QnrD

qnrD is a plasmid-borne gene that encodes a pentapeptide repeat protein, QnrD, a member of the Qnr family that contributes to reduced susceptibility to quinolones. The QnrD protein protects bacterial DNA gyrase and topoisomerase IV from inhibition by fluoroquinolones, allowing bacteria to survive at higher drug concentrations than they would otherwise.

The gene was first described in 2009 as a novel quinolone resistance determinant identified in Raoultella planticola isolates. Since then, multiple qnrD variants have been reported, including qnrD1, qnrD2, and qnrD3, in a variety of Enterobacterales species. qnrD has been detected in clinical isolates as well as in environmental and animal-associated bacteria, indicating a broad distribution across different habitats.

Genetically, qnrD is typically located on small plasmids, which can be transferred between bacteria through conjugation or mobilization. The gene is often found in conjunction with other resistance determinants, including additional qnr genes or beta-lactamase genes, creating multi-drug resistance contexts. The presence of qnrD can elevate the minimum inhibitory concentrations (MICs) of several fluoroquinolones by modest amounts (commonly 2- to 8-fold), which can facilitate the selection of higher-level resistance when combined with chromosomal mutations in gyrA or parC.

Clinically, qnrD contributes to quinolone resistance and is one factor among the diverse mechanisms that limit fluoroquinolone efficacy. Its detection is part of molecular surveillance efforts to monitor the spread of plasmid-mediated resistance.

---