The host is home to an ecological community of trillions of symbiotic microorganisms that are essential for maintaining the homeostasis and health in the host. In 2001, Joshua Lederberg named the consortium of microbes as the “Microbiome” (12). The commensal bacteria or the component of the “human flora” is the dynamic residents. The development of commensal microbiomes (such as bacteria, archaea, fungi, and viruses) is influenced by external and internal factors occurred in the host throughout the early life (13). So far, the gut microbiome is one of the most popular and emerging systems that appeal to researchers, especially its role in regulating the host’s health.

The human gut contains more than 3x10 (13) bacteria, including a repertoire of five phyla with Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria and Verrucomicrobia. The gut microbiome is 150 times larger than the entire human genome, considered as the “second genome” of human, as its vital role in the health of host. The composition of the microbiota is determined and influenced by many factors such as host genetics, different lifestyle, incidence of diseases, and exposure to antibiotics (14). The structure of host microbiome has been developing during the initial years of life. Since then, the composition of the gut microbiota and other epithelial barriers remain relatively stable throughout the adulthood. However, diet, changes in lifestyle, diseases and associated treatments can affect the composition of gut microbiota. The microbiota locates in the epithelial barrier of the gut, which can influence the local and systemic metabolic functions, inflammation, and immunity. Therefore, it involves in regulating the initiation and progression of tumor, and the response to anticancer therapies (15).

Emerging research highlights the diverse functions of the gut microbiota in the health of host. Gut microbiota regulates many essential biological processes, such as monitoring epithelial development, metabolic functions, and innate immune responses including activation and maturation of immune cells, prevention of systemic migration and infiltration of enteric pathogens (16). However, disrupting the existing gut microbiota is referred as microbial dysbiosis. The gut microbial imbalance is strongly associated with many metabolic diseases such as cardiovascular diseases, diabetes, obesity, and even a variety types of cancer (17). It is reported that commensal bacteria, such as Clostridium difficile and vancomycin-resistant Enterococcus, acquire pathogenicity by interrupting the ecology of intestine (18). This suggests that the imbalance of gut microbiota can lead to the occurrence of diseases.

Interestingly, the pathogenic microbiota in the gut of host often cause more than 20% of cancers, such as pathogenic Escherichia coli (E. coli) which contributes to colorectal cancer (CRC) (19). It deserves further studies to demonstrate the association and functional impact of gut microbiota on the health of host. Significant alterations of microbial community have been revealed by metagenomic sequencing in patients living with cancer, compared with those in control. Moreover, intestinal microbes colonize in tumors and play an important role in the pathogenesis, especially by inducing the synthesis of nutrients in the host through their metabolites, and regulating the efficacy of anti-tumor therapy and the clinical outcome (20). Furthermore, commensal flora can regulate the occurrence and development of tumors through modifying the immune system of host, as the immune deficiency is one of the important markers of cancer pathogenesis (21).

The interactions between microbes and host cells are essential for the local and systemic health and the regulation of many physiological functions (20). Importantly, the gut microbiota can also systematically influence the metabolism, immunity and homeostasis of host to affect cancer development and treatment response (22) ( Figure 1 ). The subtle and well-managed balance of system functions result from the delicate interactions between the host and gut symbionts, such as energy balance, metabolism, inflammation and immunity. Various members of gut microbiota, including Bacteroides, Lactobacillus, and Bifidobacterium, are closely associated with the metabolism of dietary fiber that prove difficult to catabolize in the gastrointestinal tract of the host.

Many studies have shown that many diseases, including cancer, may be induced by the dysbiosis of the microbiota. Importantly, microbial pathogens are responsible for high morbidity of cancers (23). Therefore, it is of great significance to investigate the role of these microbial pathogens in tumorigenesis and development. An imbalanced gut microbiota is associated with numerous host pathologies, and a well-balanced composition of gut microbiota is essential for the host’s health. Thus, disturbances in the gut microbiome can exacerbate the development of cancers ( Table 1 ).

