Question
Bacteroides fragilis
Answer
Bacteroides fragilis is a Gram-negative, non-spore-forming, obligate anaerobic bacillus, and the most commonly isolated anaerobic pathogen from human clinical specimens — part of the normal colonic flora but an important opportunistic pathogen when it escapes the gut lumen.
Pathogenesis/virulence factors: possesses a distinctive polysaccharide capsule, which is antiphagocytic and also promotes abscess formation (a relatively unique virulence property among anaerobes — capsular polysaccharide complex directly stimulates abscess formation even in the absence of viable bacteria); produces various enzymes (e.g., superoxide dismutase and catalase, conferring some aerotolerance and helping the organism survive brief oxygen exposure during spread from the gut) and a potent fragilysin (Bacteroides fragilis toxin, BFT) in enterotoxigenic strains, implicated in diarrhoeal disease and possibly colorectal carcinogenesis.
Clinical significance: the leading cause of anaerobic intra-abdominal infections — following bowel perforation, surgery, or trauma, it causes peritonitis and intra-abdominal/pelvic abscesses, often as part of a polymicrobial infection together with facultative anaerobes (e.g., E. coli); can also cause bacteraemia, particularly from an abdominal or pelvic source.
Laboratory diagnosis: requires strict anaerobic culture technique (anaerobic jar/chamber, appropriate transport media to prevent oxygen exposure); characteristic growth on selective media containing kanamycin and vancomycin (which many other anaerobes are susceptible to but B. fragilis resists); identification by biochemical profile, gas-liquid chromatography of metabolic products, or MALDI-TOF.
Treatment: notably resistant to penicillin (produces beta-lactamase); metronidazole, carbapenems, beta-lactam/beta-lactamase-inhibitor combinations (e.g., piperacillin-tazobactam), and clindamycin (though resistance is increasing) are used, generally combined with agents active against accompanying facultative Gram-negative organisms in mixed intra-abdominal infections.