Which penicillin is penicillinase resistant?

• A. Dicloxacillin
• B. Amoxicillin
• C. Augmentin
• D. Ampicillin

Answer: (A)

Penicillinase-resistant penicillins have bulky side chains that shield the beta-lactam ring from beta-lactamase enzymes produced by certain bacteria, especially Staphylococcus species. Dicloxacillin has such protective bulky groups, so it remains active even when penicillinase is present. Amoxicillin and ampicillin lack this protective feature and are readily inactivated by penicillinase. Augmentin combines amoxicillin with clavulanic acid, a beta-lactamase inhibitor; this pairing extends activity by inhibiting the enzyme, but the underlying antibiotic isn’t intrinsically penicillinase resistant. Therefore, the penicillin in this set that is penicillinase resistant is dicloxacillin.