Numerous virulence factors are produced by Helicobacter pylori, including the oncoproteins cytotoxin-associated gene A (CagA) and vacuolating cytotoxin A (VacA), which have been identified as the primary virulence factors involved in the pathogenesis of gastric cancer (35). Additionally, Helicobacter pylori causes gastric atrophy and gastric acid shortage, which promote excessive microbial growth in the stomach, resulting in more nitrogen derivatives in the diet that can be converted into carcinogens (36).

It was reported that Fusobacterium are abundant in patients with colorectal cancer (CRC) (37). Surface adhesion protein (FadA), the primary factor controlling the adherence and invasion of Fusobacterium, can bind to β-catenin and lead to its activation, which in turn triggers inflammation and tumor growth (38). Additionally, many bacterial metabolites may also cause genomic instability, leading to tumorigenesis. For instance, Enterobacteriaceae produce colibactin, which stimulates the overexpansion of intestinal epithelial cells and aids in the development of CRC by generating DNA damage, mutation, and genomic instability (39). Enterococcus faecalis can induce double-stranded DNA breaks and promote the development of CRC in mice by producing superoxide free radicals (40). Similarly, secondary bile acids produced by gut bacteria can affect the mitotic process of intestinal epithelial cells, induce DNA damage, and increase the risk of CRC (41). The metabolism of gut bacteria also produces other cancer-promoting substances such as glucuronidase that can transform the precarcinogens in food or drugs into carcinogens (42); or produce carcinogenic chemicals such as N-nitroso compounds (NOCs) (43) and hydrogen sulfide (H₂S) (44).

A growing body of literature points to an increased abundance of lipopolysaccharide (LPS)- producing bacteria (Neisseria, Enterobacteriaceae, and Vermicella) in patients with hepatocellular carcinoma (HCC) (45–47). It is reported that LPS can activate the toll-like receptor (TLR) 4 and NF-κB pathways and trigger the production of cytokines such as TNF-α and IL-6 that promote cancer progression (48). Secondary bile acids can make the intestinal mucosa more permeable, allowing intestinal pathogens to translocate to the liver and cause HCC (49, 50). Through the portal venous system, the liver is connected to intestinal bacterial components and their metabolites, which may cause inflammatory alterations, hepatotoxicity, and finally, HCC. For instance, alterations in the gut microbiota raised the levels of hepatobiliary acid, which caused hepatocarcinogenesis in an obesity-induced human liver cancer xenograft mice model (51).

Fusobacteria is one of the most common microorganisms found in the esophagus (52). Matrix metalloproteinases (MMPs) can be secreted by intestinal epithelial cells when Fusobacterium is present (53). Activated MMPs can encourage the growth of tumor cells by increasing the breakdown of the esophageal extracellular matrix, stimulating tumor angiogenesis, and controlling cell adhesion and motility (54). Fusobacterium simultaneously triggers the IL-6/p-STAT3/c-MYC signaling pathway and encourages M2-type differentiation of macrophages through a TLR4-dependent mechanism, promoting tumor growth (55). Therefore, Fusobacterium is a potential target for the treatment of esophageal cancer.

Bifidobacterium augmented anti-CTLA-4 checkpoint blockade in mouse models (58), whereas antibiotic-treated or sterile mice showed no response to this blockade. More abundant bacterial species such as Enterococcus faecium enhance anti-PD-L1 treatment in mouse models (18). Recent research has shown that the use of probiotics as adjunctive therapy for Helicobacter pylori infection can effectively inhibit the progression of gastric cancer (59, 60). Probiotics administered to patients with gastric cancer following total gastrectomy were found to improve immune function and reduce inflammation (61).

