Which organism predominates in the fecal flora as part of the indigenous colon microbiota?

• A. Escherichia coli
• B. Bacteroides fragilis group
• C. Lactobacillus
• D. Clostridium difficile

Answer: (B)

In the healthy colon, the indigenous fecal flora is dominated by obligate anaerobes, with the Bacteroides fragilis group being the most abundant component. These bacteria are well adapted to the anaerobic, bile-rich environment of the large intestine and play a major role in breaking down complex polysaccharides and helping maintain colonization resistance. Escherichia coli is a common resident but is a facultative anaerobe and does not outnumber the Bacteroides group in the colon. Lactobacillus species are more characteristic of other niches, such as vaginal or upper GI mucosa, and are not the major inhabitants of colonic lumen. Clostridium difficile is not part of the normal indigenous flora; it is a potential pathogen that can overgrow when the normal balance is disrupted. Thus, the Bacteroides fragilis group best represents the predominant fecal flora of the indigenous colon microbiota.