The variability of microbial population of gastrointestinal tract is currently correlated to the occurrence of different disorders including colorectal cancer. Though several advanced researches have been conducted in developed countries by using genomic and metagenomic approaches to characterize microbial cells in feacal or mucosal specimens, there is no any published data related to the overall microbiota profile of mucosal or feacal specimens of CRC patients in Ethiopia. Therefore, the current study was aimed at determining the distribution of at least cultivable aerobic bacterial microbiota of cancerous and normal-featuring tissues of CRC patients.
The dysbiosis of bacterial microbiota abundance and distribution in malignant tissues from adjacent normal biopsies is currently become an indicative in the diagnosis and prognosis of CRC patients. These alterations are also demonstrated in our study by the presence of abundant bacterial microbiota in normal biopsies [x̄=4.0x105 CFU/ml] while much smaller bacterial population [approximately 2.0x105 CFU/ml less] from malignant tissue biopsies of CRC patients (Figure 1).
In this study, we found higher abundance of bacterial composition of phyla; Proteobacteria (55.6%), Firmicutes (33.3%) and Fusobacteria (11.1%) in normal biopsies of CRC patients (Figure 2). However, it is much different from a study reported by Eckburg et al. (14), in which 90% of bacterial composition of normal luminal microbiota belongs to the phyla; Firmicutes and Bacteriodes, the remaining minor constituents were Proteobacteria and Actinobacteria. On the other hand, a review study by Villeger and colleagues (15) presented a clear increase in both Proteobacteria and Bacteroidetes in tissues from patients with colorectal adenoma compared with tissues from healthy volunteers.Among members of phylum Fusobacteria (16), only a genus Streptobacillus in the family Leptotrichiaceae was isolated from normal tissues of CRC patients (Figure 2). It could be due to the alternative method we employed, candle jar for fastidious bacterial cultivation, probably supported the growth of microaerophilic bacteria. Other most fusobacterial members strictly require anaerobic environment to grow (17) and are associated greatly with cancer tissues than in normal tissues (18). Despite the genera Bacteriodes (19), Leptotrichia species (20)(21) and Fusobacteria (19)(22) were the most frequently identified and reported bacteria from malignant tissues of colorectal cancer, our study didn’t show any of the above mentioned species while we employed candle jar cultivation.
According to the author Lau et al. (23), Streptobacillushongkongensis is a novel bacterial species that permanently found in human oropharynx and there might be more other Streptobacillus species probably also residing in human oropharynx. This genus might get easy access to the lumen of the colorectal regions (24).
The microbial abundance of family Bacillaceae in malignant biopsies (26%) was lower than the abundance in non-malignant tissues (39%) while the family Enterobacteriaceae, a member of phylum Proteobacteria (25) was over-represented (45%) from malignant group of tissue (Figure 4). This observation could be supported by the fact that family Enterobacteriaceae is considerably a member of the carcinogenic bacteria that constitute Lipopolysaccharides (LPS), D-Lactate and other bacterial components which positively correlated with the incidence and progression of inflammatory bowel diseases (IBD) as well as colorectal cancer (26)(27)(28).
More recently, studies (15)(29) indicated a drastic difference in the microbial composition of colitis-associated CRC patients, with an increase in the family Enterobacteriaceae compared with the mucosa of sporadic CRC patients. For instance, colibactin-producing E. coli were more frequently identified in microsatellite stable CRC and higher colonization by negative-colibactin E. coli bacteria were detected in patients with microsatellite instability CRC (15).
Our study also revealed that significant abundance of family Enterococcaceae was identified only from malignant biopsies (Figure 4). This finding supports previously reported evidences that patients with ulcerative colitis and Crohn’s disease have larger members of family Enterococcaceae than healthy controls (30)(31)(32). Furthermore, genus Enterococcus is among certain bacterial species that has been identified to play a key role in the incidence and development of colorectal cancer (15)(29). In this genus, Enterococcusfaecalis can even trigger macrophages and other immune cells to produce procarcinogenic enzymes capable of damaging target cell DNA that contribute to CRC carcinogenesis (33)(34).
The imbalance of these bacteria and their gene products (35)(36) that underlies mucosal surface of intestinal microvilli would facilitate the replication of opportunistic pathogens which might have direct contribution in the onset and progression CRC. Hence, these findings could be a base line information for future investigations focusing on potential pro-oncogenic pathogens of gastrointestinal cancers in the study area.
The current study used intact biopsies of both malignant and adjacent normal-appearing tissues of study participants where most faecal specimen microbiome studies might not show typical characteristics of adherent gut microbiota of the colorectal cancer patients.
Since our study employed a culture-based aerobic cultivation, huge segment of mucosal-associated microbiota; obligate anaerobes, fungal agents and uncultivable microbes were not addressed. Microbial distributions in relation to anatomic positions of colorectal biopsies, cancer stage, anticancer or antibiotic use, comorbid diseases and long term dietary habit were not well considered. However, with these limitations, the study will provide base line information for future development of culture independent studies of gut microbiota in the study area.