Due to its high acidity, the stomach was once regarded as a sterile organ and unsuitable for microbial colonization. However, the discovery of HP led to the realization that some microbes could indeed colonize the stomach (2). Advancements in high-throughput sequencing technologies in microbiology have facilitated the discovery of an increasing number of exogenous and endogenous microbiota. The normal human stomach harbors a diverse microbiota, with over 130 lineages representing 7 to 13 bacterial phyla, of which 5 are the most dominant, including Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, and Fusobacteria (10, 11). Fungi, a common yet easily overlooked component of gastric microbiota, have also been identified. Investigations of the gastric environment in healthy individuals, patients with chronic gastritis (CG), and gastric ulcers have detected hundreds of fungal strains in the stomach. Fungi may possess an effective acid tolerance mechanism, enabling them to proliferate more efficiently in a gastric environment with a pH of only 1.4 compared to bacteria. Researchers have detected fungal species, such as Candida tropicales and Candida lusitanae, in addition to the most common Candida albicans. Fungi usually colonize areas like the gastric mucosa and stomach contents (12, 13).

Both bacteria and fungi in the human stomach play crucial roles in various physiological activities, including energy metabolism, nutrient absorption, immune response, and nervous system regulation (14). However, some studies have observed differences in the flora of the gastric mucosa and gastric juice (8, 10, 11, 15, 16). Due to swallowing in the oral cavity and reflux in the duodenum, the gastric juice may contain more bacteria than the gastric mucosa (17). Although most current gastric flora studies have sampled the gastric mucosa, bacteria in the gastric juice should not be overlooked. These bacteria can contribute to the proper functioning of the gastrointestinal system but may also cause some adverse reactions.

The results are diverse for changes in the diversity and abundance of microbiota in gastric cancer. Some studies showed that the abundance and diversity of the gastric microbiota did not change gradually with the development of gastric cancer (18, 19). Other studies have taken the opposite view, suggesting that the abundance and diversity of the microbiota decrease as gastric cancer progresses, which is what most studies have concluded (20–22). Another study has shown a significant increase in the abundance and diversity of the microbiota in cancerous tissues compared to paracancerous tissues. They also found that oral bacteria (such as Peptostreptococcus, Streptococcus, and Fusobacterium) dominated the cancerous tissues, and lactic acid-producing bacteria (such as Lactococcus lactis and Lactobacillus brevis) were enriched in the paracancerous tissues (23). However, Liu et al. found that Prevotella melanogenic, Streptococcus anginosus and Propionibacterium acnes were enriched in cancerous tissues compared to normal and paracancerous tissues. In contrast, the abundance of HP, Prevotella copri and Bacteroides uniformis was significantly reduced in cancerous tissues (24).

Many current studies exist on the microbiota of gastric cancer patients and controls. However, their results varied widely, and some were even contradictory, possibly due to different sample types, study populations, sequencing technologies and analysis methods ( Table 1). However, it is undeniable that there is a significant difference in the microbial composition of gastric cancer patients compared to controls. In general, the microbiota of gastric cancer patients shows a decreased abundance of HP, a significant enrichment of the oral bacteria, and reduced diversity and abundance of the microbiota.

HP is a gram-negative bacterium that parasitizes the human stomach. The colonization of HP can lead to dysbiosis. The diversity and abundance of the gastric microbiota decrease after infection with HP (27). HP-positive gastric cancers demonstrate an abundance of Lactobacillus, Achromobacter, Citrobacter, Clostridium, and Rhodococcus but lack HP and Neisseria compared to other gastric diseases (26). A recent meta-analysis suggests that eradicating HP can restore the diversity of the gastric microbiota and reduce the risk of gastric cancer (28). However, some studies contradict this result by suggesting that eradicating HP does not entirely prevent the development of gastric cancer, which indicates that other microorganisms in the stomach besides HP are also associated with gastric cancer carcinogenesis (29). Lertpiriyapong et al. demonstrated this hypothesis with INS-GAS mice. They found that HP mice colonized by complex or restricted microbiota were more susceptible to HP-induced gastric cancer than germ-free mice (30). This result suggests that after HP infection of the gastric epithelium causes dysbiosis, HP acts synergistically with the dysbiotic microbiota to promote gastric carcinogenesis.

