A 58-year-old woman with intra-abdominal sepsis grows Bacteroides fragilis from blood cultures. The surgeon plans combination therapy but asks the physician about aminoglycosides. Why is gentamicin an inappropriate choice for this organism?
- A Aminoglycoside uptake across the bacterial inner membrane is an oxygen-dependent process that cannot occur in obligate anaerobes ✓
- B Bacteroides fragilis has an impermeable outer membrane that excludes all protein synthesis inhibitors
- C Bacteroides fragilis produces aminoglycoside-modifying enzymes constitutively
- D Gentamicin is inactivated by the beta-lactamases produced by Bacteroides species
Correct answer: A. Aminoglycoside uptake across the bacterial inner membrane is an oxygen-dependent process that cannot occur in obligate anaerobes
Explanation
Aminoglycosides enter bacteria by passive diffusion followed by energy-dependent, oxygen-requiring active transport across the inner membrane. Obligate anaerobes lack oxidative metabolism, so transport fails and the drug never reaches its ribosomal target. This makes all aminoglycosides intrinsically inactive against anaerobes regardless of enzyme production. Enzymatic modification (option C) matters for aerobes such as enterococci, not for anaerobic permeability failure.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.