Eubacterium, Lactobacillus, and Streptococcus can release multiple metabolites that influence the immune system, such as SCFAs, which are positively connected to the anti-PD-1/PD-L1 response (62). SCFAs alter inflammation, including the activation, proliferation, and differentiation of anti-inflammatory T regulatory (Treg) cells or pro-inflammatory Th1 and Th17 cells, and affect the polarization of pro-inflammatory M1 and anti-inflammatory M2 macrophages (63). Furthermore, SCFAs cause tumor growth by activating mitogen-activated protein kinase and PI3K (phosphatidylinositol-3-kinases) signaling by increasing somatomedin C (IGF-1) levels. In addition, virulence factors are important components of gut flora that affect CRC immunotherapy (64). TcdB produced by Clostridium difficile inhibits TH and memory B-cell differentiation (65). Therefore, modulating the Clostridium difficile count is beneficial for immune recovery. Interestingly, in some microflora such as Bacteroides fragilis, the cross-reaction between bacterial antigen and tumor neoantigen can activate antitumor T cells (66). In contrast, the enterotoxins BFT (Bacteroides fragilis enterotoxin) and IL-17 produced by Bacteroides fragilis induce the differentiation of monocytic myeloid-derived suppressor cells into intestinal epithelial cells, which can selectively upregulate arginase 1 (Arg1) and type 2 NO synthase (NOS2) to produce NO, inhibit T cell proliferation, and promote CRC generation (67).

The function of dendritic cells can be enhanced by oral Bifidobacterium administration, which increases CD8⁺ T cell accumulation in HCC tissue (68). Similarly, SagA, an enzyme expressed by Enterococcus faecium, enhances the effect of immunotherapy in HCC (69). Microbial metabolites, including amino acid derivatives and secondary BAs, are also involved in immunoregulation. Lactobacillus is able to convert tryptophan into indole and its derivatives, which are major aromatic hydrocarbon receptors (AHRs) that play a pivotal role in intestinal immunobarrier function (70). In addition, the bacterial metabolites lactic acid and pyruvate, enhance the immune response by inducing GPR31-mediated dendritic cell differentiation (71).

Immune checkpoint inhibitor (ICI) therapies have been evaluated for their effect on specific microbes in esophageal cancer, including cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death 1 (PD-1)/PD-1 ligand (PD-L1) inhibitors (72). Vetizou et al. found that anti-CTLA-4 therapy was effective only when B. fragilis and/or B. thetaiotaomicron and Burkholderiales populations were present and are therapeutic when T cells are specific for B. fragilis and B. thetaiotamicron (58). In addition, the reintroduction of B. fragilis cells and/or polysaccharides or adoptive transfer of B. fragilis-specific T cells restored therapeutic efficacy and reduced immune-mediated colitis through activation of Th1 cells with cross-reactivity to bacterial antigens and tumour neoantigens. These results indicate that reconstruction of intestinal flora is beneficial for esophageal cancer treatment through immune regulation.

According to their natural properties, intestinal microbiota can be divided into nine phyla, of which Firmicutes (64%), Bacteroides (28%), Proteus (8%), and Actinomycetes (3%) account for 98% of the flora (83). Dietary compounds indirectly affect immune responses by regulating the microbiota composition. For instance, insoluble dietary fiber extracted from barley leaves increased Parasutterella and Alistipes abundance to a certain extent and decreased Akkermansia abundance, as well as markedly relieved acute colitis symptoms and decreased levels of inflammatory factors such as IL-6, TNF-α, and IL-1β in colitis mice. In addition, short-term rice bran consumption reduces the Firmicutes: Bacteroidetes ratio in humans but may increase it in the long run (84). Therefore, there may be a time difference in the impact of dietary compounds in reducing CRC risk (85). Additionally, genetically modified mice with the same epigenome but two different gut microbes were fed four equal-calorie diets with the same dietary fiber composition. Integrated transcriptomic and metabolomic analyses showed that the metabolic results of the final amino acids and lipids in each group were different, indicating that gut microbiota structures also affected metabolism of dietary compounds (86).

The metabolism of dietary compounds by gut microbiota is mainly catabolic, which reduces the molecular weight of drugs, weakens polarity, and increases fat solubility and the efficacy. According to research on ginseng metabolism, microbiota can metabolize the ginsenosides Rb1 and RD into compound K, which has a potent anticancer effect (87). The oral bioavailability of food components can also be improved by gut flora. Curcumin can prevent tumor growth in vivo by increasing the chemical sensitivity of HCC cells to 5-FU via blocking the G2/M phase of the cell cycle, and reducing the activation of downstream protein kinases in the PI3K/AKT/mTOR signaling pathway (88). Glycyrrhizin is a flavonoid food component that can be metabolized by gut flora to yield three potential metabolites: pantothenic acid (M3), resorcinol (M4), and M5 to achieve antitumor activity (89, 90).

