Colorectal cancer (CRC) is a type of human malignancies associated with the gastrointestinal tract (GI) in which the proliferation and invasion of colon epithelial cells and GI become uncontrollable (1–3). Due to the high metastatic properties of CRC cells and the lack of effective treatment, people with CRC usually die (4). Based on available knowledge, depending on genetics and family history, all people are at risk for CRC, but people over 50 are more likely to develop CRC than other age groups (5, 6). Regarding risk factors for predisposition to CRC, studies have reported that mutations in certain genes, heredity and family history of gastrointestinal malignancies, high-fat and fiber-free diets, obesity and diabetes, gastrointestinal adenomatous polyps, inflammatory bowel disease (IBD), smoking, alcoholism, and GI microbiome composition can increase the susceptibility to CRC (7–10). The origin of CRC is usually polyps formed in the large intestine, which are small noncancerous masses, and over time, under the influence of other predisposing factors, some of these masses can become cancerous tissue (11, 12). For the treatment of CRC, various therapeutic approaches such as traditional therapies including surgery, chemotherapy, radiation therapy, and novel and targeted therapies such as immunotherapy are used to treat patients with CRC (13, 14).

Moreover, the GI is home to trillions of highly diverse microorganisms, collectively termed microbiomes, that can play an important role in regulating biological events and human health (15). It has been shown that the microbiome can be associated with nutrient absorption, digestion and metabolism as well as the host immune system components and responses. Based on recent findings, scientists have discovered that microbiome pattern alteration can lead to diseases such as obesity, diabetes, IBD and cancer (15, 16). It has also been documented that changing the colon environment by manipulating the gut microbiota can reduce the side effects of cancer therapy. For instance, various studies have shown that some probiotic strains can improve chemotherapy-induced mucositis, diarrhea, weight loss. These changes can also reduce inflammation, regenerate and improve intestinal epithelial barriers, and prevent intrinsic apoptosis (17). The microbiota pattern can also be effective in immunotherapy (18). In this regard, researchers have reported that distinct species of Bacteroides might be involved in the antitumor effects of cytotoxic T-lymphocyte associated antigen 4 (CTLA4) blockade. Furthermore, specific T cells responses against Bacteroides fragilis or Bacteroides thetaiotaomicron were associated with greater treatment efficacy with anti CTLA4 (19).

Therefore, considering the importance of the role of microbiota and its possible effect on immunotherapy outcomes, this review discussed the pathogenesis of CRC and its immunopathogenesis and the effect of gut microbial patterns on immunotherapy-based CRC treatment.

Colorectal cancer is a disease in which the growth and proliferation of epithelial cells in the colon or rectum is out of control (20). In some cases, the growth of abnormal masses termed polyps in the colon or rectum can cause CRR. To prevent CRC, screening tests can lead to early detection of polyps and allow them to be removed before they become cancerous. Screening tests can also detect CRC in early stages, just as treatment approaches work best (21). Studies have shown that there are different types of CRC, the most common of which is adenocarcinoma. Other types of CRC include gastrointestinal stromal tumors, colorectal lymphoma, and carcinoid tumors.

Additionally, hereditary colorectal cancers are categorized as familial adenomatous polyps (FAP) and hereditary nonpolyposis colon cancer (HNPCC) (22). It has been revealed that aging, especially in the fifth decade, increases CRC risk. One of the most important risk factors is the presence of IBDs, such as ulcerative colitis (UC) or Crohn’s disease. It is also important to have CRC or colorectal polyps, HNPCC, and FAP cases in the family (23). Lifestyle-related factors include lack of regular physical activity, a low-fruit and vegetable diet, a low-fiber and high-fat diet, a diet high in processed meats, overweight and obesity, tobacco and alcohol consumption, and could increase the risk of CRC (8, 23).

In addition, in around 15% of sporadic CRCs, defective DNA mismatch repair (MMR) could be occurred. Several retrospective investigations have shown that MMR-deficient CRC patients have a more promising stage-adjusted prognosis than those who have MMR-proficient CRC (24).

Based on the findings of previous studies, colorectal polyps and CRC are not always asymptomatic, especially at the beginning of the disease. Many people with CRC or polyps may not be aware of the disorder, so regular screening tests are essential for the early detection of CRC (25). Clinical signs of CRC usually include changes in bowel habits, blood in the stool, diarrhea, constipation or feeling of complete bowel emptying, chronic abdominal pain or cramping, as well as unexplained weight loss (26). Therefore, it is recommended that adults 45 to 75 years of age be screened for CRC. Screening tests, including periodic stool tests, CT colonography, and flexible sigmoidoscopy, can help diagnose polyps or CRC (27).