Studies have found significant differences in the composition of gut microbiota between patients with CRC and healthy controls. The structure of gut microbiota in patients with CRC is in dysbiosis, in which the relative abundance of Fusobacterium sp., Pavimonas micra, Peptostreptococcus stomatis were significantly over-represented (30, 31). A meta-analysis of 526 fecal metagenomic data reveals seven major clusters of enriched bacteria in patients with CRC, namely, Bacteroides fragilis, Fusobacterium nucleatum, Porphyromonas asaccharolytica, Parvimonas micra, Prevotella intermedia, Alistipes finegoldii, and Thermanaerovibrio acidaminovorans (32). Studies demonstrate that E. coli, Bacteroides fragilis, and Peptostreptococcus anaerobius have oncogenic properties through inducing genotoxic stress, cholesterol biosynthesis, and activating Th1 immune responses. However, the abundance of butyric-producing bacteria Clostridium butyricum and lactic-acid-producing bacteria Streptococcus thermophiles are down-represented (33). Preclinical studies have also shown that the gut microbiota plays a crucial role in the development of CRC. The known carcinogen, Cycads, failed to induce tumors in the germ-free rats. In addition, when 1, 2-dimethylhydrazine was administered to rats, 93% of normal rats developed tumors, while only 21% of germ-free rats developed cancers (34). Subsequent studies have shown that the gut microbiota, including specific E. coli strains, Enterococci, Bacteroides, and Clostridium genera, promote the development of cancer. In addition, transplantation of fecal stools from patients with CRC into germ-free mice can promote tumor formation (35). There are 21 bacterial strains enriching significantly in patients with gastric carcinoma, including Fusobacterium nucleatum (F. nucleatum), Parvimonas micra, Streptococcus anginosus and Peptostreptococcus stomatis. In patients with pancreatic cancer (PC), the abundance of Porphyromonas, Streptococcus, Bifidobacteria and Fusobacteria were over-represented, which was demonstrated by 16S rRNA sequencing. A metagenomic study found that the presence of oral bacteria, Haemophilus, Porphyromonas, Cilia, and Fusobacteria is correlated with the increased risk of PC (36). These studies identify the vital role of gut microbiota to induce cancer.

Propionibacterium acnes (P. acnes), is the most abundant microorganism in prostate tissues. Cavarretta et al. reveal that P. acnes is over-represented in prostatic tumor tissues, compared to non-tumor tissues (37). Cohen et al. demonstrate a significantly higher degree of inflammation in P. acnes-positive prostate tissues (38). P. acnes promotes the proliferation of prostate epithelial cells by secreting proinflammatory cytokines such as interleukin-6 (IL-6), interleukin-8 (IL-8) and tumor necrosis factor-α (TNF-α) (24). Therefore, studies have revealed that P. acnes infection may be a cause of a long-term of pathogenic cascade, an inflammatory process in the prostate tissues.

Helicobacter hepatica (H. hepaticus) is the specie isolated from the liver and intestine of inbred mice (39). Accumulating evidence indicates H. hepaticus and related species play important roles in promoting intestinal dysbiosis. H. hepaticus-induced intestinal dysregulation can promote the development of breast and prostate cancers, possibly by modulating the innate immunity (40). Ge et al. have revealed that infection of H. hepaticus may trigger precancerous lesion of CRC (41). Powrie et al. demonstrate that H. hepaticus produces a large soluble polysaccharide that can induce IL-10 production by intestinal macrophages that activates a specific anti-inflammatory signaling pathways (42). It is reported that the colonization of H. hepaticus contributes to the promotion of aflatoxin- and hepatitis C virus (HCV) transgene-induced hepatocellular carcinoma (43). In addition, the cytolethal distending toxin (CDT), as the main virulence factor of H. hepaticus, results in DNA double-strand breaks, may contribute to the expression of oncoprotein (25). As the pathogenic bacterium, H. hepaticus is a strong risk factor associated with hepatocellular carcinoma, prostate cancer, and breast cancer (44).

Helicobacter pylori (H. pylori), as a cancer-promoting microorganism, can lead to the formation of gastric cancer (26). Its main functional mechanism includes secretion of virulence factors, vacuolar toxin A (vacA) and cytotoxin-related gene A (CagA), and activation of multiple pathways, including epidermal growth factor receptor (EGFR) pathway, S6 kinase pathway and cell cycle progression pathway, to abnormally transform cell proliferation, cell cycle transition and cell death, thereby promoting cancer development (45). Other indirect mechanisms are reported as well. For example, H. pylori and Enterococcus faecalis (E. faecalis) can induce the generation of oxidative stress, leading to carcinogenesis. This helps promote sudden genomic instability and manipulate host immune responses, thereby tumor cells can evade immune surveillance. E. coli produce toxins, such as Colibactin, exhibit DNase activity causing DNA damage, ultimately leading to abnormal cell cycle and tumor progression (28).