Although HP is recognized as a type 1 carcinogen by the World Health Organization (WHO), not all gastric cancers are caused by HP infection. HP-negative gastric cancers also account for a percentage ranging from 0.7% to 47.8% of gastric cancer patients (31). The role of microbiota in the development of HP-negative gastric cancer has attracted increasing attention. In HP-negative gastric cancer, Kim et al. found there was a significant increase in the abundance and diversity of Lactobacillaceae, Streptococcaceae, and Prevotellaceae and a decrease in the numbers of Burkholderiaceae, Haemophilus, and Campylobacter (32) ( Figure 1). These findings are inconsistent in related studies, but the increased abundance of Lactobacillaceae was detected in almost all patients with gastric cancer. Lactobacillaceae is a nitrosating bacterium, and Haemophilus is a nitrate-reducing bacterium. An increase in the abundance of Lactobacillaceae and a decrease in the abundance of Haemophilus can increase the formation of N-nitroso compounds, which can cause persistent inflammation and promote the development of gastric cancer (32). Ding et al. also concluded that HP-negative gastric cancer occurs due to disruption of the normal microbiota, with elevated levels of some bacteria with pro-cancer activity and decreased levels of cancer-suppressing bacteria (33). In other words, it is not a certain kind of bacteria that causes gastric cancer alone. Instead, the microbiota in the stomach undergoes dysbiosis, and the various dysbiotic microbiota work together to participate in the development of gastric cancer.

The oral and gastric environments are linked through swallowed saliva and ingested food. The oral microbiota is a major source of gastric microbiota (95). Many studies have found that dysbiosis of the oral microbiota is associated with gastric carcinogenesis. Oral bacteria such as Parvimonas, Eikenella and Prevotella-2 were significantly enriched in gastric cancer compared to other precancerous stages (18). The abundance of oral bacteria such as Peptostreptococcus, Streptococcus, and Fusobacterium was higher in cancerous tissues than in paracancerous tissues (23). Network analysis revealed that oral microorganisms such as Peptostreptococcus stomatitis, Slackia exigua, Parvimonas micra, Streptococcus anginosus, and Dialister pneumosintes might play critical roles in the development of gastric cancer (25). Under normal conditions, most of the oral microbiota entering the stomach is either destroyed by gastric acid or protected by the mucus-bicarbonate barrier from invading the gastric epithelium. However, certain disease states, such as gastric ulcers and gastric cancer, can damage the gastric mucosa and neutralize gastric acid, allowing oral bacteria to colonize the gastric mucosa from gastric juices (17). Recently, some studies have also been on the interaction between oral microbiota and HP. On the one hand, HP infection can disrupt the homeostasis of the oral microbiota (96). On the other hand, ectopic colonization on the gastric mucosa by the oral microbiota may, in turn, be necessary for HP-induced ecological dysbiosis (95).

Since the pH in the intestine is more conducive to microbial colonization, intestinal bacteria are nearly 10⁷ times more abundant than those in the stomach (16, 97). Approximately 99% of the 10¹⁴ microorganisms that comprise the human microbiome reside in the intestine (98). Numerous studies have demonstrated that intestinal flora can directly and indirectly affect the development, treatment, and prognosis of gastric cancer.

In a study conducted in Shanxi province, researchers investigated changes in the gut microbiota of 116 patients with gastric cancer and 88 healthy controls. They observed an increase in the abundance of intestinal flora, a decrease in butyrate-producing bacteria, and a significant enrichment of Lactobacillus, Escherichia, and Klebsiella in patients with gastric cancer (99). According to Sarhadi et al., Enterobacteriaceae were abundant in stool samples from patients with all types of gastric cancer (100). Enterobacteriaceae may serve as a potential biomarker in the diagnosis of gastric cancer.