Pectin, a soluble fiber extracted from plant cell walls, can be used to block the proliferation cycle of tumor cells, thereby inhibiting CRC (97, 119). The gut flora of CRC subjects in tumor-bearing mice was dosed orally with pectin, which significantly improved the anti-PD-1 monoclonal antibody effect (82). Moreover, mice treated with gut flora from patients resistant to anti-PD-1 antibody showed a similar effect. Inulin is a plant-derived polysaccharide fiber present in Jerusalem artichoke and chicory tubers. It can be adsorbed into intestinal mucosa folds through the thickening effect, which is conducive to its interaction with gut flora. Akkermansia, Lactobacillus, and Rosebacilli are the main SCFA-producing bacteria, and their abundance increased significantly after inulin gel administration. Notably, high-dose inulin intake for 14 days (approximately 450 mg per day) slows tumor growth in mice before tumor modeling but does not work in synergy with anti-PD-1 (98). This may be related to a reduction in gut flora diversity caused by a single diet. Approximately 90% CRC tumors exhibit abnormal Wnt/β-catenin pathway activation (120). Albuca Bracteate Polysaccharides (ABP) treatment inhibits β-catenin expression. Furthermore, ABP and 5-FU (first-line drugs for CRC treatment) synergistically affected CRC tumor cell growth, migration, and invasion, and the antitumor effect of their combination was better than that of 5-FU or ABP alone. Fucoidan significantly increased Lactobacillus levels and decreased Fusobacterium levels in CRC mouse models, which alleviated macrophage and T cell infiltration, and reduced colonic inflammation (100). Moreover, fucoidan also inhibited the Wnt/β-catenin pathway (121). Dietary fibers with similar effects also contain cellulose and apple and jujube polysaccharides, which reduce the abundance of differential bacteria correlated with IL-6, IL-1β, and TNF-α concentrations to inhibit colon tumor formation (101, 102, 122).

Corylin is one kind of flavonoids isolated from the fruit and seed of Psoralea corylifolia (104). Corylin improved intestinal homeostasis to further reduce tumor cell-induced inflammation by inhibiting the TLR4/p38/AP-1 pathway, inflammatory factors, and the contact between bacteria and epithelial cells in CRC mice. Apigenin has been reported to prevent atrophic gastritis and subsequent gastric cancer caused by Helicobacter pylori. Apigenin also treats tumors by regulating gut flora associated with SCFA production, such as Bifidobacterium and Lactobacillus (103, 123). Bound polyphenol of the inner shell (BPIS) sharply restores Lactobacillus and Bifidobacterium abundance in the intestine of CRC mouse models, with an increase in various lymphoid subgroups such as CD4⁺, CD8⁺T cells, and NKT cells in the blood (105). BPIS also increases the number of microbiota products, including SCFAs and indole derivatives, which boost intestinal junction recovery and alleviate inflammation in tumor-bearing mice (124). The above result shows that BPIS can regulate oncogenic inflammation. The initial response to harmful stimuli is acute inflammation, with chronic inflammation potentially resulting from the persistence of inflammatory factors (125). Inflammatory cells and cytokines act as tumor promoters during chronic inflammation, affecting cell survival, proliferation, invasion, as well as angiogenesis. Moreover, the effect of inflammation on most cancers is double-edged, since cancer also affects inflammation. Inflammation has a close relationship with tumors, making inflammation an important target for anticancer treatment (126). Activating anti-cancer immunity cells can improve the cancer-killing ability of the immune system (127, 128). These results showed that BPIS may become a new cancer drug through microbial restoration and immune regulation. Other flavonoid compounds with anti-tumor effects include carnosic acid (106) and baicalin (129).

An early study showed that Ginsenoside Rk3 exerts antitumor effects in vitro, without toxic effects on normal cells and lymphocytes (105). Later, it was found that a crucial factor in avoiding HCC is the LPS-TLR4 signaling pathway that Ginsenoside Rk3 inhibits by enhancing gut microbial imbalance (107). The mechanism of Rk3 against esophageal cancer is also related to this pathway. Neohesperidin (NHP), found in citrus fruits, upregulates Firmicutes and Proteobacteria and downregulates Bacteroides abundance. Furthermore, NHP treatment significantly increases IFN-γ expression and CD4⁺ and CD8⁺ T cell infiltration in mouse tumor cells, whereas the antibiotic cocktail (ABX) suppresses this effect (108). Consequently, NHP, a microecological regulator, induces antitumor immunity by improving immune checkpoint efficacy as a glycoside.

