Novel ecological niche of Cetobacterium somerae, an anaerobic bacterium in the intestinal tracts of freshwater fish

Aims: This study was conducted to clarify the taxonomic status of Bacteroides type A strains with high vitamin B12-producing ability that is widely distributed in the intestinal tracts of freshwater fish. Methods and Results: Seventeen strains of Bacteroides type A isolated from five fish species were all rod-shaped and gram-negative. The strains were positive for esculin hydrolysis, nitrate reduction, resistance to bile, acid phosphatase, and negative for the production of catalase and urease and the susceptibility to vancomycin. The G+C content of DNA from the 17 strains was 29·1–31·9 mol%, and 16S rDNA sequence analysis revealed a close phylogenetic relationship between Bacteroides type A strains and Cetobacterium somerae sharing 99·7–100% sequence similarity. In addition, strains were capable of producing vitamin B12 at a rate of 1·82–13·98 ng ml−1 in 48 h. Conclusion: Phenotypic and phylogenetic characteristics indicated that all isolates previously classified as Bacteroides type A strains belong to C. someare. Significance and Impact of the Study: This study provided the important finding of novel niche of vancomycin-resistant bacteria such as C. somerae in the intestinal tract of freshwater fish.