(a) Schematic illustration of the combination of chemotherapy with gut microbiota regulation for CRC treatment via a prebiotic xylan-stearic acid conjugate (SCXN). (b) Drug release profiles of free Scap and SCXN in phosphate-buffered saline (PBS, pH 7.4) with or without MCC. (c) In vivo antitumor effects in MC38 tumor-bearing mice treated with multiple doses of different formulations. (d) Tumor weights of the dissected tumors after diverse treatments on day 18. (e) Tumor growth curves of HT29 tumor-bearing mice treated with different formulations. (f) Representative photograph showing the effects of the treatments. Reprinted with permission from Refs. (a–c) [132] 2023 Springer Nature and (d–f) [133] 2022 ELSEVIER.

The presence of F. nucleatum in the community of CRC accounts for its resistance to chemotherapy. To overcome this drawback, Yan et al. conjugated lauric acid and a platinum oxaliplatin prodrug to hyperbranched polyglycerol and then added cucurbit [7]uril to trigger supramolecular assembly, resulting in the preparation of a multilayer supramolecular nanomedicine (PG-Pt-LA/CB(7)). In principle, lauric acid is used to eliminate F. nucleatum, thereby enhancing the efficacy of oxaliplatin [133]. This mechanism not only effectively overcomes the chemotherapy resistance induced by F. nucleatum but also significantly enhances the therapeutic effect through targeted drug release (Fig. 1d–f). In another study, Zheng et al. isolated F. nucleatum-specific bacteriophages and linked them with irinotecan-loaded dextran Nps to fabricate a phage-guided nanomedicine. This nanosystem demonstrated good safety in piglets; it was enriched in CRC, prolonged the survival of CRC mice, decreased F. nucleatum levels, and increased the abundance of Clostridium butyricum in the colon [134]. The phage-guided targeting strategy offers a novel and nontoxic method to address chemotherapy resistance and enhance therapeutic efficacy.

Overall, these studies demonstrate the potential of combining advanced Np-based drug delivery systems with gut microbiota modulation to increase the efficacy of chemotherapy in patients with CRC. By regulating the gut microbiota with prebiotic Nps, not only can drug bioavailability be improved and the half-life of chemotherapy agents prolonged, but the drug concentration within tumor sites can also be increased, thereby enhancing the therapeutic effect. Furthermore, targeting F. nucleatum and promoting the growth of beneficial probiotics can effectively overcome the chemotherapy resistance caused by the gut microbiota, significantly improving therapeutic outcomes. These innovative approaches allow for more precise drug delivery while modulating the microbiota, offering a more personalized and effective treatment strategy and providing new hope for CRC therapy.

6.2.2. Radiotherapy synergy with Nps

Nanotechnology-based approaches offer novel possibilities for improving the therapeutic effects of radiotherapy and reducing its side effects. Owing to the immunosuppressive nature of the tumor microenvironment, radiotherapy-mediated release of tumor antigens does not fully activate the immune response [[135], [136], [137]]. Wang et al. demonstrated that genetically attenuated Salmonella (VNP20009) coated with antigen-adsorbing cationic polymer NPs significantly enhanced the efficacy of radiotherapy [138]. Compared to radiotherapy alone, the composite therapy achieved greater inhibition of primary tumor growth and delayed secondary tumor progression by 13 days (Fig. 2a–c), showcasing amplified abscopal effects through improved tumor antigen capture and delivery. Sequential tumor rechallenges (days 152/208) further revealed durable immune memory, with composite-treated mice exhibiting resistance to tumor regrowth, whereas control groups experienced rapid relapse (Fig. 2d and e). Nevertheless, although the effects of genetically modified Salmonella have been attenuated, its long-term safety still requires further investigation. These findings underscore the potential of engineered bacteria to enhance radiotherapy efficacy and induce long-term immune protection.

Fig. 2.