It has been revealed that the early-onset CRCs show differential molecular, clinical, and pathological features than later-onset CRCs (28). Early-onset colorectal cancer (CRC) occurs in people below 50 years has been growing worldwide, particularly in high-income countries (29). It has been reported that the exposome and early-life environmental exposures, including western diets, red and processed meat, obesity, antibiotics, stress, synthetic dyes, monosodium glutamate, titanium dioxide, high-fructose corn syrup, birth mode, breastfeeding behaviors, and maternal stress, could affect microbiome health and development (30). Moreover, some of the mentioned exposures may lead to genetic and epigenetic modifications in CRC cells and affect the gut microbiota and host immunity. Additionally, the gene-by-environment interactions throughout life may play a pivotal role in the early-onset CRC etiology. Moreover, hypomethylation of long-interspersed nucleotide elements1 (LINE1) is sporadically detected in CRC (31). A wide range of exposures such as physical inactivity, high BMI, pesticides, ionizing radiation, smoking, benzene have been accompanied by LINE1 hypomethylation in blood cells (32–34). Regarding the relationship between tumor LINE1 hypomethylation and younger age at CRC diagnosis, it is probable that gene-by-environment interactions along with mentioned exposures, particularly in early life, play an etiological role in early-onset CRC (35, 36). Therefore, evaluating early-life exposures, gene-by-environment interactions, and genetic germline polymorphisms could improve the screening effectiveness and early diagnosis of early-onset CRC (28).

These data collectively indicated that improving lifestyle and diet to develop a healthy microbiota may contribute to early-onset CRC prevention and a more favorable treatment response.

As we know, the TME of solid tumors has complex properties and special conditions, and due to the presence of hypoxia, high acidity, different signals as well as the presence of immune system cells, different mediators, endothelial cells, fibroblasts and other growth spectrum cells provides tumor cell development (37, 38). Immune cells in the TME can have pro and antitumor roles based on their phenotypes (39, 40). The main components of the innate immune system include physical epithelial barriers, circulating plasma proteins, phagocytes, dendritic cells (DCs), natural killer (NK) cells, and lymphatic cells. These innate immune system components are usually present in all tissues (41). However, understanding the function of these cells in the TME remains unknown (42). For instance, researchers have long believed that NK cells are an effective antitumor agent in CRC, but novel investigations have shown that despite the secretion of chemokines and adhesive molecules associated with the recruitment and homing of NK cells in tumor tissue, there is a surprising absence of these cells are in the TME (43–46). Correspondingly, due to the predominance of myeloid cells over lymphocytes and even tumor cells, different myeloid cell origins and unknown differentiation programs for myeloid subtypes add to the complexity of understanding the components of innate immune function in the site of tumor (47–49).

Chronic inflammation subsequent from tumor function or the immune system to control the tumor is one of the hallmarks of human malignancies such as CRC, which promotes and maintains cellular transformation and tumor development in CRC (50). The source of this chronic inflammation can be persistent infections, autoimmunity, and sterile inflammation, in all of which the innate immune system cells are the main performers (51). Although the extent of these cells’ involvement in the initial inflammatory response is not fully understood, the TME’s signals intelligently manage these innate immune cells and their mediators in favor of the cancer development. Inflammatory signals include heat shock proteins (HSPs) and toll like receptors (TLRs), apoptotic cells, damage-associated molecular patterns (DAMPs), cytokines, or free DNA molecules that are responsible for dysregulation of immune system responses (52–55). Following these signals, the secretion of chemokines leads to the recruitment of more immune cells to the TME, which can act as a suppressor or activator of antitumor responses and ultimately lead to tumor development or inhibition (38).

