Bacteroides fragilis

1. GC 105 Expert Practice Tip 6: "Consider adding metronidazole in patients with severe intra-abdominal sepsis to cover Bacteroides fragilis" — this exact tip also appears in the Gen Clerk Microbiology Summary as point #6. This is extremely high-yield for MCQs asking about empirical antibiotic coverage in intra-abdominal sepsis ^[1][2].

2. GC 105: B. fragilis is specifically highlighted as a "GI tract coloniser often found in complicated intra-abdominal infections" with "increasing antimicrobial resistance" ^[1]. Expect MCQs testing that B. fragilis is resistant to standard penicillins and cephalosporins but susceptible to metronidazole and carbapenems.

3. GC 105: Anaerobes are described as "part of polymicrobial infections & abscesses" — B. fragilis is the prototypical anaerobe in mixed intra-abdominal infections. The polymicrobial nature is a key discriminator from SBP (monomicrobial) ^[1][3].

4. Acute cholangitis bacteriology (Felix Lai Medicine & Surgery notes): B. fragilis is listed as one of the causative Gram-negative organisms alongside E. coli, Klebsiella, and Enterobacter. Acute cholangitis questions commonly ask for the bacteriology — do not forget the anaerobic component ^[4].

5. Secondary vs spontaneous bacterial peritonitis (senior notes): Polymicrobial culture growth (including Bacteroides) is the most important feature distinguishing secondary bacterial peritonitis from SBP. SBP = single organism that translocated; secondary = direct bowel inoculation ^[3].

6. Aminoglycosides do not cover anaerobes — aminoglycosides require aerobic electron transport for drug uptake. This is a commonly tested principle and appears in the context of PD peritonitis and intra-abdominal infection antibiotic choices.

• B. fragilis vs Fusobacterium: Both are G−ve anaerobic rods. Fusobacterium → Lemierre's disease (internal jugular vein thrombophlebitis after pharyngeal infection); B. fragilis → intra-abdominal infections. GC 105 lists both side-by-side ^[1].
• SBP vs secondary peritonitis: If a cirrhotic patient's peritoneal fluid grows polymicrobial organisms (especially with anaerobes like Bacteroides), this is secondary peritonitis → look for a surgical cause (perforation) ^[3]. SBP grows a single organism and is treated medically.
• Metronidazole vs carbapenems: Metronidazole is the classic drug for anaerobic coverage. Carbapenems (except ertapenem for Pseudomonas) also cover anaerobes but are broader — exam may test why metronidazole is "added" rather than switching to a carbapenem for cost/stewardship reasons.
• β-lactamase production: B. fragilis produces chromosomal β-lactamases → resistant to ampicillin, penicillin, and most cephalosporins. This contrasts with above-diaphragm anaerobes (e.g. Peptostreptococcus) which are generally penicillin-susceptible.
• Common wrong answer: Choosing an aminoglycoside to cover B. fragilis — aminoglycosides require oxygen-dependent uptake and have zero activity against obligate anaerobes.

No directly relevant past paper questions specifically testing Bacteroides fragilis as the primary focus were identified in the indexed past paper material. The organism appears in clinical context within senior notes (cholangitis bacteriology, secondary peritonitis, liver abscess microbiology) but no indexed MCQ, SAQ, or minicase stem from the 2016–2025 Fourth Summative papers specifically asks about B. fragilis identification, treatment, or virulence as the central question.

Students should anticipate B. fragilis appearing as part of broader stems on: (1) empirical antibiotic selection for intra-abdominal sepsis, (2) cholangitis bacteriology, or (3) distinguishing SBP from secondary peritonitis (polymicrobial culture).