A 55-year-old diabetic man undergoes drainage of a foul-smelling foot abscess. Empiric therapy includes gentamicin along with anaerobic cover. Cultures grow Bacteroides fragilis, yet gentamicin shows no activity against this isolate despite reported susceptibility of other isolates to gentamicin in the same laboratory batch. The most likely explanation is:

• A Bacteroides fragilis has an impermeable outer membrane that excludes all aminoglycosides
• B Bacteroides fragilis constitutively produces an aminoglycoside-modifying enzyme
• C Aminoglycoside uptake into bacteria is an oxygen-dependent process that fails in strict anaerobes ✓
• D Gentamicin is degraded by the beta-lactamase produced by Bacteroides fragilis

Explanation

Aminoglycosides enter bacteria by an active, energy-dependent transport system that requires oxygen for electron transport driven uptake. Strict anaerobes lack oxidative metabolism, so transport fails and the drug cannot reach its 30S ribosomal target regardless of susceptibility testing artifacts. This is why aminoglycosides are never used alone for anaerobic infections such as Bacteroides abscesses. Enzymatic modification and membrane barriers cause resistance in aerobic Gram-negatives, but they do not explain total inactivity against all anaerobes.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.