F. nucleatum, a kind of oral bacteria, can promote cancer growth by activating Wnt signaling pathway (46). Actually, the virulence factor FadA (for Fusobacterium adhesin A) of F. nucleatum increases the expression of annexin A1 through E-cadherin signaling. Importantly, FadA upregulates c-Myc and cyclin D1 through activating the Wnt/β-catenin signaling pathway. In addition, other virulence factors, Fap2 and Lipopolysaccharide (LPS) from F. nucleatum, can also promote the transformation of normal epithelial cells into tumor cells (29). In addition, F. nucleatum attenuates T cell-mediated immune responses in CRC. The outer membrane protein Fap2 can also activate Toll-like receptor 4 (TLR4), leading to activation of nuclear factor-κB (NF-κB) and subsequent expression of oncogenic microRNA21 (47). Studies have revealed that supplementation with F. nucleatum for Apc^Min/+ mice have smaller intestinal and colorectal tumors, compared with controls. Importantly, the activated NF-κB pathway leads to the over-expression of multiple proinflammatory cytokines, such as TNF, IL-6, IL-8, and interleukin-1β (IL-1β). Recent studies have shown that F. nucleatum significantly upregulates the expression of lncRNA keratin 7-antisense (KRT7-AS) and keratin 7 (KRT7) in CRC, thereby promoting cell migration and metastasis (48). Therefore, the abundance of F.nucleatum in feces may be a biomarker for noninvasive screening of CRC. In addition, the detection of serum IgA or IgG antibodies against F. nucleatum may provide the strategies of diagnosis. However, recent study has shown that F.nucleatum can enhance the efficacy of tumor immunotherapy (49).

In addition to its role in tumorigenesis, the gut microbiota has been implicated as the mediators in the response and outcome of tumor therapy. Studies have shown that regulating of gut microbiota can improve the efficacy of chemotherapy and reduce the side effects of chemotherapy.

Chemotherapy is one of the main therapies for various malignant tumors. These drugs can inhibit the growth and proliferation of tumor cells. Chemotherapy can kill tumor cells as well as normal tissue cells, which gives rise to some adverse effects such as nausea, vomiting, abdominal pain, diarrhea, intestinal mucosal ulcer bleeding.

Recent studies have shown that chemoradiotherapy can modulate the balance of intestinal microecology and induce intestinal damage. 16S rRNA sequence analysis demonstrates that the composition of gut microbiota is dysregulated in patients between patients before and after radiotherapy, of which the relative abundances of Bacilli and Actinomycetes are over-represented (50). Meanwhile, chemotherapy agents for cancer patients can directly destroy the structure of gut microbiota, leading to the imbalance of gut microbiota. Chemotherapy agents significantly reduce the abundance of Lactobacillus and Bifidobacterium, and increase the abundance of pathogenic bacteria, such as E. coli, which can lead to adverse effects of digestive tracts. Chemotherapy-induced pain occurs in 30% of cancer patients. However, the sensitivity of oxaliplatin-induced mechanical pain is found alleviated in germ-free and antibiotic-pretreated mice. And the fecal microbiota transplanted to germ-free mice lost its protective effect (50).

The gut microbiota modulates the responses to cancer therapy through a variety of mechanisms, including immune regulation, bacteria translocation, and enzymatic degradation (51). Translocation refers to the migration of commensal microbiota and pathogenic bacteria that crosses the intestinal epithelial barrier to induce systemic effects and regulates the efficacy of chemotherapy. Paulos et al. firstly report the beneficial role of the gut microbiota in cancer therapy, showing that whole-body radiotherapy induces translocation of gram-negative microbiota and promotion of LPS release, thereby inducing TLR4-dependent activation of antigen-presenting cells and enhancing the efficacy of adoptive T-cell therapy (52).

