In 2022, there are expected to be 191,830 new cancer cases and 609,360 cancer deaths in the United States (1), which is increasing year by year (2, 3), consequently, more targeted cancer control interventions in cancer are needed to reduce cancer mortality. Usually, only 5-10% of all cancer cases are considered to be attributed to genetic defects (4), the remaining 90-95% have their roots in the environment and lifestyle that include smoking, diet, alcohol, sun exposure, environmental pollution, infection, stress, obesity and lack of exercise (5, 6). In addition, over the past few decades, the link between microorganisms and the development of several cancers has also been generally recognized. Therefore, it is important to understand the role of microorganisms in tumor prevention and treatment. Helicobacter pylori in gastric cancer (7), the human papillomavirus (HPV) in cervical cancer (8), and the hepatitis B and C viruses (HBV and HCV) in hepatocellular carcinoma (9) are the best evidence that the microbiota is not passenger or bystander. The influence of the gut microbiota on the development of certain tumors and the specific mechanisms have recently received much attention. Bacteria are found in many tissues and organs of the body, especially in the digestive tract, and their numbers and species are constantly changing (10). Host genetics, dietary components, drugs, chemicals, aging and stress have been shown to regulate the dynamic balance of the intra-host microbiome (11, 12). Microbial dysbiosis in the gut has been connected to the development of tumors both inside and outside the gastrointestinal system, including colon cancer, esophageal cancer, liver cancer, and breast cancer (13–15). Strategies against the gut microbiota may influence the course of disease in which dysbiosis is observed. Various approaches, including dietary interventions, the use of probiotics and antibiotics, and fecal microbiota transplantation (FMT) have been used to modulate the gut microbiota to prevent or treat different cancer pathological processes (16). As research on intestinal flora continues to progress, FMT as a novel treatment modality has been used for the restoration of gut microbial dysbiosis.

FMT, which originated in Chinese medicine at least 1700 years ago, is the transfer of fecal microbiota from a healthy donor into the patient’s intestine to rebalance the flora (17). It can target and modulate the human gut microbiota, which has been widely studied in the treatment of various diseases (18). Given that FMT maintains microbial diversity without disrupting the natural balance of the microbial gut, it exhibited significant advantages over other treatment modalities. FMT is commonly used for the treatment of Clostridioides Difficile infections (CDIs) (19, 20). More than that, promising results have indicated that FMT also is effective in other digestive diseases, including colonized with multidrug-resistant organisms (MDROs), inflammatory bowel disease (IBD), and irritable bowel syndrome (IBS), and neurological disorders including multiple sclerosis (MS), hepatic encephalopathy (HE), Parkinson’s disease, and diabetic neuropathy (18, 21–25). Meanwhile, more evidence shows that FMT also has an antitumor effect which will be an emerging treatment modality for cancer such as melanoma (26, 27). The regulatory and antitumor effects of FMT on the host intestinal microbiota are achieved by affecting the immune system and inflammatory response, changing microbial metabolites, affecting cell signaling pathways, inhibiting DNA damage, and acting on extraintestinal parts through blood circulation (28). In this review, we will focus on the role and mechanisms of human gut microbiota in tumor development. Likewise, we will discuss the antitumor effect and clinical application of FMT by modulating or reconstructing intestinal microbiota, particularly in oncotherapy, including immunotherapy, chemotherapy, and radiotherapy.

Humans first acquire a large microbiota from their mothers at birth, and the microbiota is constantly changing as they grow and develop (29, 30). There is a bidirectional feedback loop between microbiota and host health. On the one hand, many studies revealed significant correlations of the gut microbiome with host genetics, environment, changes in lifestyles, and diet (11, 12, 31). Since the genome of the host can affect the make-up and function of the microbial community, human and mouse genetic motifs are being mapped by genome-wide correlation studies (32). For example, a microbial trial in the Netherlands involving 7,738 individuals examined the association of 207 of these taxa and 205 genome-wide entries representing microbial composition and function and found two signals associated with microbial taxa in the vicinity of the LCT and ABO genes and replicated in two independent cohorts (11). In addition, a long-term specific diet of the host can make some specific microbiota dominant, and some microorganisms may even become extinct (33). In that study, successive generations of rats fed a low-fiber diet revealed a gradual loss of microbiota diversity, with some taxa being undetectable. On the other hand, the complex and unique make-up of the gut microbiome community influences host metabolism, immunity, and elimination of harmful substances (10, 34). Our gut microbiota not only helps us absorb nutrients from our food, but also contributes to the body’s immune system (35). A recent study through mouse models and brain imaging technology has proved that intestinal microorganisms can affect brain function and metabolism (36). As long as the gut microbiota is functioning properly and maintaining a balance (symbiosis), the host physiology is maintained and protective effects are obtained. Moreover, a standard and healthy eubiosis cannot be standardized due to individual differences, and maintaining a balanced microbial composition in one individual is not the best choice for others. Nevertheless, it is almost certain that gut microbiome diversity is associated with human health (37, 38).