Antigen-capturing bacteria enhance immune responses and trigger abscopal effects in CT26 tumor-bearing mice. (a) Therapeutic treatments for inhibiting the growth of primary and rechallenged CT26 tumors. The irradiated tumors were labeled "primary tumors (1°)" and treated with antigen-capturing bacteria. The rechallenged tumors were labeled "secondary tumors (2°)" and were not treated. (b, c) Average tumor growth curves for primary (b) and secondary (c) CT26 tumors posttreatment. (d) Survival of CT26 tumor-bearing mice (RT, RT + B^mPEG, RT + B⁻, n = 5; saline, B^mPEG, n = 6; B⁺, B⁻, RT + B⁺, n = 7, biologically independent animals). (e) Tumor growth curves for successively rechallenged mice treated with RT + B⁺. The mice were rechallenged at the tumor site on the opposite flank at days 152 and 208 (RT + B⁺, n = 3; control, n = 4 or 5, biologically independent animals). (f) Schematic illustration of the SP@AMF synthetic protocols and radioprotective mechanisms: I. SP protects AMF from gastric degradation. II-IV. The SP@AMF gradually degraded, releasing AMF slowly throughout the small intestine (duodenum, jejunum, and ileum). V. SP@AMF shields intestinal tissues from radiation-induced epithelial injury, inflammation, and fibrosis. VI. SP@AMF support the health of the gut microbiota. (g) Quantification of surviving colonies of CT26 cells (colorectal cancer cells) irradiated with 0, 2, 4, or 6 Gy X-rays in different treatment groups (PBs, SP, AM, and SP@AMF). Reprinted with permission from Refs. (a–e) [138] 2022 Springer Nature and (f–g) [139] 2022 Springer Nature.

In another study, Zhang et al. developed an innovative oral drug delivery system by loading astaxanthin-encapsulated poly(d,l-lactide-co-glycolide)/polyvinyl alcohol nanoparticles (AMF) onto Spirulina platensis (SP), resulting in the SP@AMF composite system [139]. Notably, SP@AMF demonstrated protective effects against radiation-induced intestinal damage, mitigating epithelial injury, inflammation, and fibrosis, while also maintaining gut microbiota homeostasis (Fig. 2f). Furthermore, SP@AMF significantly reduced the clonogenic survival of CT26 CRC cells exposed to high doses of X-ray radiation compared to other treatment groups (Fig. 2g). This dual functionality highlights the potential of SP@AMF as a therapeutic agent capable of simultaneously shielding healthy tissues from radiation toxicity and preserving the radiosensitivity of cancer cells. The innovative design of SP@AMF showcases a promising approach to improving cancer treatment outcomes while minimizing adverse side effects.

Overall, these findings underscore the promising potential of Np-based systems in enhancing the therapeutic efficacy of radiotherapy while simultaneously addressing the immunosuppressive challenges within the tumor microenvironment. By leveraging Nps to facilitate antigen delivery and activate dendritic cells, these strategies not only improve the immune response but also prolong survival in preclinical models. The integration of Nps with genetically modified bacteria or natural compounds such as Spirulina represents a novel approach for overcoming the limitations of traditional radiotherapy, such as the insufficient activation of immune responses due to the immunosuppressive tumor microenvironment. Continued research into the safety, efficacy, and long-term impact of these Np-based interventions, particularly in clinical settings, will be crucial for translating these innovative concepts into effective cancer therapies.

6.2.3. Immunotherapy synergy with Nps

Current drug delivery systems face challenges such as low delivery efficiency and severe off-target toxicity, which are particularly prominent when precisely modulating gut inflammation and the microbiota during CRC treatment. To address these challenges, recent studies have shown significant potential for Np-based therapeutic strategies. For example, Ma et al. designed the P3C-Asp nanodrug system, which combines an aspirin prodrug with the prebiotic dextran, demonstrating stronger efficacy than free aspirin in alleviating the immunosuppressive tumor microenvironment and enhancing CD8⁺ T-cell activity by improving gut microbiota diversity (Fig. 3a and b) [140].

Fig. 3.

