Amoxicillin Resistance Mechanism — MFDS Part 1 MCQ

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ModeratePharmacologyAmoxicillin Resistance MechanismMFDS Part 1

An amoxicillin-resistant Prevotella isolate from a dentoalveolar infection becomes susceptible when clavulanic acid is added during laboratory susceptibility testing. Which resistance mechanism best explains this finding?

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Correct answer: E — Production of beta-lactamase that hydrolyses the beta-lactam ring

The correct answer is E. Beta-lactamases hydrolyse the beta-lactam ring and inactivate amoxicillin. Clavulanic acid inhibits susceptible beta-lactamases, so restoration of susceptibility when it is added is the decisive finding. Beta-lactamases, including CfxA and other classes, occur in oral Gram-negative anaerobes such as Prevotella. Altered DNA gyrase primarily causes fluoroquinolone resistance, while ribosomal RNA methylation is associated with resistance to agents acting on the ribosome, including macrolides and lincosamides. Efflux and reduced permeability can contribute to resistance in Gram-negative bacteria, but neither is specifically reversed by clavulanic acid.

Reference: Electronic Medicines Compendium. Co-amoxiclav 500 mg/125 mg film-coated tablets: Summary of Product Characteristics, section 5.1 Pharmacodynamic properties. Text revised 19 September 2025. https://www.medicines.org.uk/emc/product/101680/smpc