LPS is an essential component of the cell wall in Gram-negative bacteria (GNB). An examination conducted in 2020 on bacterial populations present in pan-cancerous tumors demonstrated that LPS has a crucial role in promoting inflammation in different types of cancer (34). Using polymyxin B to clear GNB from the intestines or using TAK-242 to block LPS activation of Toll-like receptor 4 (TLR4) can both alleviate the immunosuppressive microenvironment of colorectal cancer and facilitate T-cell infiltration into the tumor (35). Thus, LPS has the potential to trigger long-term inflammation, leading to the advancement of cancer. Long-term inflammation caused by LPS results in T-cell exhaustion, an increase in the expression of the PD-1/programmed death-ligand 1 (PD-L1) axis and the buildup of myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs). This leads to the development of a microenvironment that suppresses the immune system and promotes the proliferation and advancement of tumors (36).

Furthermore, LPS promotes the secretion of various cytokines that aid in tumor cell proliferation and metastasis. Activation of the NF-κB signaling pathway by LPS results in the secretion of IL-6, which activates STAT3 in an autocrine manner., promoting prostate cancer progression in mice (37). LPS also stimulates TLR4 to induce the expression of vascular endothelial growth factor (VEGF) C, which promotes tumor cell metastasis and lymphangiogenesis (38). Macrophages, which are integral components of the TME, exert a marked influence on the evasion of the immune system by tumors. Research has shown that stimulation of the TLR4 signaling pathway by LPS leads to an increase in the production of CCL5 by macrophages. Consequently, this obstructs the ability of T cells to destroy colorectal cancer cells and enables the cancer cells to avoid detection by the immune system. In addition, CCL5 regulates the process of removing ubiquitin molecules from PD-L1 and maintaining its stability (39).

SCFAs have been shown to possess antitumor capabilities (40). However, SCFAs may also promote tumor progression and metastasis via immunomodulatory effects, signal transduction regulation and inflammation modulation, all of which can remodel the TME. In the intestine of gnotobiotic mice colonized with the F. nucleatum Fn7-1 strain, Fn7-1 metabolism produces SCFAs that combine with GPR43 to increase colonic T helper 17 (Th17) cell frequency and promote IL-17A and IL-17F expression, which leads to chronic inflammation and increases the risk of colorectal cancer (41). Furthermore, SCFAs enhance the activation of the MAPK and PI3K signaling pathways in the neighboring prostate region by upregulating the production of insulin-like growth factor-1, hence facilitating the proliferation and multiplication of prostate cancer cells (42). The most prevalent SCFAs in the human body include acetic acid, propionic acid and butyric acid. Although the majority of studies concentrate on these three molecules, it is crucial not to disregard the potential influence of formate on the advancement of tumors. Genetic interference has been found to effectively decrease tumor cell invasion by inhibiting the synthesis of formate (43). Additionally, the accumulation of formate has been shown to increase the expression of the formyl peptide receptor, which enhances the invasive ability of cancer cells (44). Formate also plays a role in promoting colorectal cancer tumor invasion and enhancing the population of tumor stem cells through the activation of AHR signaling (45). Formate treatment induces a rise in Th17 levels (45), which are inflammatory regulatory cells that promote chronic inflammation. Therefore, formate may enhance the progression of colorectal cancer by promoting chronic inflammation.

When considering the relationship between secondary bile acids and different tumor types, including colorectal cancer and liver cancer, it is important to investigate therapeutic approaches involving secondary bile acids to advance the prevention and treatment of these tumors (46,47). The gut microbiome produces bile salt hydrolases (BSHs) to deoxygenate primary bile acids into secondary bile acids, primarily deoxycholic acid (DCA) and lithocholic acid (LCA). These compounds can contribute to tumor progression by reshaping the TME (48). Unconjugated DCA and LCA buildup in the intestine and stimulate GPRs in regions where BSH is highly expressed. As a result of this activation, there is an elevated production of CCL28, which is regulated by β-catenin, in the tumor. Activation of the β-catenin/CCL28 axis leads to increased amounts of Tregs in the TME, which in turn creates an immunosuppressive microenvironment (49). In Apc^min/+ mice (a commonly used mouse model for colon cancer), the accumulation of DCA activates VEGF receptor 2, promoting vasculogenic mimicry and epithelial-mesenchymal transition (EMT) (50).