As mentioned, the distribution and abundance of immune cell subpopulations in the TME of solid tumors are very diverse. In this regard, it has been found that there is a close relationship between the number of immune cells and the clinical course of CRC (56–58). For example, the high frequency of effector T cells with antitumor properties is associated with a good prognosis, and conversely, the high frequency of infiltrated myeloid cells in TME is associated with a poor prognosis leading to tumorigenesis (59–61). Evidence suggests that the adaptive immune system components, including T cells and B cells, play an important role in CRC and can learn to detect tumor cells and contribute significantly to the course of the disease (38). The presence of effector T cells in the TME is typically considered a sign of inflammation, while regulatory T cells (Tregs) are considered a sign of immunosuppression (62–64). However, in CRC, the high frequency of Tregs is associated with a better prognosis, which contradicts the negative association of FOXP3⁺ tumor-infiltrated Tregs in other cancers (61). In CRC metastatic liver lesions, the composition of the TME is mainly determined by the chemokine and cytokine pattern, and regularly a small number of FOXP3⁺ Treg and NK cell cells are recruited to the tumor site (65). It has been reported that a small number of CRC patients with unstable microsatellites (MSIs) tumors that contain impaired DNA repair-related proteins, immunity-related mutations are increased, resulting in a remarkable upsurge in the presence of adaptive antitumor immune cells such as lymphocytes, leading to promising prognosis. While the number of infiltrated cells in microsatellite stable CRC tumor tissue (MSS) is less than half, these patients respond to immunotherapy much better than the MSI subgroup (66, 67).

Since the different phenotypes of different B cells, the role of these cells in the CRC TME is highly controversial (68–70). The formation of tertiary lymphoid structures seems to occur with the juxtaposition of B cells and T cells in CRC, indicating an increase in effector immune cells’ penetration and, ultimately, a more favorable prognosis. However, some studies show that mutations in the B-Raf proto-oncogene, serine/threonine kinase (BRAF) gene are associated with the formation of tertiary lymphoid structures (71–74).

The tumor escape mechanisms from the immune system are very diverse and intelligent. Tumors can induce the production of cytokines that enhance Tregs and MDSCs and inhibit the function of CD8⁺ cytotoxic T cells. These occurrences can lead to the suppression of CD4⁺ and CD8⁺ effector T cells that can no longer detect or respond to tumor antigens. Tumor cells can also reduce the expression of MHC-I so that T cells can no longer recognize these cells. In addition, tumors can induce the expression of immunosuppressive molecules such as CD274 (PDL1), CTLA4 (CD152), lymphocyte activating 3 (LAG3), and T-cell immunoglobulin domain and mucin domain 3 (TIM3), which lead to exhaustion of effector T cells as well as inhibition of malignant cell apoptosis (75–77). Targeting these same immune checkpoint molecules forms the basis of immunotherapy using the blockers of these inhibitors.

Evidence designates that lifestyle, diet, nutrition, the microbiome, the environment, and other exogenous factors could be responsible for the formation of pathologic states and affect the genome, epigenome, transcriptome, proteome, and metabolome of cancerous and noncancerous cells, as well as immune cells. Nowadays, the importance of studying big data is felt more and more, and this requires the transformation of pathology to epidemiology, biostatistics and bioinformatics data science fields. The Molecular Pathological Epidemiology (MPE) research framework is able to reveal the advantages and strengths of interdisciplinary integration that has been used to study several human malignancies such as CRC, lung, breast, and prostate cancers. The MPE research model offers new insights into the interactions between the environment, the tumor, and the host and could open new research frontiers (78).

In this context, the combination of tumor immunology assessments with the MPE approach can evaluate the impact of endogenous and exogenous factors on tumorigenic processes leading to CRC by assessing antitumor immune responses (79, 80). This integrative immuno-MPE field is able to fill a research gap between epidemiology and tumor immunology, and it signifies a future direction for cancer investigations (79, 81, 82). The immune status analyses in the TME are gradually being unified into large-scale epidemiological cohorts (83–88).

Based on studies on the use of immunotherapy-based methods in the treatment of patients with CRC, the success of these methods compared to traditional therapies can be recognized to some extent, although sometimes for different reasons and specific conditions of tumor cells and TME along with the unpredictable behavior of the immune system, immunotherapy also faces major challenges (76). Immunotherapy in MSI-H or dMMR patients has been performed and approved, and currently, immune-checkpoint blockers such as programmed cell death 1 (PDCD1) inhibitors are being studied more than other approaches. About 80% of cases of MSS or MMR-p can be detected in the pathology of advanced stages of metastatic CRC, and it is possible that combination therapies using immune-checkpoint blockers and other tumor-inhibiting factors such as radiotherapy, chemotherapy, and cancer vaccines together can lead to strong antitumor responses from the patient’s immune system (76, 89, 90). However, combining these methods has not led to satisfactory clinical outcomes in several studies (91).

