Which is the most common species within the Bacteroides fragilis group that causes human infection?

Introduction:
Bacteroides are anaerobic, gram-negative rods that are part of normal gut flora but can cause serious opportunistic infections. Within the fragilis group, one species is most frequently implicated in human disease.

Given Data / Assumptions:

• Asked to identify the most commonly pathogenic species in the fragilis group.
• Clinical relevance focuses on isolates from intra-abdominal and soft tissue infections.

Concept / Approach:
Bacteroides fragilis possesses a polysaccharide capsule and multiple virulence factors that promote abscess formation, immune evasion, and tissue invasion, making it the leading pathogen in the group.

Step-by-Step Solution:
Identify the clinical setting: intra-abdominal sepsis and abscesses frequently yield B. fragilis. Compare species within the group: B. vulgatus, B. ovatus, and B. merdae are less frequently isolated as primary pathogens. Select the species most consistently associated with human infections: Bacteroides fragilis.

Verification / Alternative check:
Standard microbiology references consistently list B. fragilis as the most clinically significant member of its group.

Why Other Options Are Wrong:

• B. vulgatus / B. ovatus / B. merdae: Gut commensals, less often primary pathogens.
• B. thetaiotaomicron: Important commensal; pathogenic less frequently than B. fragilis.

Common Pitfalls:
Equating prevalence in the gut with clinical dominance in infections; virulence traits, not mere abundance, determine pathogenic prominence.

Final Answer:
Bacteroides fragilis is the most common human pathogen in the fragilis group.