Additionally, secondary bile acids also induce the expression of cytokines that influence tumor progression. LCA activates Erk1/2 MAPK and inhibits STAT3 phosphorylation to promote IL-8 expression in colorectal cancer cells (51). LCA induces the progression and metastasis of colorectal cancer via promoting IL-8 expression (52). Furthermore, DCA activates CD4⁺ T cells and induces IL-10 secretion to induce Treg cell differentiation, causing an immunosuppressive microenvironment (53). Furthermore, the release of IL-10 stimulates the polarization of tumor-associated macrophages (TAMs) towards the M2 phenotype, hence facilitating the proliferation and metastasis of colorectal cancer (54). The two derivatives of LCA are 3-oxo LCA and isoallo LCA. 3-oxo LCA hinders Th17 cell differentiation by binding to retinoid-related orphan receptor-γt, whereas isoallo LCA encourages the formation of Tregs by producing mitochondrial reactive oxygen species (ROS) (55).

TMAO has been linked to a range of diseases, such as cardiovascular disease and liver, pancreatic and colorectal cancer (64-67). While the exact method by which TMAO contributes to tumor progression remains unclear, it is known that elevated levels of amines, as a metabolite, lead to oxidative stress (68). Oxidative stress can lead to an increase in intracellular DNA mutations and genomic instability, which in turn promotes cancer incidence and tumor progression. Recently, several investigations have established a connection between TMAO and different cancer types. For instance, TMAO produced by the gut microbiota during the breakdown of food has been found to stimulate macrophages, enhance the response of T cells and decrease the number of tumor cells in pancreatic ductal adenocarcinoma (PDAC) by increasing the production of IFN-Ⅰ (66). Due to the connection between the pancreatic duct and the duodenum, microbial metabolites from the intestines can reach the pancreatic duct and surrounding tissues. This could potentially have a role in the development and treatment of PDAC. Immunotherapy has had little effectiveness in treating triple-negative breast cancer (TNBC) (69). A study that conducted a multi-omics analysis of patients with TNBC discovered that TMAO was linked to a cohort that received immunotherapy and exhibited an improved immune response. TMAO also triggers tumor cell pyroptosis by activating the endoplasmic reticulum stress kinase, PERK, while also boosting CD8⁺ T cell-driven antitumor immunity in the host (70). Meanwhile, a study revealed a correlation between elevated blood levels of TMAO and colorectal cancer (71). However, the precise pathophysiology remains uncertain, but it is likely associated with the stimulation of persistent gastrointestinal inflammation. Hence, it is crucial to regulate the concentration of TMAO when employing TAMO for research or cancer therapy. Several strategies, including the suppression of crucial metabolic enzymes, dietary regulation and the use of antibiotics, have been suggested to manipulate the gut microbiome to produce TMAO (72-74).