(a) Schematic illustration of the role of oral P3C-Asp in shrinking primary CRC. P3C-Asp was fabricated by conjugating PBAsp (aspirin prodrug) with DEX-N3 (dextran-graft-3-azido-1-propylamine) via a “click reaction.” The resulting P3C-Asp readily self-assembled into stable nanoparticles. Upon oral administration, P3C-Asp accumulated in primary CRC tissues, where it underwent assembly dissociation with SA and the release of prebiotics (dextran) while scavenging tumor-associated ROS. This process alleviated the immunosuppressive tumor microenvironment, which was characterized by a reduction in M2 macrophages, MDSCs, and COX-2, as well as an increase in CD8⁺ T cells. Owing to the synergistic effects of inflammation relief and homeostasis restoration, P3C-Asp effectively inhibited CRC. (b) Relative abundance of the gut microbiome. Phylum- and family-level taxonomies are presented as percentages of total sequences, n = 5. (c) The Amuc_2172 protein, released from Akkermansia muciniphila-derived extracellular vesicles, acetylates histone H3 at Lys14, promoting H3K14 acetylation. This stimulates the secretion of HSP70, which enhances T-cell function and reprograms the tumor microenvironment to inhibit cancer. (d) Images of tumors from allograft tumor models initiated with CT26 cells after treatment with or without Amuc_2172. (e) A schematic illustrating the F. nucleatum-mimicking nanomedicine for the selective elimination of tumor-colonizing bacteria and enhancing immunotherapy in patients with colorectal cancer. (f) The average tumor growth curves of CT-26 tumors in mice treated with different regimens. Reprinted with permission from Refs. (a–b) [140] 2024 ELSEVIER, (c–d) [142] 2023 The British Medical Journal and (e–f) [143] 2024 John Wiley & Sons, Inc.

In addition, the use of biomimetic materials has also shown new therapeutic potential [141]. For example, Jiang et al. developed macrophage membrane-coated Amuc_2172 Nps, which increases the expression of HSP70, promoting the function of CD8⁺ cytotoxic T lymphocytes and thereby increasing immune responses against tumors (Fig. 3c and d) [142]. Chen et al. developed an F. nucleatum-mimicking nanomedicine that targets F. nucleatum to colonize tumors without affecting the gut microbiota, alleviating the F. nucleatum-associated immune-suppressive microenvironment and enhancing the efficacy of immune checkpoint inhibitors (Fig. 3e and f) [143].

In another study, Li et al. developed 6-gingerol-loaded magnetic mesoporous silica Nps, which enhanced the uptake of Nps by CRC cells and induced apoptosis and ferroptosis through oxidative stress and magnetic hyperthermia while increasing the abundance of beneficial bacteria [144,145]. Song et al. used LPS-targeting fusion proteins incorporated into a lipid–protamine–DNA Np system to mitigate the tumor immunosuppressive microenvironment induced by LPS from gram-negative bacteria, increasing the proportions of CD8⁺ and CD4⁺ T cells as well as dendritic cells [146].

These studies demonstrate the tremendous potential of Np-based strategies in enhancing immunotherapy synergy, particularly in terms of tumor microenvironment modulation, gut microbiota regulation, and immune response enhancement [[147], [148], [149]]. As these technologies continue to develop, they may form the foundation for novel clinical treatment strategies for CRC and other cancers influenced by the gut microbiota and immune modulation.

6.2.4. PTT synergy with Nps

PTT is a non-invasive treatment method that employs photothermal agents (PTAs) for light-to-heat energy conversion to eliminate tumor cells effectively when exposed to external light sources, such as near-infrared (NIR) light [[150], [151], [152], [153]].

Presently, many studies have focused on exploring PTAs, leading to the emergence of various new materials; however, the primary focus remains on nanoscale materials. For example, Wang et al. developed an oral Np platform (namely, CTPB) consisting of hollow mesoporous copper sulfide and asymmetrically coated TiO₂ as a self-thermophoretic matrix (Fig. 4a and b) [154]. Under NIR laser irradiation, CTPB improved the intestinal mucus penetration efficiency by 2.7-fold and reduced the interception of pathogenic bacteria by 3.5-fold through thermophoretic propulsion. Notably, after 3 weeks of treatment, CRC orthotopic mice presented a 99.4 % tumor suppression rate, indicating the potential of CTPB to effectively penetrate the intestinal mucus and reduce capture by pathogenic bacteria.

Fig. 4.