Certain metabolites of intestinal bacteria, such as acetic acid and butyrate, also influence the development of gastric cancer. Acetic acid and butyrate are the major short-chain fatty acids, and intestinal bacteria such as Eubacterium, Clostridium, Ruminococcus, and Coprococcus can produce butyrate (35). Increasingly, research has found that butyrate plays a crucial role in inhibiting gastric cancer development. First, butyrate inhibits the Warburg effect of gastric cancer by binding pyruvate kinase M2 (PKM2), increasing the content of glucose intermediates in mitochondria, preventing the conversion of tricarboxylic acid cycle intermediates to ATP, and ultimately depriving tumor growth of sufficient energy, thus inhibiting gastric cancer development. Second, butyrate can directly interfere with the mitochondria of gastric cancer cells, upregulate oxidative stress, and significantly increase the level of ROS. Moreover, butyrate can promote caspase 9 production and inhibit BCL-2 synthesis, leading to the apoptosis of gastric cancer cells (101). Lastly, a recent study discovered that butyrate could inhibit the growth, migration, and invasion of gastric cancer cells and aerobic glycolysis by blocking the Wnt/β-catenin/c-Myc signaling pathway (102).

Probiotics, such as Bifidobacterium, Lactobacillus, and Saccharomyces, are active microbes that benefit the host’s health (107, 108). There is growing evidence that probiotics may inhibit the growth of gastric cancer to some extent. Probiotics can significantly reduce inflammatory responses, enhance the immune system, promote tumor cell apoptosis, restore gut microbiota homeostasis, and inhibit cancer signaling pathways (35, 109, 110). Lactobacillus can inhibit the development of gastric cancer by reducing the expression of NF-κB and the phosphorylation of the PI3K/Akt/mTOR signaling pathway (111). Bifidobacterium can directly induce DC maturation and cytokine (IFNγ, TNFα, IL-10, IL-17) production, thus promoting anti-tumor immunity and anti-PD-L1 efficacy and almost inhibiting tumor progression after combination therapy with PD-L1 monoclonal antibody (mAb) (104). Bacteroides thetaotaomicron and Bacteroides fragilis (B. fragilis) are also potential probiotics. Vétizou et al. found that the antitumor and immunotherapeutic effects of CTLA-4 blockade were associated with Bacteroides thetaotaomicron and B. fragilis. Oral administration of Bacteroides thetaotaomicron or B. fragilis to germ-free mice enhances the antitumor effects of CTLA-4 blockers (112). B. fragilis also increase macrophage phagocytosis, prompting them to polarize towards an M1 phenotype (113).

Probiotics have become a prominent research topic due to their beneficial effects on gastric cancer. Immunotherapeutic probiotics could be developed to improve the efficacy of immunotherapy (114). However, contrary to the aforementioned findings, it is relatively easy to identify inconsistencies. The role of Lactobacillus in gastric cancer remains unclear. In addition to Lactobacillus, there are inconsistent findings on the effects of other probiotics on gastric cancer. Based on human microbiome studies and animal models, researchers have cautioned against the direct use of Lactobacillus in treating patients with gastric cancer (81).

FMT is transplanting fecal material from a healthy donor into a recipient’s intestine, ultimately altering the recipient’s intestinal flora to match the donor’s microbial profile (115). Several methods of fecal microbiota transplantation exist, including oral, nasal, and rectal administration. Rectal administration is preferred, as nasal administration may cause pulmonary or gastrointestinal complications, and although oral administration is convenient, careful avoidance of first-pass effects is necessary (116, 117). Given FMT’s numerous benefits, there is growing interest in its potential for enhancing cancer immunotherapy.

Routy et al. discovered that the aberrant composition of intestinal flora results in primary resistance to immune checkpoint inhibitors (ICIs) in patients with late-stage cancer and that antibiotics interfere with the therapeutic efficacy of ICIs in patients. Their findings revealed that the anti-tumor effects of PD-1 inhibitors are more effective in germ-free mice or those not treated with antibiotics after FMT from cancer patients who respond to ICIs. However, this was not the case in the FMT of patients who did not respond to ICIs. They also analyzed the metagenomics of fecal material from patients and found a correlation between the efficacy of ICIs and the abundance of Akkermansia muciniphila (A. muciniphila). After FMT, patients who did not respond were given A. muciniphila can restore the efficacy of PD-1 inhibitors (103). Fecal material from immunotherapy responders was transplanted into germ-free mice, resulting in slower tumor growth and increased immunotherapy efficacy (105). In summary, FMT can enhance patient responses to immunotherapy.