SCFAs are important metabolites generated by the intestinal microbiota during the digestive process, mainly consisting of acetic acid, propionic acid and butyric acid. SCFAs promote the metabolism of intestinal epithelial cells and enhance the intestinal barrier function (75). Certain research suggests that increasing the levels of butyric acid in the stomach through pectin supplementation can promote the infiltration of T cells in the TME and improve the efficacy of ICIs in treating colorectal cancer (76). SCFAs have shown strong antitumor effects in multiple types of cancer, particularly colorectal cancer (77). The ability to act through different routes is made possible by the diversity of SCFAs and their derivatives. The antitumor actions of SCFAs are achieved through their binding to many GPRs, such as GPR41, GPR43 and GPR109a. Through the gut-hepatic axis, acetic acid binds to GPR43 in the liver, limiting IL-6 release and obstructing the JAK1/STAT3 signaling pathway to stop liver cancer from progressing (78). Moreover, a study found that the combination of propionic acid and GPR43 also inhibited the proliferation of liver cancer cells (79). In addition, propionic acid can inhibit the Hippo/Yap and MAPK signaling pathways by binding to GPR41 and GPR43, ultimately inhibiting the metastasis of breast cancer cells (80). Butyric acid acts on GPR109a in colon macrophages and dendritic cells, inducing the differentiation of Tregs and CD4⁺ T cells and inhibiting the development of colitis and colorectal cancer (81). Moreover, a study has shown that GPR43 deletion accelerates the development of colorectal cancer by exhausting CD8⁺ T cells and hyperactivating dendritic cells (82). In summary, SCFAs stimulate the proliferation and specialization of immune cells by attaching to GPRs. SCFAs also hinder the advancement and metastasis of tumors by attaching to GPRs on tumor cells. Additionally, SCFAs suppress long-term inflammation by activating GPR signaling, which prompts the development of Tregs.

Tumor progression has been closely linked to epigenetic reprogramming. Tumor cells frequently exhibit elevated levels of histone deacetylases (HDACs), which lead to increased histone deacetylation. This alteration affects the control of gene expression and facilitates the advancement and metastasis of tumors (83). Additionally, HDACs suppress both the immune response and inflammatory events in immune cells (84). HDAC inhibitors enhance the antitumor therapeutic efficacy of ICIs by modifying the immunosuppressive function of TAMs and impeding the ingress of MDSCs into tumors, thereby remodeling the immunosuppressive microenvironment (85). In a recent study, inhibition of HDAC by butyric acid caused an elevation in CD25 expression, INF-γ and tumor necrosis factor-α and greatly enhanced the antitumor function of cytotoxic T lymphocytes in pancreatic cancer and melanoma models (86). Propionic acid also inhibits IL-17 and IL-22 secretion by γδ T-cells via inhibition of HDAC, which could inhibit colitis-associated colorectal cancer development (87). A study has also demonstrated that butyric acid inhibits the growth and promotes the apoptosis of P815 mouse breast cancer cells by inhibiting HDAC, in addition to the effects of SCFAs on immune cells (88).

The presence of Trp metabolites in organisms is influenced, either directly or indirectly, by the gastrointestinal microbiome. Metabolites and enzymes associated with Trp have been identified as prospective targets for pharmaceutical advancements in the treatment of various disorders, such as psoriasis, atopic dermatitis and hepatic fibrosis (89-91). Trp metabolites have been reported to act selectively on AHR (92). Indole-3-aldehyde (I3A) activates the AHR of CD8⁺ T cells in the melanoma TME, promoting CD8⁺ T-cell differentiation and IFN-γ production (93). This enhances ICI-induced antitumor immunity. However, in PDAC, Trp metabolites bind to the AHR of TAMs and reduce the frequency of intratumoral CD8⁺ T cells, creating an immunosuppressive microenvironment (94). This implies that the effects of Trp metabolite-mediated AHR activation may vary among tumor types or be influenced by heterogeneity in the TME.

Epigenetic modifications are of the utmost importance in the regulation of gene expression and are critical in the development, progression and treatment of numerous types of cancer. An examination of the effects that microbial metabolites have on epigenetic modifications will establish a solid theoretical basis for the advancement of novel approaches in the field of tumor immunotherapy. Tumor immunity can be regulated by Trp metabolites via epigenetic modifications. Indole-3-lactic acid alters chromatin accessibility to initiate the antitumor activity of CD8⁺ T cells and directly enhances the secretion of IFN-γ and granzyme B by tumor-infiltrating CD8⁺ T cells to kill tumor cells, and also promotes IL-12a secretion by binding to dendritic cells (95). IL-12a in turn further induces the proliferation of CD8⁺ T cells and promotes tumor immunity (96).