Anthocyanins are polyphenolic compounds widely present in plants. Bilberry anthocyanin extracts (BAE) assist in systemic nutritional status and immunity improvement by increasing the levels of Lachnospiraceae johnsonii in Firmicutes (109). Meanwhile, BAE induces T cell responses and increases SCFA production by upregulating Clostridia to ferment resistant starch and nonstarch polysaccharides. Simultaneously, the ratio of aerobes decreased remarkably, and the proportion of anaerobes increased with BAE, showing that oxygen content reduction may help BAE improve the microflora environment. Recombinant phycoerythrin (RPE) is a light-harvesting pigment that greatly reduces tumor weight, increases the incubation period of tumor cells, and inhibits cancer growth in H22-bearing mice. Moreover, RPE improved the probiotic level and sharply reduced the pathogen level compared to the cyclophosphamide group (110). Safflower yellow (SY), the main active ingredient from Carthamus tinctorius, modulates microbiota composition in BC mice, including increased Bacteroides fragilis and Clostridium counts, which could suppress pro-inflammatory factor activity and induce CD8⁺T cell and butyric acid production. This finding provides evidence that SY improves the immune microenvironment by affecting the immune cell components of the liver and modulating the abundance of inflammation-related gut microbiota (111).

Furthermore, combined treatment with multiple dietary compounds requires more attention. Combined GLP and GPS treatment significantly improved the intestinal barrier by blocking colonic polyp growth, transforming M1 to M2, effectively adjusting epithelial–mesenchymal transition markers and cutting carcinogenic signaling molecules in Apc^Min/+ mice. Their combination also greatly promotes SCFA-producing bacteria and inhibits sulfate-reducing bacteria (130). However, some highly disconcerted effects were also observed. Soluble fermentable fiber inulin may lead to gut flora imbalance and icteric hepatic cellular cancer (131). Excessive inulin increases SCFA production and stimulates immune cells to produce inflammatory factors, including IL-1α, IL-1β, IL-6, and IL-10, which cause acute lamellar inflammation and laminitis. Therefore, the application of dietary compounds in immunotherapy requires more comprehensive investigations.

GQD is a classical TCM formula, and its active compounds, including baicalin, Glaxo, and berberine, greatly reduce the inflammatory response and oxidative stress both in vivo and in vitro, which have been used in ulcerative colitis therapy (132, 133). Additionally, GQD and anti-PD-1 combination therapy downregulates PD-1 and increases IL-2, indicating that the combined treatment restores T-cell function to a certain extent by suppressing the checkpoint blockade. Combination therapy also increased Bacteroides groups, which reduces the pro-inflammatory activity of the mouse small intestine and exerts immune regulation by releasing extracellular bacterial DNA. Notably, combination therapy changes glycerophospholipid and sphingolipid metabolism, which could be used as biomarkers for monitoring patients with CRC (134).

Experimental results have indicated that sporoderm-broken Ganoderma reverses the tumor xenotransplantation-mediated microbiota structural shift (135) by increasing the immunoactivity-related genera, and by reducing microbiota, such as Bacteroides, which cause immunologic suppression and carcinogenic effects (136). The transformation of microbiota leads to changes in a series of key metabolites, including several amide acids necessary to form SCFAs. The Yiyi Fuzi Baijiang decoction (YFBD), composed of Coix seed and Patrinia villosa, is a TCM used to treat gastrointestinal disorders. Coix seeds and Patrinia villosa demonstrated antitumor hyperplasia effects in several carcinoma cell lines (137, 138). YFBD inhibited CRC cell proliferation and development in Apc^Min/+ mice without significant weight changes or immune recovery. YFBD also changes Apc^Min/+ mice intestinal bacteria, such as Bacteroides fragilis and Trichospiroideae, which regulate the Treg/Th17 ratio to control carcinogenesis (139).

Siwu-Yin inhibits esophageal precancerous lesion occurrence by increasing Turicibacter abundance, regulating bile acid synthesis and secretion metabolic pathways, and improving macrophage polarization (117). Accordingly, TCM has great potential in preventing digestion cancer progression in addition to its application in tumor immunotherapy. Furthermore, Danggui Buxue decoction (DBD) significantly improves bone marrow suppression-mediated anemia after CRC chemotherapy, while treating CRC by increasing butyric acid-producing bacterial abundance (118). This suggests a novel use of TCM for treating the postoperative side effects of digestive cancer.