Since FMT has a more substantial impact on intestinal flora than gastric flora, most current research focuses on the effect of FMT on intestinal diseases. However, researchers should also consider FMT as a treatment for gastric cancer and precancerous lesions in the stomach. As a safe and effective therapy, FMT may treat gastric cancer in the future by interfering with the composition of the intestinal flora. However, we need further studies to evaluate FMT’s safety and reduce the risk of side effects.

Bacteriophages are viruses that selectively infect bacteria (118) and are more than 100 times more abundant in the gut than in bacteria and human cells (119). Given that most bacteriophages are particular to specific pathogenic bacteria but do not disrupt the homeostasis of normal flora (120), many studies have constructed bacteriophage-based biotic–abiotic hybrid nanosystems to accurately remove tumor microorganisms (121).

In order to precisely regulate intestinal flora to treat colorectal cancer, Zheng et al. loaded irinotecan inside dextran nanoparticles (122). And through bioorthogonal reaction, it was covalently linked with azide-modified phages that could specifically lyse F. nucleatum, which could not only effectively inhibit the proliferation of F. nucleatum, but also reduce the toxic side effects caused by the non-targeted release of conventional chemotherapeutic drugs. Moreover, phages can also remodel TME. F. nucleatum selectively amplifies MDSCs, thereby blocking the body’s anti-tumor immune response. Through phage display technology, Dong et al. screened M13 phage that can specifically bind F. nucleatum (123). They further assembled silver nanoparticles on the surface capsid protein of the M13 phage (M13@Ag). Subsequently, it was also demonstrated that M13@Ag could specifically clear F. nucleatum and inhibit the proliferation of MDSCs, reversing the immunosuppressive TME. In addition, the M13 phage utilizes its coat protein to activate antigen-presenting cells, which further activates the host immune response and inhibits CRC. Animal experiments demonstrated that M13@Ag combined with immune checkpoint inhibitors or chemotherapeutic agents significantly prolonged the survival of CRC mice. A recent study on phage strategies targeting HP may open new avenues for treating HP-positive gastric cancer (124).

Diet has an important influence on the composition of the gut microbiota. Different dietary habits can lead to differences in the composition of the gut microbiota (125). Compared to omnivores, the intestinal flora of strict vegetarians was characterized by a higher abundance of Bacteroidetes and a lower abundance of Firmicutes (126). However, the opposite alteration of the intestine flora was observed in people on chronic high-fat diets (127). Therefore, the composition of the intestinal flora can be adjusted by dietary interventions, thereby creating a more favourable microecological environment for the host. Dietary interventions such as fasting and calorie restriction have been used in clinical trials for various cancers, and the results have shown that dietary interventions can alter the systemic metabolism of cancer patients, improve anti-tumor immunity (128), and inhibit tumor growth. For example, in an ongoing clinical trial (NCT01642953), researchers restrict patients’ diets after gastric cancer surgery to assess whether fasting promotes recovery and reduces mortality and adverse events (129). Dietary interventions can also inhibit inflammation, improve anti-cancer immune surveillance (130) and prevent chemotherapy-induced toxic side effects (131).

WHO defines the concept of prebiotics as “an inactive food ingredient that promotes the health of the organism by modulating the composition of the microbiota (107).” Common prebiotics are fructooligosaccharides (FOS), galactooligosaccharides (GOS) and inulin (132). Prebiotics can help the body resist pathogenic infections, maintain stable gastrointestinal function, promote the production of short-chain fatty acids by beneficial bacteria, regulate immunity and energy metabolism, and increase the absorption of micronutrients, thus improving the efficacy of cancer treatment (132–134). To demonstrate the role of prebiotics in cancer treatment, many preclinical studies and clinical trials are underway or have been completed. One clinical trial in the United States (NCT04682665) is investigating eicosapentaenoic acid’s role in treating liver metastases from colorectal cancer (135). Furthermore, It has been shown that a high intake of Ganoderma spp (136). Polysaccharides and raffinose (137) potentially reduce the risk of gastric cancer, but more extensive independent studies and clinical trials are needed to confirm this.