Despite excellent in vitro activity against many facultative Gram-negative bacilli, aminoglycosides show no clinically useful activity against obligate anaerobes such as Bacteroides fragilis. The pharmacological basis for this gap in the spectrum is:

• A Aminoglycoside entry into the bacterial cell is an oxygen-dependent, energy-requiring active transport process ✓
• B Obligate anaerobes lack the 30S ribosomal subunit target of aminoglycosides
• C Anaerobes possess constitutively expressed aminoglycoside-modifying enzymes
• D The low pH of anaerobic environments irreversibly inactivates aminoglycosides

Explanation

Aminoglycosides cross the bacterial envelope by an active, carrier-mediated transport that requires oxidative metabolism and is inhibited under anaerobic conditions or by a low extracellular pH. Obligate anaerobes therefore cannot accumulate the drug, and the ribosomal target itself is intact and susceptible. This is why aminoglycosides are never used alone against anaerobic infections and why activity is reduced inside abscess cavities, where pH is low and conditions are hypoxic.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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