Gram-negative bacillus bacterium species, and an obligate anaerobe of the gut.
B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections (especially those that originate from the gastrointestinal flora).
B. fragilis is the most prevalent organism in the B. fragilis group, accounting for 41% to 78% of the isolates of the group.
These organisms are resistant to penicillin by virtue of production of beta-lactamase, and by other unknown factors.
B. fragilis is involved in 90% of anaerobic peritoneal infections.
Important considerations: The choice of an agent should be based on local antimicrobial sensitivities, site of infection, cost, and comorbid conditions. Generally, the most common agents/regimens are listed first.