In contrast to eubiosis, there is an imbalanced gut microbiome or altered community structure in various disease states, which is called dysbiosis, resulting in the production of large amounts of harmful metabolites. Dysbiosis, implying disturbances in microbial composition and metabolism, is relevant to a wide range of disorders and a therapeutic target (39). For example, lifestyle changes and high-fat diets can cause dysbiosis of intestinal flora, increasing LPS expression levels and decreasing miR-145, leading to metabolic inflammation and metabolic disorders (40, 41). Of note, an imbalance in the gut microbiota can also contribute to the progression of cancer. Dysbiosis implies the production of harmful metabolites by the microbiota as well as causing immune dysfunction in the body, which leads to several diseases such as IBD and IBS. In addition, long-term metabolic disorders and inflammation can lead to the development of tumors (42). For instance, colon cancer may progress from chronic inflammation caused by dysbiosis (15, 43, 44). One very important recent finding indicates that patients with esophageal cancer also had dysbiosis of the gut microbiota (n = 10) (13).

Intestinal goblet cells maintain the epithelial barrier by secreting mucus (45), which provides a habitat for commensal bacteria and prevents bacterial infiltration and inflammation, thus maintaining barrier integrity (46). The intestinal microbiome is essential in preserving the integrity of the gut barrier (47). Furthermore, the gut microbiota aids digestion, helps with vitamin production and regulates physiological functions (36, 48, 49), including regulation of metabolism, blood production, immune enhancement, and protection against cancer (13–15, 36, 50, 51). Conversely dysbiosis of the microbiota can also promote tumors. In this review, we pay attention to the microbial mechanisms that may be involved in tumorigenesis and progression ( Figure 1 ).

Experiments in mouse models have confirmed that microbiota can cause cancer and a wide range of other diseases, and microbiota can also be used to therapeutically treat cancer. Dietary interventions, probiotic supplementation, antibiotics, and FMT are a few of the methods currently available to alter the microbiota in the gut and reverse intestinal dysbiosis ( Figure 3 ) (16, 85). Consuming dietary fiber-rich foods and prebiotics early in life, minimizing red meat intake, and keeping body weight in the normal range can help reduce the incidence of tumors (86, 87). Similarly, the intake of probiotics can restore intestinal microbial balance and prevent colonic infection (88–90). However, there is a lack of evidence that probiotics can colonize the intestinal mucosa, and especially the role of probiotics in humans after the use of antibiotics is more elusive (91, 92). On the other hand, probiotics also affect the efficacy of ICB therapy in patients with cancer (93). It is no doubt that the administration of antibiotics can suppress or kill pathogenic micro-organisms within the host. Nevertheless, the unregulated use of broad-spectrum antibiotics may lead to antibiotic resistance, which in turn can lead to dysbiosis and even cancer development (94).

In addition to the above therapies, FMT, which originally originated from TCM theory more than 1700 years ago, has also become an acceptable method of modulating the gut flora to treat disease today (17). In the process of fecal microbiota transplantation, fecal suspensions from healthy individuals are transferred to the patient’s gastrointestinal tract in a variety of ways to re-establish new intestinal flora for the treatment of intestinal and extraintestinal diseases ( Figure 4 ) (95). On the one hand, it can be administered through the upper gastrointestinal tract such as the duodenal tube or oral capsules, on the other hand, it can be administered through the lower gastrointestinal tract, by colonoscopy or enema (96–99). Over the last decade, FMT has become relatively mature and received increased attention (100). Until now, FMT has published case reports for many conditions treated, ranging from celiac disease, constipation and cystitis (101–103). More than that, promising results have indicated that FMT also is effective in other digestive diseases, including colonized with MDROs, IBD, IBS, and neurological disorders including MS, HE, Parkinson’s disease, diabetic neuropathy (18, 21–25).

