Morphology: Gram-negative anaerobic rod, bile-resistant.
Anaerobe
Typical drugs
Empiric therapy when resistant
Metronidazole remains highly active. Clindamycin resistance is common (>30% in some centers) — don't use clinda for B. fragilis.
Resistance mechanisms
Enzymatic degradation
cfiA carbapenemase (rare)
Example: Carbapenem-resistant B. fragilis.
Resistance notes
Clindamycin resistance widespread — not reliable for below-diaphragm anaerobes anymore.
Pearls
Below-diaphragm anaerobic flora: intra-abdominal, pelvic, deep abscess. Above-diaphragm anaerobes covered by clindamycin / amp-sulb.