Therefore, studies are not limited to the use of immune-checkpoint blockers and other treatments such as chimeric antigen receptor (CAR) T-cell therapy and cancer vaccines, as well as interventions in the microbiome pattern of patients with CRC evaluated.

CAR-T cells, especially the third generation and later, have successfully treated human blood malignancies such as acute B-cell lymphoblastic leukemia by targeting specific antigens (92, 93). However, in solid tumors, due to various problems such as lack of penetration and ineffective trafficking, the tumor inhibitory TME has not yet achieved satisfactory consequences (94). In metastatic CRC mouse models, targeting the guanylate cyclase 2C (GUCY2C) antigen, expressed by tumor cells, could eliminate these cells and inhibit metastasis (95). Clinical trial studies are also underway on CAR-T cells in treating patients with CRC. In this context, targeting the epidermal growth factor receptor (EGFR) antigen and the design of CAR-T cells against this receptor are underway (NCT03152435). Despite the challenges, with the advancement of science and the identification of a wider range of tumor-specific antigens, a moral prospect for this treatment for CRC can be envisioned.

Vaccine therapy is another type of immunotherapy that can also treat cancer. Based on current theories and backgrounds of vaccines and their effect on the innate and adaptive immune system, cancer vaccines may lead to more effective antitumor responses by inducing tumor-related antigens to be targeted by the immune system (76, 96, 97). Various studies have been performed on anti-metastatic CRC vaccines, including peptide vaccines, DC vaccines, and autologous vaccines (98–100). However, the available evidence suggests that cancer vaccines are generally not as effective as other immunotherapy-based methods and traditional therapies such as chemotherapy and surgery in increasing patients’ survival for a variety of reasons, including the genome-wide tumor mutations and the development of neoantigens, as well as high tumor heterogeneity (101, 102).

As mentioned earlier, genetic and environmental factors play an important role in increasing the incidence of CRC (103). In developing countries, changing lifestyles and unhealthy nutritional habits such as low-fiber or high-fat diets, consuming processed meats and red meat, along with physical inactivity, alcohol and smoking, can alter the gut microbiota pattern and influence several physiological processes, immune system responses, and even cancer treatment (104–107). Contrary to popular belief, the gut microbiome is not limited to bacteria, but fungal species and viruses are also components of the gut microbiome. For example, the DNA load of the virus in tumors is much higher than in noncancerous tissue. Studies of viral infections such as human poliovirus, human herpes, and human papillomavirus have also shown that these viruses can be involved in the pathogenesis and risk of CRC (108, 109) ( Figure 1 ).

Recent investigations have reported that microecology plays a critical role in the effectiveness of CRC treatment by antitumor agents such as chemotherapy as well as immunosuppressive agents (180, 181) ( Table 1 ). The gut microbiota has the ability to regulate the antitumor effect of chemotherapy drugs commonly used in chemotherapy to eliminate CRC cancer cells. It has been shown that 5-fluorouracil (5-FU), a chemotherapeutic drug routinely used to treat CRC and other human malignancies, can increase the effectiveness of tumor cell killing under the effects of metabolites produced by gut microbial (182). Correspondingly, eating foods rich in probiotics or supplements containing probiotics, prebiotics, and symbiotics can reduce the risk of CRC (183, 184).

Nowadays, cancer immunotherapy includes approaches such as immuno-binding site-blocking therapy, adoptive immunotherapy, indoleamine 2, 3-dioxygenase 1 (IDO1) inhibitors, cancer vaccines, and nonspecific immunomodulators (180, 185, 186). The combination of cancer immunotherapy and microorganisms was first proposed in the early 19th century, and experimental outcomes showed that the combination of heat-killed Serratia and Streptococcus pyogenes could effectively increase the survival of sarcoma patients. This increase in survival was probably due to the development of a sustained antitumor immune response (180, 187, 188).

Immune checkpoint inhibitors with PDCD1/CD274 axis inhibitory capability can induce stable clinical outcomes in several patients with malignancy. It has been shown that primary resistance to immunosuppressive inhibitors can be attributed to the abnormal pattern of the gut microbiome because antibiotics can reduce or even inhibit the clinical benefits of immune checkpoint inhibitors in patients with advanced cancer (189). Evidence suggests that immunotherapy is effective in only one subset of people with CRC. In this context, anti-PDCD1 is approved only for use in dMMR/MSH metastatic CRC, which may be due to the expression of higher amounts of neoantigens by this type of CRC. How to respond to the anti-PDCD1 therapy and understand the mechanisms leading to improved immunotherapy performance in dMMR/MSH CRC have not been fully elucidated (190–192).

