Tons of studies indicated that CRC-related microbiota can provide valuable insights into cancer occurrence, progression, and treatment response[13]. Disparities between TT and NT were primarily arise from individual taxonomic variations on the taxonomic profiling. In this study, by constructing community structures known as co-abundant groups (CAGs), we grouped all samples and examined the association of clinical characteristics with CAGs.
Compared with NT, a higher diversity of organisms in TT was observed as indicated by the Chao1 index of alpha diversity. PCoA analysis based on Bray-Curtis distance also demonstrated a significant differentiation between TT and NT. Similar findings were found in Loke’s studies (Loke et al., 2018)[14]. However, non-significant differences in microbial diversity (α- and β-diversity) between TT and NT were also reported (Liu et al., 2021; Li M. et al., 2020)[15, 16]. These discrepancies can be partially attributed to variations in geographical location and tumor heterogeneity. Regarding taxonomic profiling and discriminant taxa, we identified distinct taxa that can differentiate TT from NT. Specifically, Escherichia-Shigella, Fusobacterium, Streptococcus, Peptostreptococcus, Parvimonas, Klebsiella, and Gemella were significantly enriched in TT at the genus level. The enrichment of Fusobacterium and Streptococcus in TT has been consistently reported across numerous studies, highlighting their important role in tumor initiation and progression[17, 18]. Notably, Parvimonas exhibited a significant positive correlation with the host gene PARVB, which is highly expressed in CRC tissues[19]. Furthermore, Escherichia-Shigella, Peptostreptococcus, and Klebsiella were found to be enriched specifically in CRC patients compared to healthy volunteers in an investigation focusing on intestinal flora composition[20]. Gemella which predominantly resides within the oral cavity and upper gastrointestinal tract, was reported to be associated with oral squamous cell carcinoma[21]. In summary, the above findings suggest subtle differences in microbial diversity between TT and NT. Furthermore, both TT and NT exhibit unique taxonomic profiles, each characterized by a dominant genus.
Recognizing that a single taxonomic group might not fully capture microbial differences between TT and NT, we applied hierarchical clustering based on Bray-Curtis distance to construct four co-abundance groups (CAG 1–4). These constructed CAGs were then used for unsupervised clustering of TT and NT samples, resulting in the classification of all samples into six major categories (sample group 1–6). CAG 2 was notably enriched in TT tissues, while CAG 4 was enriched in NT. Sample group 3 and sample group 5 contained predominantly NT, whereas sample group 4 and sample group 6 contained predominantly TT. The CAGs level analyses revealed that sample group 4 and sample group 6 exhibited a higher abundance of CAG 2, sample group 3 had an increased abundance of CAG 3, and sample group 5 was enriched with CAG 4. Further examination revealed that the abundance of CAG 1 in sample group 1 was exceptionally high, nearly 100%, resulting in a low abundance of the remaining CAGs. To address this question, we excluded sample group 1 and conducted the same analysis with the remaining samples. In the remaining samples, we found that CAG 3 was significantly increased in NT. Additionally, CAG 3 and CAG 4 exhibited a positive correlation, though not statistically significant, and both were negatively correlated with CAG 2. Previous studies have demonstrated the feasibility of classifying experimental subjects using bacterial abundance or CAGs. For instance, in a study on colorectal cancer and adjacent normal tissues, K-means clustering was employed to divide the samples into three distinct subgroups[12]. Another similar study clustered the Operational Taxonomic Unit (OTU) hierarchy into six CAGs, subsequently categorizing the samples into multiple distinct subgroups, a process replicated in two additional cohorts[10]. In our study, CAG 1 comprises a substantial number of nonpathogenic or opportunistic pathogens that were widely found in nature or the human body, including Bacteroides, Delftia, Enterococcus, Klebsiella, Proteus, and Pseudomonas[22–27]. CAG 2 includes Fusobacterium, Streptococcus, Peptostreptococcus, and Parvimonas, which were reported to promote the occurrence and progression of CRC in various studies. Interestingly, these four genera were assigned to the same CAG which was considered a pathogenic bacterial cluster in the study by Flemer et al.[10]. Campylobacter in CAG 2 was reported to be associated with colorectal and esophageal cancer[28]. In CAG 3 and CAG 4, we identified more bacteria that are considered to be probiotic or nonpathogenic such as Akkermansia, Alistipes, Bifidobacterium, Blautia, Collinsella, Faecalibacterium, Parabacteroides, Prevotella, Bacillus, and Lactobacillus[29–38]. Hence, human diseases can be attributed not only to a single pathogen but also to overall changes in the microbiota[39]. For instance, a study on breast cancer described the combination of estrogen in the liver, excretion into the gastrointestinal cavity, conjugation by bacterial β-glucuronidase, reabsorption as free estrogens through the enterohepatic circulation, and distribution to different organs like the breast. These metabolites, produced by several bacteria from the Clostridia and Ruminococcaceae families through estrogen metabolism, may collectively have breast cancer-causing potential[40]. These findings offer insights into flora changes during the transformation from NT to TT in a higher dimension, explore bacterial interaction from the bacterial clusters, and provide clues to the mechanism of the multi-bacterial joint promotion of CRC occurrence and development.
Based on the previously constructed CAGs, we further investigate the association of CAGs in TT with clinical characteristics. CAG 2 was found to be associated with the microsatellite status of tumors, exhibiting higher abundance in TT associated with unstable microsatellites. Previous studies conducted in Japan and the United States have demonstrated a significant correlation between the positive expression of F. nucleatum and unstable microsatellite status[41]. Notably, Fusobacterium was exactly in our CAG 2. Furthermore, CAG 4 exhibited a positive correlation with CA199 levels. CA199 is a typical marker for gastrointestinal tumors and has high sensitivity for pancreatic cancer diagnosis, as well as aiding in rectal cancer, colon cancer, and primary liver cancer detection[42]. In intrahepatic cholangiocarcinoma cases, Bacillus anthracis and P. azotoformans were observed to be positively associated with CA199 levels[43]. Notably, bacillus was exactly in our CAG 4.
In addition to compositional changes in bacterial taxa, we also observed predicted functional alterations across different groups. We found that the following metabolic pathways including nucleotide metabolism, lipid metabolism, enzyme metabolism, energy metabolism, carbohydrate metabolism, and amino acid metabolism were enriched in the NT group. Similar findings were reported in previous studies[44–46]. Our findings suggest that microbial changes may impact multiple metabolic pathways including amino acid, lipid, and carbohydrate metabolisms which could potentially underlie the transition from NT to TT.
Our research boasts a relatively substantial sample size, contributing to the generation of robust and reliable findings. However, several limitations still require attention. First, in our cluster analysis, two sample groups could not be accurately classified, possibly due to the heterogeneity of tumor samples in terms of location and subtype. Previous studies have highlighted differences in microbial composition between CRC originating from different locations or subtypes[47, 48]. Second, cross-sectional studies emphasize the need for prospective trials to fully elucidate the role of microbiota in CRC. Lastly, as we employed 16S rRNA gene sequencing for microbiota analysis, we were unable to determine species-level composition and actual genetic functions. Further investigations utilizing shotgun metagenomic sequencing are warranted to unravel the mechanisms underlying CAGs and CRC.
In summary, our research will deepen our understanding of the interactions among multiple bacteria and offer insights into the potential mechanism of NT to TT transition.