A modest elevation in oxidative stress and suppression of EMT can greatly impede the growth and metastasis of tumor cells. Indoxylsulfate (IS) suppresses the activity of nuclear factor erythroid 2-related factor 2 and stimulates the production of inducible nitric oxide synthase, leading to the occurrence of oxidative and nitrosative stress. Additionally, IS hinders EMT, resulting in a notable decrease in the metastasis of breast cancer cells to adjacent tissues (97). The metabolism of Trp is not only regulated by the gut microbiome but also by tumor cells (98). Tumor cells often exhibit heightened activity in Trp metabolic pathways, such as indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase, which facilitate the conversion of Trp into kynurenine. This process increases the number of Tregs in the TME and boosts the expression of PD-1 on CD8⁺ T cells, helping tumor cells avoid detection by the immune system (99). An extensive examination of the regulatory mechanisms of Trp metabolism will provide new targets and strategies for tumor therapy, given the intricate nature of the Trp metabolic pathway.

Chemotherapy is a popular cancer treatment that can halt cancer from progressing via several methods, but drug resistance and adverse effects remain a problem. As such, reducing the adverse effects and medication resistance linked to chemotherapy is a major task. As demonstrated in Fig. 3, the combined use of microbial metabolites and various chemotherapy drugs can regulate the TME through multiple mechanisms, promoting tumor cell apoptosis.

UroA is a product of gut microbial metabolism. UroA has been reported to downregulate the expression of breast cancer resistance protein (a drug efflux transporter protein), which would contribute to the sustained efficacy of mitoxantrone in cancer cells (107). Meanwhile, in a recent study, UroA and its structural analog, UAS03, were combined with the chemotherapeutic drug, 5-fluorouracil (5FU), and investigated for their anticancer effects. The results showed that UroA/UAS03 re-sensitized 5FU-resistant colorectal cancer to chemotherapy by modulating the FOXO3/FOXM1 axis (108). These findings indicate that the use of UroA in combination with low-dose chemotherapeutic drugs can provide similar antitumor effects as high-dose chemotherapeutic drugs, potentially minimizing the negative side effects associated with chemotherapy. Furthermore, UroA can suppress the growth of gastric cancer cells, hinder their capacity to invade and metastasize and stimulate their programmed cell death (109). This indicates that UroA may emerge as a novel adjuvant cancer therapy medication. However, further investigations are necessary to confirm the safety and effectiveness of UroA, as well as to identify the ideal therapeutic concentration and clinical protocol.

Butyrate is a SCFA produced by gut microbiota, typically generated as a byproduct during the fermentation of dietary fibers in the colon. Butyrate has demonstrated potential efficacy as an adjuvant treatment drug in chemotherapy by modulating cellular metabolism, reducing oxidative stress and preserving liver function through various mechanisms (110-112). The coadministration of butyrate and irinotecan induces programmed cell death in colorectal cancer cells by modulating the BAX/BCL-2 ratio, while also augmenting the responsiveness of cancer cells to irinotecan (113). Meanwhile, butyrate and oxaliplatin can synergistically inhibit the proliferation, invasion and metastasis of colorectal cancer cells and promote the apoptosis of these cells (114). Furthermore, the intestinal bacterium, F. nucleatum, promotes resistance to chemotherapy in colorectal cancer by modulating autophagy (115). Butyrate downregulates the expression of adhesion-associated outer membrane proteins and inhibits F. nucleatum growth, enrichment and adhesion in colorectal tissues, thereby reducing F. nucleatum colonization and mitigating F. nucleatum-induced chemoresistance (116). Gut microbiota can produce indole-3-acetic acid (3-IAA) by metabolizing tryptophan from the diet. Neutrophil-derived myeloperoxidase (MPO) is essential for the joint action of 3-IAA and chemotherapy. MPO catalyzes the oxidation of 3-IAA. When combined with chemotherapeutic drugs, the oxidative products of 3-IAA can reduce the production of enzymes that degrade ROS. This leads to an increase in ROS levels and a decrease in tumor cell autophagy, ultimately inhibiting the development of tumor cells (117). Therefore, appropriate intervention in the nutrition of patients with cancer during treatment may have a positive impact on chemotherapy.