Moreover, it has been shown that transplantation of fecal microbiota (FMT) from responder cancer patients to immune checkpoint inhibitors to sterile or antibiotic-treated mice potentiates the antitumor effects of PDCD. In contrast, FMT from non-responder patients has no effect on treatment with PDCD1 blockers. Metagenomics studies of at diagnosis patient fecal specimens have disclosed an association between clinical responses to immune checkpoint inhibitors and the relative abundance of Akkermansia muciniphila. On the other hand, oral supplementation of non-responder patients with A. muciniphila upon FMT restored the anti-PDCD1 efficacy via IL12 by increasing the recruitment and infiltration of CXCR3⁺, CCR9⁺, CD4⁺ T cells into the mouse tumor site (193). According to recent studies, the composition of the intestinal flora can predict the effectiveness of allogeneic stem cell transplantation, and this feature confirms that the gut microbiome is effectively involved in the formation of systemic immune responses (194).

As discussed before, gut microbiota can modulate immune responses in the TME. In this context, an investigation on a patient with cutaneous melanoma reported an association between infiltrating CD8⁺ T cells, intratumor bacteria (Lachnoclostridium, Gelidibacter, Flammeovirga, Acinetobacter), and patients’ survival. The outcomes revealed that intratumor Lachnoclostridium could modulate CCL5, CXCL9, and CXCL10 levels, affecting the infiltration of CD8⁺ T cells (195). A phase 1 clinical trial was recently performed to evaluate the feasibility and safety of FMT and reinduction of anti-PDCD1 immunotherapy in patients with anti-PDCD1 refractory metastatic melanoma. The findings showed clinical responses in some patients.

Interestingly, patients under FMT treatment experienced promising alterations in expressing several genes involved in antitumor responses and infiltration of effector immune cells in the TME and gut lamina propria (196). Similarly, another clinical trial in this field reported that FMT in combination with anti-PDCD1 in patients with PDCD1 refractory melanoma was well tolerated and accompanied by clinical benefits in about half of these patients. The outcomes demonstrated that responder patients exhibited elevated taxa associated with response to anti-PDCD1 immunotherapy, activation of more effector CD8⁺ T cell, and reduced the number of myeloid cells responsible for release IL8 (197).

In general, the findings of these studies suggest that microbiota may play a key role in regulating the host’s immune system, which is directly related to the success or failure of cancer immunotherapy. As discussed, one of the most important immunotherapy-based approaches in CRC is checkpoint blockers using anti-inhibitory molecules mAbs against CTLA4, PDCD1, and CD274 (198). In this regard, researchers studied mice and showed that Bifidobacteria have antitumor properties.

Interestingly, administration of oral Bifidobacteria alone can have the same effect as CD274 inhibitors. In addition, combination therapy with CD274 inhibitors and Bifidobacteria can nearly completely inhibit the growth and development of cancer cells (199). This treatment can augment DC function, enhancing antitumor T CD8⁺cell priming and infiltration in the TME. As a result, Bifidobacterium enhances antitumor immune responses and improves the effectiveness of anti-CD274 immunotherapy through increased trafficking, penetration, and infiltration of effector T CD8⁺ cells into tumor tissue which is considered as one of the most important challenges in cancer immunotherapy.

Bacteroides are considered another bacterial species effective in immunotherapy (200). The antitumor property of CTLA4 inhibitors has been documented to be dependent on the presence of Bacteroides or B. fragilis, and this association may be reciprocal because the administration of CTLA4 inhibitors contributes to the growth of B. fragilis. To prove this, it has been shown that sterile mice or mice treated with antibiotics that lacked the itemized bacterial species did not respond to treatment with anti-CTLA4 but responded to this type of treatment by feeding the animals with B. fragilis (19). Of course, most of the findings in this field are in malignancies such as melanoma, and perhaps due to the lower response of CRC to checkpoint blockers, the pattern of gut microbiota cannot significantly affect this type of immunotherapy and support it.

In contrast, a recent study on melanoma immunotherapy using immune checkpoint blockers has disclosed that not all patients respond to this treatment, and numerous patients with melanoma who have been treated have potentially life-threatening side effects (irAEs) and some gut microbiota bacterial species such as Bacteroides dorei and Bacteroides vulgatus may be involved in causing these complications (201).