FMT has its unique advantages, as it not only upsurges microbial diversity but also establishes the cross-border balance between intestinal bacteria, viruses and fungi. In a clinical trial, patients with active left UC were randomly divided into FMT or 5-ASA enema groups showed that the gut microbial diversity was at a similar level in both groups, however, FMT remained effective over three months compared to 5-ASA (104). Similarly, a study using single-nucleotide variants in the subgenome monitored the strain population in fecal samples of patients with metabolic syndrome after FMT. It was also found that donor and recipient strains coexisted widely and lasted for 3 months after treatment. Although the success of colonisation of the recipient’s intestine by existing strains was higher than that of the newer species, the latter was also located within the fluctuating levels seen in similar time frames in health individuals (105). These both indicated that the donor bacterial strain persists in the recipient’s intestine for up to 3 months. Therefore, FMT is certainly an efficient means to modulate the host intestinal microbiota, which is considered to be a breakthrough in medical progress in recent years (18, 106). In particular, animal studies in the anti-cancer treatment show that FMT can be widespread applied (107–109). For example, FMT maintains the stability of the intestinal environment and improves DSS-induced colonitis in mice (110). In addition, Rosshart et al. reported that FMT enhanced host resistance to DSS or azomethane-induced colorectal tumorigenesis in 2017 (53).

Chemotherapy is a common method of treating cancer (132). Microorganisms and chemotherapeutic agents such as 5-fluorouracil and cyclophosphamide can interact with each other in both directions. On one hand, chemotherapy alters the composition of the patient’s microbial community, which may cause serious side effects in immunocompromised cancer patients (133, 134). On the other hand, the microbiome can metabolize drugs and modify anticancer drug efficacy (16, 135a; 136b). There is growing evidence that improving drug efficacy, increasing antitumor effects, and reducing toxic effects are the three main effects of intestinal microbes on chemotherapeutic drugs (111). Cyclophosphamide is an alkylating agent used in cancer therapy and is widely used in the treatment of leukemia, lymphoma, multiple myeloma, rheumatoid arthritis, and before bone marrow transplantation (109, 134). Gram-negative bacilli increase the antitumor effect of cyclophosphamide by increasing T cells infiltration in tumor sites (115, 137). In certain settings, the microbiota may also become a drug target to improve the side effects of many chemotherapeutic drugs on the gastrointestinal tract. Although the underlying molecular and cellular mechanisms of FMT are unknown, it may involve direct donor-intestinal microbiota-host interactions, thereby mediating the observed effects on host physiology, the intestinal mucosal barrier, and the immune system (138). In addition, animal experiments have shown that FMT improves the recovery rate of chemotherapy mice (139). Thus, FMT can effectively control the intestinal microbiota, enhance the efficacy of chemotherapy drugs, and reduce inflammation and toxic reactions.

Radiation therapy also performs an irreplaceable part in the treatment of many types of cancer (140). However, exposure to radiation can also damage healthy surrounding tissues (141, 142). Intestinal microbes have a key role in radiation-induced intestinal damage (143). In a mouse model, gavage of intestinal microorganisms attenuated and protected against radiation-induced injury. Specifically, FMT was able to improve irradiated mice recovery rate (107). The results of the current study suggest that gut microbial transplantation-mediated fluctuations in lncRNA expression profiles may play an important role in mitigating radiation-induced injury, which warrants further investigation. Thus, as a potential treatment to mitigate radiation toxicity, FMT may be used in oncologic radiotherapy to improve prognosis (107).

Although FMT is safe and easy to implement, concerns have been raised after a patient died from the procedure. The cause of death in patients receiving FMT is an invasive multi-drug resistant E. coli infection, although this E. coli is inherent in the feces of healthy donors (144). Identifying such pathogenic species and understanding mechanisms that promote their cohabitation is necessary for the development of effective and safe microbiome-based therapies, as exemplified in the present study (145). The composition of intestinal flora varies from patient to patient, and the effect of FMT treatment varies (146), which also suggests that the long-term impact of FMT not yet known and should be used with caution. For example, studies have shown that rumenococci affect the efficacy of FMT since donors with higher levels of rumenococci are more likely to fail the procedure (110). Meanwhile, when FMT is considered a treatment option, its safety remains an important issue (147). In a clinical trial on the safety of FMT, adverse effects such as abdominal pain, cramping or pressure, diarrhea, or constipation were reported in about 20% of patients in the FMT group, in contrast to an average of 2% in those on placebo. Moreover, two patients developed diverticulitis compared with none on placebo (148). It will be essential to conduct future controlled research studies exploring the safe, duration, dosing, formulation, administration route, and combinations of FMT to determine whether FMT can be used for cancer treatment (149).