(a) Schematic illustration of the self-thermophoretic nanoparticles of CTPB (Np platform), which enhance intestinal mucus penetration and reduce pathogenic bacterial interception in CRC through the following cascade processes: 1. Biomimetic chemotactic colonization of CRC intestinal segments; 2. Self-thermophoretic-driven detachment of pathogenic bacteria; 3. Autonomous penetration of intestinal mucus. (b) Tumor weights of the mice in each group (n = 5) after three weeks of treatment were recorded as follows: 1. Saline; 2. Cisplatin (CP); 3. CP@CuS/TiO2; 4. CP@CuS/TiO2@PEG2000-transferrin; 5. CP-loading CTPB; 6. CTPB + near-infrared (NIR) spectroscopy; 7. CCTPB + NIR. (c) Schematic showing the E. coli@Cu2O microbial nanohybrid designed to enhance antitumor immune responses through tumor microenvironment-activatable NIR-II photothermal therapy that induces ferroptosis and cuproptosis. (d) Temperature changes in mice injected with various formulations under laser illumination (1 W cm⁻²). (e) Representative thermal images of mice after injection with various formulations under laser irradiation. Reprinted with permission from Refs. (a–b) [154] 2023 John Wiley & Sons, Inc. and (c–e) [155] 2024 John Wiley & Sons, Inc.

Engineered microbial nanocomposites refer to the microorganisms modified via nanotechnology that exhibit specific functions or properties. Ruan et al. prepared an engineered microbial nanocomposite, namely, E. coli@Cu2O, which could accumulate at the CRC site after intravenous injection, resulting in strong photothermal conversion in the NIR-II biological window (Fig. 4c–e) [155]. Mechanistically, E. coli@Cu2O induced ferroptosis and cuproptosis by inactivating glutathione peroxidase 4 and aggregating dihydrolipoamide S-acetyltransferase, which resulted in the promotion of dendritic cell maturation and T-cell activation, leading to the reversal of immune suppression in CRC. However, the use of engineered microbial nanomaterials such as E. coli@Cu2O can considerably increase treatment costs and may restrict access to the associated treatment modalities in resource-poor regions.

Unlike Ruan et al., Kim et al. employed a one-pot biosynthesis method in which Lactobacillus kimchicus DCY51T was used to load ginsenoside compound K (CK) onto gold Nps (Au Nps), resulting in the formation of DCY51T-AuCKNps [156]. In vitro experiments demonstrated that DCY51T-AuCKNps significantly enhanced apoptosis in CRC cells (HCT116, HT29, AGS) upon laser treatment, outperforming CK alone. This research highlights the innovative synergy between gold Nps and the gut microbiota in CRC treatment, offering enhanced therapeutic efficacy through synergistic photothermal effects and chemotherapy, along with a simplified and cost-effective synthesis process.

These studies suggest that bacteria in PTT can act as therapeutic targets and serve as carriers or enhancers of therapeutic effects. However, further studies are warranted to address the following key challenges: (1) identifying bacteria that can influence the efficacy of PTT; (2) developing cost-effective and highly stable PTAs; (3) addressing potential immune responses or toxicity issues in combination therapy; and (4) resolving the risks associated with laser intensity.

6.2.5. SDT synergy with Nps

SDT is an emerging cancer treatment approach that uses ultrasound to stimulate sonosensitizers to produce ROS, thereby inducing apoptosis in cancer cells and damaging or killing pathogenic bacteria [157,158].

SDT exploits the penetrability of ultrasound through tissues and safety, and it has shown great potential as a complementary approach to traditional cancer therapies [[159], [160], [161], [162]]. For example, Qu et al. designed an ultrasound-triggered Np (namely, Au@BSA-CuPpIX) using bovine serum albumin as a carrier and stabilizer (Fig. 5a–e) [163]. Under ultrasound stimulation, Au@BSA-CuPpIX generated ROS, reduced the abundance of F. nucleatum within CRC cells and the levels of apoptosis inhibitor proteins, thereby increasing ROS-induced apoptosis. Studies on xenotransplantation and in situ CRC models revealed that Au@BSA-CuPpIX exhibited high SDT efficiency, effectively inhibiting tumor growth. This method further presents the advantages of reduced phototoxicity caused by the accumulation of metal porphyrins in the skin; thus, it can facilitate the prevention of severe inflammation and skin damage.

Fig. 5.

