Colorectal cancer (CRC) is the third most commonly diagnosed cancer in both men and women and the second leading cause of cancer-related deaths worldwide (Sung et al., 2021). The vast majority of CRC cases (1.9 million cases per year) are sporadic and can be attributed to various environmental factors (Islami et al., 2018). Cancer incidence in the large intestine is estimated to be 12-fold higher than that in the small intestine, which has been partially attributed to the greater bacterial density in the large intestine (Sun and Kato, 2016). In addition to host genetic factors, the gut microbiota plays an important role in CRC. An imbalance in the normal intestinal microbiota can promote chronic inflammation and carcinogenic metabolite production, ultimately leading to neoplasia (Marchesi et al., 2011).

Several bacterial species, including Helicobacter pylori, Escherichia coli, Bacteroides fragilis, Salmonella enterica, and Fusobacterium nucleatum, have been implicated in the development of CRC (Sun and Kato, 2016). A meta-analysis revealed a consistent increase in the prevalence of F. nucleatum in the tissue and fecal samples of patients with CRC compared to controls. Moreover, a high abundance of F. nucleatum in colorectal tumors was associated with poorer overall survival (Gethings-Behncke et al., 2020).

The anaerobe B. fragilis is a colonic symbiote that prefers mucosal colonization and accounts for only a small proportion of fecal microbiota (approximately 0.5%–1%). There are two molecular subtypes, nontoxigenic B. fragilis (NTBF) and enterotoxigenic B. fragilis (ETBF). According to some studies, ETBF is associated with both colitis and CRC (Basset et al., 2004; Toprak et al., 2006a; Dadgar-Zankbar et al., 2023). A review summarizes existing evidence for the association between ETBF and CRC as well as the current state of knowledge about the molecular mechanisms by which the B. fragilis toxin (BFT) influences the etiology of CRC (Scott et al., 2022). However, despite the increasing research on the relationship between ETBF and CRC, its role in the development of colorectal cancer remains largely uncertain (Zamani et al., 2020; Oliero et al., 2022). To our knowledge, no systematic reviews with meta-analyses have fully investigated the potential role of ETBF in CRC development.

This systematic review and meta-analyses of the published scientific literature aimed to assess (1) the prevalence of ETBF colonization in biological samples from individuals with CRC compared to healthy controls or adjacent normal tissues and (2) the relationship between the prevalence of ETBF and various stages of CRC.

After identifying 2126 references, we excluded 480 duplicate publications and 1595 irrelevant studies, leaving 51 potentially eligible studies ( Figure 1 ). Finally, 14 cohort studies (Toprak et al., 2006b; Boleij et al., 2015; Viljoen et al., 2015; Keenan et al., 2016; Haghi et al., 2019; Jasemi et al., 2020; Zamani et al., 2020; Khodaverdi et al., 2021; Piciocchi et al., 2021; Shariati et al., 2021; Oliero et al., 2022; Périchon et al., 2022; Matsumiya et al., 2023; Zhou et al., 2023) conducted between 2006 and 2023 were considered for the meta-analysis. Table 1 summarizes the general characteristics of the included studies. A total of 1692 patients were involved in these studies, with trial sizes ranging from 30 to 197 participants. Among these studies, two were from Europe, two from North America, six from West Asia, two from East Asia, one from South Africa, and one from New Zealand. Regarding the sample type, five studies used fecal samples, while nine used tissue samples. The detection method used was real-time polymerase chain reaction (PCR) in two studies, PCR in four studies, and quantitative PCR (qPCR) in eight studies. According to the quality assessment criteria, 11 studies were rated as good quality and 3 as fair quality.

The association between ETBF and CRC has attracted increasing interest. In this first comprehensive systematic review with meta-analyses of published literature, we aimed to investigate the relationship between ETBF and CRC, shedding light on its potential role in CRC development and progression.

Our findings suggest that ETBF is more prevalent in patients with CRC than in healthy controls, particularly in mucosal tissue or fecal samples from different individuals as shown by the subgroup analysis. The included studies revealed that ETBF colonization was more common in patients with CRC (6.1%–88.5%) than in healthy controls (3.8%–64.9%). This is consistent with previous research suggesting an association between ETBF colonization and CRC pathogenesis (Nouri et al., 2022). ETBF pathogenicity is attributed to BFT, a 20-kDa zinc-dependent metalloprotease toxin with three isotypes (BFT1, BFT-2, and BFT-3) (Sears, 2009). BFT binds to a specific colonic epithelial receptor, activating the Wnt and NF-κB signaling pathways, resulting in increased cell proliferation, epithelial release of proinflammatory mediators, and DNA damage (Sears, 2009; Goodwin et al., 2011), whereas ETBF promotes tumor formation in experimental animals (Wu et al., 2009; Goodwin et al., 2011).

Our analysis suggested that the detection rate of ETBF did not differ significantly between adjacent colorectal tissue samples and CRC tissue samples, but the next conclusion showed significant differences in ETBF prevalence between stage I/II and stage III/IV CRC. A previous study showed that ETBF supports the progression of malignancy as well as tumorigenesis (Kim and Lee, 2022). This suggests that ETBF may play a role in CRC initiation, and could possibly correlate with disease progression or severity. Therefore, this finding should be confirmed in larger cohorts.

Gut microbiota is a complex ecosystem that evolves in tandem with hosts and is influenced by their physiological environment. The composition and function of gut microbiota are closely associated with dietary habits and regional differences. Human dietary patterns have a direct impact on the abundance and diversity of gut microbiota. Diet is an important modifiable factor influencing the gut microbiome (Leeming et al., 2019). Furthermore, the proportion of plant-based and animal-based foods in the diet influences gut microbiota composition. The alteration in the abundance and diversity of gut microbiota caused by dietary changes has been associated with colorectal carcinogenesis (Appunni et al., 2021; Levy et al., 2021; Rebersek, 2021; Zygulska and Pierzchalski, 2022). Moreover, recent research has indicated that transitioning from a traditional to Western diet increases the abundance of CRC-associated bacteria (Ahmad Kendong et al., 2021). Another study demonstrated that switching from a traditional to Western diet increases the risk of CRC (Le Marchand and Kolonel, 1992). In our subgroup analysis of comparison between patients with CRC and healthy controls, the findings confirmed that individuals from different regions may have different outcomes due to differences in dietary habits.

These findings have two important implications. First, the increased prevalence of ETBF in the mucosal tissue or fecal samples of patients with CRC suggests that it can be used as a biomarker for CRC screening and diagnosis. Detection of ETBF may serve as an adjunctive tool in existing screening protocols to improve the sensitivity and specificity of CRC detection methods. Second, the consistent detection of ETBF in various stages of CRC highlights the need for additional research to determine its precise role in CRC pathogenesis. Understanding the mechanisms underlying ETBF-induced carcinogenesis may pave the way for targeted therapeutic interventions that disrupt the ETBF–CRC axis. However, a limitation is that the small number of studies prevented a formal assessment of publication or reporting bias, which may reduce the robustness of some meta-analyses involving subgroup analyses.

There is consistent evidence that ETBF is more prevalent in the fecal and tissue samples of patients with CRC than in healthy controls. Further prospective studies into the role of ETBF as a causal factor or predictive biomarker for CRC promotion and development are warranted.