Microbial metabolites can both increase and impair the efficacy of chemotherapy. A study has demonstrated that Lactobacillus residing in tumors can lead to resistance to radiotherapy and chemotherapy. This resistance is achieved by modifying the metabolism of the tumor and the signaling pathways related to lactate (118). Furthermore, the metabolites produced by microbes as they metabolize chemotherapy drugs can lead to undesirable responses. A randomized controlled trial conducted in 2021 revealed that bacterial glucuronidase can break down the inactive metabolite of irinotecan, SN-38G, into its active form, SN-38. This process subsequently leads to damage in the intestinal mucosa (119).

ICIs have been used with great success in cancer treatment, but many adverse effects (such as gastrointestinal reactions) that occur during treatment have limited their wider application (120). Nevertheless, the integration of ICIs with microbial metabolites could potentially resolve this issue. I3A was found to have a protective impact on enteritis generated by ICIs in mice. Additionally, I3A protected enteritis induced by ICIs in mice that received FMT from I3A-treated mice via modifying the composition and function of the gut microbiome (121). The TME consists of a complex network of various cytokines, immune cells and cancer cells. Fig. 4 illustrates that UroA, SCFAs, LPS and inosine act on components of the TME to activate the immune system and promote the transition from a 'cold' TME (TME lacking immune cell infiltration and activity) to a 'hot' TME, thereby enhancing the efficacy of immunotherapy.

At present, a considerable proportion of individuals with cancer do not demonstrate a strong reaction to ICI therapy due to cancer cells employing various strategies to avoid detection by the immune system (122-125). Therefore, activation of immune cells within the microenvironment has become a crucial research avenue to enhance the efficacy of ICIs. In a recent study, UroA alone led to a notable decrease in M2-like macrophages and an elevation in CD4⁺ and CD8⁺ T-cell infiltration within the TME, as well as downregulation of PD-1 expression and a rise in the overall survival rate of mice with cancer (126). Moreover, the reciprocal impacts of microbial metabolites can potentially lead to a reduction in the effectiveness of ICIs. Despite their anticancer properties, SCFAs hinder the effectiveness of anti-CTLA-4 antibodies in treating melanoma. This is due to their ability to decrease the presence of tumor-specific and memory T cells, while simultaneously boosting the proportion of Tregs (127).

Prior research has demonstrated that cytokines exert influence on the proliferation, progression and therapeutic interventions of cancer by regulating the function of both immune cells and tumor cells. Tumors with excessively active cytokine signaling pathways frequently experience increased tumor proliferation and invasion, significantly reducing the effectiveness of ICIs (128-130). Targeted binding against cytokines and their receptors may also leading to new strategies for tumor therapy (131-133). Microbial metabolites may indirectly influence the efficacy of ICIs by modulating cytokines. LPS upregulates IL-6 levels, which leads to tumor cell proliferation by activating JAK/STAT signaling (134). Furthermore, elevated levels of IL-6 in the TME not only diminishes the effectiveness of ICIs but also intensifies the negative side effects caused by ICI treatment (135). In addition, IL-12 can reprogram the TME, directly bind to the IL-12 receptor on CD8⁺ T cells and activate these cells, while promoting the infiltration of CD4⁺ T cells and downregulating the number of Tregs, which ultimately significantly improves the efficacy of ICIs (136,137). A variety of microbial metabolites modulate IL-12 secretion. Research has shown that LPS induces IL-12 secretion, which is inhibited by butyrate (138).

In summary, microbial metabolites can modify the immunological status of the TME, converting it from a 'cold' to a 'hot' environment, and control the relative amounts of different cytokines, all of which will increase the effectiveness of ICIs. Microbial metabolites also have the potential to influence cytokine receptors in addition to immune cells and cytokines. It has been found that inosine binds to A_2A in the presence of exogenous IFN-γ to promote the differentiation of Th1 cells and significantly increase the expression of the IL-12 receptor and IFN-γ in Th1 cells, which may effectively improve the efficacy of ICIs (139).