When relevant institutions do stool testing of donors, drug-resistant bacteria screening must be performed (150). Recipients most at risk for adverse events are patients with poor immune status and severe intestinal ulcers. Patients with severe immune deficiencies should be first excluded. The U.S. FDA has mandated updated screening guidelines for fecal donations for FMT and requires public fecal banks to update their screening to ensure that all FMTs are properly screened for these pathogens (151). Eligible healthy donors are screened primarily through the following eight areas: age, physiology, pathology, psychology, accuracy, timing, life circumstances, and recipient (152, 153). The safety of FMT also requires long-term follow-up. With the current increasing emphasis on FMT therapy, standardized procedures in laboratories and in clinical working processes are essential to ensure the importance of the graft microbiota, the efficiency of FMT, and the reduction of underlying risks (116). To ensure the safe delivery of FMT and to provide appropriate access for those who require it, we recommend the use of fecal banks, such as NDFB, in order to meet the basic criteria to accomplish simple, secure, and cost effective FMT treatment (154). At the same time, FMT protocols should be optimized and standardized for different indications to further determine the long-term security of FMT. More importantly, COVID-19 caused by SARS-CoV-2 infection is still prevalent worldwide currently (155). It has recently been reported that SARS-Cov-2 RNA has also been detectable in the feces of infected individuals (156, 157), which suggests that it is essential to screen for COVID-19 before performing FMT.

Shortly, the microbiota will be an important asset in the diagnosis and FMT will also be widely used as a treatment for human diseases (146, 158). Autologous fecal microbiota transplantation (a-FMT) is also a form of FMT, which means the storage of human feces in a healthy state as an alternative to avoid pathogenic allogeneic feces or related risks and is used to restore the normal function of the original intestinal flora after the dysbiosis of its intestinal flora. Compared to common FMT, which has been extensively studied, a-FMT may be an underestimated effective treatment and exhibit long-term efficacy (159). Besides, with the rise of the selective microbiota transplantation (SMT) concept, further research should focus on identifying the precise strains and specific functions, and the SMT effect of two or more strains may be the emerging direction (116, 160). Colonic TET is a new form of FMT or SMT administration that not only facilitates patient access but also improves patient outcomes. Choosing the appropriate FMT delivery method according to the patient’s specific situation can meet the patient’s needs and reduce side effects. Towards this objective, the state-of-the-art and logical delivery method for MT is the colonic transendoscopic tube (TET). Colonic TET as a new modality for FMT or SMT administration that not only facilitates patient access but also improves patient outcomes (161, 162).

Using the gut microbiota as an adjuvant to anti-cancer treatment has attracted the interest of researchers in recent years. Patient response to anticancer therapy and the incidence of adverse events is related to the gut microbiota. Accumulating evidence emphasizes that FMT can improve the efficacy of chemotherapeutic agents and reduce related undesirable events. In oncology patients, toxicity associated with radiotherapy is often accompanied by a structured microbial community. Similarly, the type and amount of gut microbiota have a non-negligible effect on the host’s reaction to anti-PD-1 and anti-CTLA-4 immunotherapies. The combination of FMT with anti-PD-1 for the treatment of refractory metastatic melanomahas recently been shown to be safe, practical, and potentially effective. Several clinical studies in patients with advanced melanoma are assessing the potential of FMT-enhanced immune checkpoint blockade therapy. Overall, FMT is a hopeful method to improve the therapeutic effect of immunotherapy while reducing the side effects of chemotherapy by modulating the gut microbiota. In future studies, it is necessary to explore the safe, duration, dosing, formulation, administration route, and combinations of FMT to determine the optimal regimen of it for cancer treatment.

HX, ZL, and YR provided direction and guidance throughout the preparation of this manuscript. CC, HX and YR wrote and edited the manuscript. JR reviewed and made significant revisions to the manuscript. HX, SW, LL, CG, LW and ZL collected and prepared the related papers. All authors contributed to the article and approved the submitted version.

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.

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