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Microbiology
- Gram-negative, obligate anaerobic bacillus (rod-shaped).
- Normal commensal of the human colon, comprising a large portion of the gut microbiota.
- Appears pleomorphic (variable in shape) and may have vacuoles.
- Aerotolerant, meaning it can survive in the presence of low oxygen concentrations due to enzymes like catalase and superoxide dismutase.
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Pathophysiology/Etiology
- An opportunistic pathogen that causes infection when the mucosal barrier of the GI tract is disrupted (e.g., surgery, trauma, diverticulitis, malignancy).
- Infections are typically polymicrobial, involving other anaerobes and facultative aerobes (e.g., E. coli).
- Virulence Factors:
- Polysaccharide capsule: Antiphagocytic and a key factor for abscess formation.
- Lipopolysaccharide (LPS): Has low endotoxic activity compared to other gram-negative bacteria, resulting in less severe septic shock.
- Succinic acid production: Inhibits neutrophil function.
- Enterotoxigenic B. fragilis (ETBF): Strains that produce B. fragilis toxin (BFT), a metalloprotease that can cause inflammatory diarrhea.
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Clinical Presentation
- Most common cause of intra-abdominal infections (e.g., peritonitis, abscesses) following a breach of the intestinal wall.
- Clinical signs often include fever, abdominal pain, and abscess formation.
- Infections are often associated with a foul-smelling discharge due to anaerobic metabolism.
- Can cause infections at other sites, including:
- Diabetic foot ulcers and other skin/soft tissue infections.
- Pelvic inflammatory disease (PID) and tubo-ovarian abscesses.
- Bacteremia, which is associated with significant mortality.
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Diagnosis
- Dx is often clinical, based on the setting of recent surgery, trauma, or underlying GI disease.
- Anaerobic culture from a normally sterile site (e.g., abscess aspirate, blood) is the gold standard. Wound swabs have low diagnostic yield due to contamination.
- Imaging (e.g., CT scan) is crucial to identify and locate abscesses for drainage.
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Management/Treatment
- Source control is critical: drainage of abscesses and debridement of necrotic tissue.
- Antibiotic therapy:
- Intrinsically resistant to penicillins via β-lactamase production.
- High rates of resistance to clindamycin.
- Effective agents include:
- Metronidazole (drug of choice for anaerobes).
- β-lactam/β-lactamase inhibitor combinations (e.g., Piperacillin-tazobactam, Ampicillin-sulbactam).
- Carbapenems (e.g., Imipenem, Meropenem).
- Empiric therapy for intra-abdominal infections must cover both anaerobes and Enterobacteriaceae.
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Key Associations/Complications
- Abscess formation is the hallmark of B. fragilis infection.
- Post-operative infections, particularly after abdominal surgery.
- Diabetic foot infections.
- Potential association between enterotoxigenic B. fragilis (ETBF) and inflammatory bowel disease (IBD) flare-ups and colorectal cancer.