A clinical microbiologist notices that an E. coli isolate from urine is resistant to trimethoprim-sulfamethoxazole, ciprofloxacin, and gentamicin, but susceptible to nitrofurantoin and fosfomycin. Genotypic testing reveals a qnrS gene. What is the mechanism of quinolone resistance conferred by qnr genes?
- A Plasmid-mediated protection of DNA gyrase/topoisomerase IV from fluoroquinolone binding ✓
- B Efflux pump upregulation encoded on chromosome
- C Mutation in gyrA leading to reduced binding affinity
- D Enzymatic acetylation of the quinolone molecule
Correct answer: A. Plasmid-mediated protection of DNA gyrase/topoisomerase IV from fluoroquinolone binding
Explanation
qnr genes (qnrA, qnrB, qnrS) encode pentapeptide repeat proteins that bind to DNA gyrase and topoisomerase IV, physically protecting these target enzymes from fluoroquinolone inhibition; this is plasmid-mediated transferable resistance. Chromosomal gyrA/parC mutations also confer quinolone resistance but are not encoded by qnr genes. AAC(6')-Ib-cr encodes enzymatic acetylation of certain fluoroquinolones, which is a separate plasmid-mediated mechanism.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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