Recently, an investigation proved that some human malignancies, such as CRC with impaired MMR, but not patients with complete chromosomal mismatch repair, respond to anti-PDCD1 therapy better, probably through the metabolic pathway of glycerol phospholipids (202). These differences in response to anti-PDCD1 therapy could lead to the emergence of therapeutic approaches using gut microbiota, thereby increasing the effectiveness of cancer immunotherapy in CRC patients with MMR by regulating the composition of the intestinal flora (202, 203).

As an anti-inflammatory mediator, IDO1 is also essential for the resistance of host-microbiome homeostasis and is produced by tumor cells and following the activation of TLR4 and TLR9 in DCs that induces immune responses against tumors (204). In the microbiome, IDO1, along with butyrate and short-chain fatty acids (SCFAs), interferes with tryptophan metabolism (189). IDO1 inhibitors may be useful in treating CRC, especially in dMMR/MSH (190, 204). In vitro and in vivo studies on CRC models of CT26 and MC38 mice have shown the use of attenuated Salmonella typhimurium carrying a small hairpin RNA plasmid targeting IDO (shIDO-ST) can remarkably reduce the expression and function of IDO proteins and thereby inhibit tumor growth and development. Moreover, epacadostat was used as a known inhibitor of IDO1, which could also reduce tumor growth by inhibiting IDO1 (205).

In mice models of colon cancer, the use of Lactobacillus acidophilus lysate in combination with anti-CTLA4 enhanced antitumor immune responses via activation of the TME-infiltrated effector T cells as well as improving homeostasis in the investigated animals (206). These findings suggest that some bacterial species may be very effective in combination therapies with other antitumor agents.

Additionally, it has been disclosed that in mice models of CRC under treatment with immune checkpoint inhibitors, specific bacterial species are associated with promising antitumor responses. In this regard, MC38 cell line treated with anti-PDCD1 or anti-CTLA4 and anti-IL10 plus CpG oligonucleotides demonstrated Ruminococcus spp. and Alistipes shahiiplayed could induce antitumor responses by modulating the TME (207). Furthermore, other spices including, Eubacterium limosum, Ruthenibacterium lactatiformans, F. ulcerans, B. uniformis, Bacteroides dorei, Parabacteroides johnsonii, Phascolarctobacterium succinatutens, Paraprevotella xylaniphila, Alistipes senegalensis, and P. gordonii might be beneficial in the improvement of immune checkpoint inhibitors’ effectiveness (189, 208).

A study on the effect of antibiotic therapy on adoptive cell therapy showed that antibiotics inhibited the innate immune system responses in irradiated mice and reduced the effectiveness of adoptive cell therapy. In CRC mouse models, the use of broad-spectrum antibiotics reduced the effectiveness of CD4⁺ T cell transplantation against implanted colorectal tumor cells (209). Only one animal study on the effect of antibiotics on CAR-T cell therapy reported that broad-spectrum antibiotics had not reduced the survival of BA20-lymphoma mice treated with CD19 CAR-T cells (209). However, the change and decrease in gut microbiome pointedly improved the duration of B cells aplasia as well as the persistence of CAR-T cells in these mice (210).

According to studies on the role of gut microbiota in tumorigenesis and also the antitumor activity of some bacterial species, it can be concluded that the gut microbiome can be of great importance in CRC immunopathogenesis by regulating immune responses. On the other hand, CRC immunotherapy has been developed in various ways, and in the case of CRC immune-checkpoint blockers, they have been studied more than other immunotherapy approaches and have had relatively promising effectiveness. However, this type of immunotherapy also faces many challenges. Therefore, the use of some bacterial species to change the pattern of the intestinal microbiome may increase the effectiveness of immunotherapy with checkpoint blockers, which can be very useful in treating CRC patients. Nevertheless, due to the complexity of microbiomes and the high diversity of microorganisms, including bacteria, viruses and fungi, and their unknown functions involved in tumorigenesis or antitumor mechanisms, further studies to identify the function of each of them and also use these microorganisms in combination with other therapeutic approaches appear to be essential.

YL: Conception, design and inviting co-authors to participate. Y-NZ: Writing original manuscript draft. D-XK: Review and editing of manuscript critically for important intellectual content and provided comments and feedback for the scientific contents of the manuscript. All authors read, revised and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.