(a) Scheme showing the synthesis of Au@BSA-CuPpIX and its mechanism for eliminating pathogenic bacteria in CRC to enhance SDT while reducing skin photosensitivity. (b) Experimental design and treatment schedule. (c) Photographs of mouse tumors. (d) Tumor growth volume curves after various treatments (PBS, Au@BSA-PpIX, Au@BSA-CuPpIX, US, Au@BSA-PpIX + US, and Au@BSA-CuPpIX + US). (e) Average tumor weights across groups. (f) Scheme illustrating the programmable bacteria-based biohybrid, where engineered bacteria are loaded with nanosonosensitizers to enhance SDT and immune responses against tumors. (g) In vivo bioluminescence imaging showing treatment effects on tumor progression. Reprinted with permission from Refs. (a–e) [163] 2023 American Chemical Society and (f–g) [164] 2024 John Wiley & Sons, Inc.

In another study, genetically engineered E. coli BL21 was designed to overexpress catalase (E. coli-pE) and carry electrostatically adsorbed nanosensitizers (PCN NPs) (Fig. 5f and g) [164]. The innovation of this approach lies not only in its ability to enhance tumor-targeted therapy but also in its ability to induce an immune memory response, effectively preventing tumor recurrence. Furthermore, the study demonstrated a distant effect (abscopal effect), where it inhibited the growth of untreated tumor sites, thus preventing metastasis. This innovative strategy combines tumor immune activation and targeted therapy, offering new directions for cancer immunotherapy.

Overall, SDT presents the combined benefits of nanobiotechnology, exhibiting excellent therapeutic efficiency and biosafety, along with significant potential for treating cancer and bacterial infections.

6.2.6. Multimodal therapy synergy with Nps

CRC cells can exhibit high complexity and heterogeneity, resulting in different tumor cells responding differently to the same treatment method. These cases highlight the need to simultaneously target multiple aspects of the tumor by combining multiple treatment approaches.

For example, Fan et al. developed an oral microrobot composed of tetragonal piezoceramic material barium titanate (BTO) nanocubes coated with an S. aureus membrane (Fig. 6a–c) [165]. Under ultrasonic stimulation, these microrobots trigger various chemical reactions at CRC sites, resulting in the production of therapeutic agents. Additionally, the catalytic activity of BTO enhanced dendritic cell maturation and promoted the polarization of M1 macrophages. Furthermore, this treatment has been shown to restore the abundance of the gut microbiota. Overall, the use of microrobots for precise delivery allows chemical reactions to be triggered at specific locations, minimizing damage to surrounding healthy tissues.

Fig. 6.

(a) Schematic diagram of a biological hybrid robot based on VA-SAM@BTO for oral targeted therapy of colorectal cancer. Schematic showing the preparation of VA-SAM@BTO. Oral administration of VA-SAM@BTO induces catalytic and immunotherapeutic effects in orthotopic CRC model mice. (b) Percent survival of orthotopic CRC model mice after treatment in different groups. The data represent the survival rates of mice subjected to various therapies, providing insights into the long-term therapeutic effects and efficacy of VA-SAM@BTO combined with ultrasound (US) irradiation. Treatment groups: I: PBS, II: SAM@BTO, III: VA, IV: US, V: SAM@BTO + US, VI: VA-SAM@BTO + US. (c) Microbial α diversity at the amplicon sequence variant level was evaluated using the Shannon index. (d) Schematic illustrating the synthesis and mechanism of CBS-CS@Lipo/Oxp/MTZ for orthotopic colorectal cancer therapy. (e) In vivo combination therapy in CT26 tumor-bearing mice. Tumor weights of CRC tissues after two weeks of treatment. Reprinted with permission from Refs. (a–c) [165] 2024 American Association for the Advancement of Science and (d–e) [166] 2024 Elsevier.

Liposomes are composed mainly of biological membrane components, offering good biocompatibility and minimizing immune responses. Niu et al. synthesized CS@Lipo/Oxp/MTZ liposomes by modifying oxaliplatin and MTZ liposomes with chitosan and combining them with C. butyricum (CBS) spores (Fig. 6d and e) [166]. CBS-CS@Lipo/Oxp/MTZ liposomes could effectively deliver chemotherapy drugs to the colon and promote CD8⁺ T-cell-mediated immune responses via SCFAs. Overall, CBS-CS@Lipo/Oxp/MTZ targeted and accumulated in the colon, inhibiting tumor growth while reducing the systemic toxicity of the drugs.

