Aminoglycosides require oxygen-dependent uptake into bacteria for their bactericidal effect. This explains their near-complete lack of activity against:

• A Obligate anaerobes such as Bacteroides fragilis ✓
• B Aerobic gram-negative bacilli such as Pseudomonas aeruginosa
• C Facultative aerobes such as Staphylococcus aureus
• D Aerobic gram-positive cocci such as streptococci

Explanation

Aminoglycoside uptake requires an electron transport chain-dependent electrochemical gradient (proton motive force) across the bacterial membrane. Obligate anaerobes like Bacteroides fragilis lack the oxidative electron transport needed to drive aminoglycoside across the cell membrane, making these organisms intrinsically resistant. This also explains their poor activity in abscesses (anaerobic, low pH, low oxygen environment). Aerobic gram-negatives and facultative organisms maintain active electron transport and are susceptible (though streptococci have additional intrinsic permeability resistance that is partially overcome by cell-wall-active agents).

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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