Cyclophosphamide (CTX) is widely used in tumor chemotherapy and exerts anti-tumor effects by interfering with the immune signaling cascades. CTX treatment is only effective in the presence of an intact gut microbiota. Preclinical studies have found that CTX-induced immune activation requires the participation of certain bacterial species, such as Lactobacillus murinus and Lactobacillus johnsonii. CTX treatment can lead to translocation of these bacteria to the lymph nodes and spleen, where can stimulates the host immune responses (53). Subsequent studies also reveal that Enterococcus hirae and Barnesiella Intestinihominis are required for the anti-tumor effects of CTX (54). In addition, antibiotic treatments result in a significantly reduction in the efficacy of chemotherapy, whereas germ-free animals do not respond to chemotherapy. Intestinal commensal bacteria, such as Lactobacillus acidophilus, play an important role in the anti-tumor effects of cisplatin and oxiplatin (55).

Clinical trials have shown that oral probiotic combinations (including Lactobacillus plantarum, Bifidobacterium animalis, Lactobacillus rhamnosus, and Lactobacillus acidophilusto) can significantly alleviate oral mucositis caused by chemoradiotherapy in patients with nasopharyngeal carcinoma (56). Intestinal mucositis, caused by chemotherapy, can be alleviated by Bifidobacterium infantis supplementation in mice. Reasonable supplementation of probiotics and prebiotics, as well as changes of dietary habits can regulate gut microbiota, can improve the efficacy of chemotherapy, and reduce chemotherapy-induced side effects.

Drug resistance, tumor recurrence and metastasis are the most common challenges in cancer therapy. Cancer can be also considered as the consequence of immune escape of the cancer cells (57). It is known that the immune system plays a dominant role in cancer defense, and its absence may greatly contribute to the onset and progression of cancer. Therefore, the main purpose of cancer immunotherapy is to improve immune response and overcome immune suppression. However, despite remarkable breakthroughs in recent years, the response to immunotherapy is heterogeneous and some immune-related adverse events have been identified. Therefore, the immunotherapy may not be effective and durable in all patients with cancer (58). Research have demonstrated the gut microbiome interacts strongly with the immune system, and the efficacy and toxicity of cancer immunotherapy are regulated by gut microbiome. The potential mechanisms underlying the unsatisfactory response to immune checkpoint inhibitors (ICIs) may result from the dysregulation of gut microbiota. However, the relationship between the gut microbiota and immunotherapy is under investigation.

Immune activation induced by CTLA-4 treatment depends on intestinal mucosal damage, microbiota dysregulation, and translocation of specific bacteria (Bacteroides thetaiotaomicron and Bacteroides fragilis), which can induce the activation of IL-12, proliferation of dendritic cells (DCs), and activation of specific Th1 cells. Bifidobacterium can activate CD11c⁺ DCs to enhance the efficacy of PD-L1 therapy. Several studies have demonstrated the vital role of gut microbiota in ICIs treatment for melanoma. Antibiotics treatment can destroy the efficacy of Immunolab, and the anti-tumor effect of the agents can be rescued by recolonizing germ-free or antibiotic-treated mice with the microorganism (59, 60). Patients with specific microbiota, such as Bacteroides and Bifidobacteria, have a better response to immunotherapy (61). Routy et al. analyze 249 patients, including types of non-small cell lung cancer (NSCLC), renal cell carcinoma, urothelial carcinoma, who have received second-line therapy with or without PD-1/PD-L1 inhibitors, and demonstrate that 69 patients who have recently taken antibiotics after receiving ICIs treatment, progression-free survival (PFS) and overall survival (OS) are significantly shortened. Furthermore, they analyze the gut microbiota in patients who was well-responded to ICIs treatment or not and the results reveal that Akkermansia muciniphila (A. muciniphila) was prevalent in patients who had achieved remission after ICIs treatment (62). Gopalakrishnan et al. involve 112 patients with advanced melanoma who have received anti-PD-1 immunotherapy into the study, analyze the diversity of gut microorganisms in patients, and reveal that the abundances of Faecalibacterium and Clostridiales are related to the efficacy of PD-1 therapy (61). Matson et al. find that compared with 26 melanoma patients who do not respond to immunotherapy, 16 responders have higher abundance of Bifidobacterium longum, Collinsella aerofaciens and Enterococcus faeciumre ( 59).

The gut immune system consists of immune cells and immune molecules in the epithelium and lamina propria, intestinal collecting lymph nodes (Peyer’s patches) and mesenteric associated lymph nodes. As a direct consequence of the mutualism between the gut microbiota and the host, gut microbiota can determine the immune system status of the gut, promote and regulate innate and adaptive immunity.