Under specific conditions, molecules order themselves via noncovalent bonds; this process is referred to as self-assembly. Zhu et al. synthesized an amphiphilic inulin-deoxycholic acid conjugate, UDIN, which could self-assemble in water to encapsulate regorafenib into prebiotic Nps, forming UIRN [167]. Oral administration of UIRN increased beneficial SCFA levels in the gut and promoted dendritic cell maturation while reducing the proportion of Tregs. This enhanced the antitumor immune response of the nanosystem, resulting in a 95.4 % tumor suppression rate. Overall, this method exploits the advantages of self-assembly, and Nps self-organize into specific structures without external intervention, simplifying the manufacturing process and reducing production costs.

Because the diversity of the gut microbiome may limit the effectiveness of any single intervention aimed at improving gut health or treating related diseases, a synergistic approach that utilizes multiple strategies is essential. Nanotechnology-based approaches can integrate various therapeutic methods, thereby creating a multifunctional platform for both the diagnosis and treatment of any disease while enhancing the effects of each component. Future studies need to focus on bridging the gap between laboratory studies and clinical applications, ensuring effective combinations of nanotechnology and gut microbiome interventions to improve overall health outcomes.

7. Conclusion and outlook

The combination of nanotechnology and gut microbiota regulation holds great promise for advancing CRC treatment. Although this field is still in its early stages, it presents significant opportunities for precision treatment.

First, the gut microbiota plays a critical role in CRC progression and the response to therapies. Microbial metabolites, such as SCFAs, have anti-inflammatory effects, whereas secondary bile acids can promote tumorigenesis. Targeting these metabolites or regulating microbial populations could offer innovative strategies to manipulate the tumor microenvironment, influence immune responses, and reduce cancer cell proliferation, providing non-invasive therapeutic options that complement traditional treatments.

Second, nanomaterials, such as liposomes, polymeric nanoparticles, and gold Nps, have inherent properties that make them ideal for enhancing CRC therapies. These materials can be engineered to interact specifically with the gut microbiota and its metabolites, improving the bioavailability and efficacy of chemotherapeutic agents. By ensuring targeted drug delivery and facilitating localized release within the tumor microenvironment, nanomaterials can help overcome challenges such as drug resistance and poor solubility, thus enhancing the effectiveness of existing therapies.

Third, nanomaterials add significant value to current CRC treatments by improving drug delivery precision, promoting immune responses, and enabling the simultaneous administration of multiple agents. For example, Nps functionalized with immune checkpoint inhibitors can target tumor cells and stimulate immune responses, whereas nanomaterial-based radiosensitizers can increase the efficacy of radiation therapy. As nanomaterials evolve, they have the potential to complement or even surpass traditional therapies, offering personalized treatment options tailored to both the tumor and the patient's microbiome.

A promising example of this integration is the work of Academician Jianlin Shi's team, which introduced a new treatment for low-stage CRC patients using the LDH/EDTA nanoplate system. This system successfully dissociated tumor cells in laboratory models by releasing EDTA into the acidic tumor microenvironment, disrupting E-cadherin connections between cells, and enabling their expulsion through feces. In preclinical trials, this approach led to tumor shrinkage and alleviated bowel obstruction in low-stage CRC patients. Although this study did not directly address gut microbiota regulation, it offers valuable insights into how nanotechnology can be further enhanced by incorporating microbiome modulation, potentially improving treatment outcomes for CRC in the future.

In conclusion, combining nanotechnology with gut microbiota regulation holds great promise for CRC treatment. This approach could enable precise targeting of harmful bacteria, modulate the gut microenvironment, promote beneficial probiotics, and enhance immune responses, paving the way for more personalized and effective therapies. While progress has been made, much work remains to be done. Continued interdisciplinary collaboration and innovation will be essential to realizing the full potential of nanotechnology and microbiome modulation, ultimately improving CRC treatment outcomes.

CRediT authorship contribution statement

Qinghe Han: Writing – review & editing, Funding acquisition, Conceptualization. Jie Li: Writing – original draft, Conceptualization. Zhuo Li: Writing – review & editing, Conceptualization. Reyida Aishajiang: Investigation, Formal analysis. Duo Yu: Writing – review & editing, Funding acquisition, Formal analysis, Conceptualization.

Funding

This work was supported by the financial aid from the National Natural Science Foundation of China (82404194) and the 2023 “Bethune Plan Project” of Jilin Province (No. 2023B09) for D. Yu.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.