Which bacterial group constitutes the predominant normal flora of the human intestinal tract? (Select the organism that is numerically dominant among obligate anaerobes in the gut.)

Introduction:
The human colon hosts a dense and diverse microbiota. This question tests recognition of the major bacterial group that predominates numerically in the large intestine and contributes to key metabolic and protective functions.

Given Data / Assumptions:

• We are comparing common gut inhabitants at the genus or group level.
• “Predominant” refers to numerical dominance in healthy adults.
• Focus is on typical conditions, not antibiotic exposure or disease states.

Concept / Approach:
Obligate anaerobes far outnumber facultative anaerobes in the colon. Among these anaerobes, species of the Bacteroides group are highly abundant and metabolically active, participating in complex carbohydrate breakdown and producing short-chain fatty acids that support colonic health.

Step-by-Step Solution:
Identify habitat: the colon is a low-oxygen environment where obligate anaerobes flourish. Compare groups: Bacteroides species form a large fraction of total bacteria, exceeding counts of facultative organisms like E. coli. Assess functions: Bacteroides metabolize dietary fibers and host glycans, generating acetate and propionate. Select the predominant group: Bacteroides best matches the description of numerically dominant flora.

Verification / Alternative check:
Microbiome surveys commonly report Bacteroides and Firmicutes as the leading anaerobic groups, with Bacteroides frequently dominant in Western-style diets, confirming the choice.

Why Other Options Are Wrong:

• Escherichia coli: Common but much fewer in number than anaerobes; often <1% of total.
• Clostridium perfringens: Present but not the numerically dominant clostridial species in healthy colons.
• Actinomyces: Primarily oral and mucocutaneous; not predominant in the colon.

Common Pitfalls:
Confusing visibility in culture (E. coli grows easily) with true abundance in vivo leads to overestimating facultative organisms.

Final Answer:
Bacteroides are the predominant normal flora of the human gut.