Pattern recognition receptors (PRRs) are one of the main mediators between gut microbiota and host immune system. In recognition of microbial antigens, PRRs activate the intestinal immune system through a downstream cascade of signaling molecules. Several of these PRRs have been implicated in colitis-associated carcinogenesis, including Toll-like receptors (TLRs), nucleotide binding oligomeric receptors, Retinoic acid-inducible gene I (RIG-I) like receptors, and the missing melanoma 2-like receptor. Specifically, F. nucleatum can activate TLR4 signaling to promote tumor development, and another over-presented bacterium, Peptostreptococcus anaerobius, can promote carcinogenesis by activating TLR2 and/or the TLR4 pathway in patients with CRC.

Gut microbiota, as the commensal microbiota of the host, is closely related to the immune environment homeostasis. As the extrinsic microorganism, microbiota contains a variety of substances, such as DNA, RNA and protein, which act as antigens to activate the innate and adaptive immunity of the host; therefore, it plays a crucial role in tumor immunity (74). The innate immune system is the frontline of defense for the host’s immune system against foreign pathogens, which is mainly composed of NK cells mononuclear macrophages cells, DCs, granulocytes (75). As the effective cytokine produced by innate immune cells, type I interferon (IFN-Ι) is known to play an important role in anti-tumor effect and the defense against infection by pathogenic microbial. Recent studies have found that gut microbiota can promote the secretion of type I interferon through the activation of cyclic GMP-AMP (cGAS)-STING-IFN-I signaling pathway, thereby enhancing the anti-tumor effect. Supplementation with Bifidobacterium can activate STING signal through its accumulation in local tumor microenvironment, thereby promoting antigen presentation of DC cells and enhancing the anti-tumor effect of CD47 inhibitor (76).

The composition of tumor microenvironment often affects the efficacy of immunotherapy. Studies have shown that the tumor microenvironment (TME) in mice lacking microbiota shows an increase in tumor-promoting macrophages, and a decrease in monocytes and dendritic cells with anti-tumor effects (64). It suggests that microbiota can reshape the TME and affect the differentiation of mononuclear macrophages. Specific bacterial flora can produce cyclic adenosine diphosphate, which can bind with STING, activate the signaling pathway, and then promote the production of IFN-I by intratumor monocytes, thereby regulating the polarization of macrophages, promoting the interaction between NK cells and DCs, and further enhancing anti-tumor effects. Since high-fiber diet is known to over-represent the probiotics, a high-fiber diet can significantly increase the abundance of A. muciniphila, which can produce C-di-AMP and enhance the efficacy of ICIs (64).

A study has found some probiotics, including Lactobacillus plantarum and Lactobacillus acidilacticii, can activate inflammatory NF-κB signaling pathways and IFN-Ι response from macrophages, which is mediated by cytoplasmic receptor antiviral STING and mitochondria signal perception. Activation of downstream signaling molecules induces the expression of IFN-Ι (77). In addition, studies have found that Lactobacillus rhamnosus can produce IFN-Ι by promoting STING signaling pathway in DC cells, enhancing the activity of tumor killing CD8⁺T cells, and further promote the efficacy of immunotherapy (78). As commensal bacteria, some pathogens often play dual roles in the occurrence and progression of tumors. Studies have found that probiotics can activate the STING pathway and play a positive role in the immunotherapy. F. nucleatum activates STING signaling pathway, then induces the expression of PD-L1 in CRC cells, increases the infiltration of IFN-γ⁺CD8⁺T cells, and improves the sensitivity of tumors to PD-L1 immunotherapy (49).

As the commensal residents in the gut, gut microbiota can interplay with the host, and contributes to the metabolism of the host. It is known that gut microbiota transforms different nutrients into small substances and obtains what they require for survival and growth. However, these microbial metabolites as well as the associated products that bacteria mediated, exert great influence on the physical health ( Table 2 ). Studies report that substances including microbes-associated small molecules and metabolites promote the tumorigenesis of the host and influence the response of treatment, through local and systemic effects (86). Metabolism function of microbiota is the vital mediator to interact the host and the microbes, which can play a profound role in the physiological function and immune environment (87). The metabolites of gut microbes derive, as well as the physical metabolism that microbes take part in, mediate the initiation and progression of different types of cancer, and the related treatments through different mechanisms.