QNR

qnr is a designation used for a family of plasmid-borne genes that confer resistance to quinolone antibiotics, including fluoroquinolones. The qnr genes encode Qnr proteins, which are pentapeptide repeat proteins that protect bacterial DNA gyrase (GyrA/B) and topoisomerase IV (ParC) from the inhibitory action of quinolones. By shielding these target enzymes, Qnr proteins reduce drug binding and allow DNA replication to proceed in the presence of the drug, contributing to reduced susceptibility.

The most commonly reported qnr families are qnrA, qnrB, and qnrS, but additional variants such as qnrC, qnrD, qnrE, and qnrVC have been described. These genes are typically located on plasmids, enabling horizontal transfer between bacteria, often alongside other resistance determinants. In many cases, qnr-mediated resistance results in low-to-moderate increases in minimum inhibitory concentrations (MICs), which can facilitate the selection of higher-level resistance when combined with chromosomal mutations in gyrase or topoisomerase IV or with other PMQR mechanisms such as aac(6')-Ib-cr or efflux pumps.

qnr determinants have been detected in a range of Gram-negative bacteria, particularly Enterobacterales, including Escherichia coli and Klebsiella species, in clinical, veterinary, and environmental settings worldwide. The presence of qnr genes is a concern for antimicrobial stewardship because they can undermine fluoroquinolone efficacy and promote the emergence of more robust resistance under selective pressure.

Detection of qnr genes is usually through molecular methods such as PCR-based assays or sequencing, often as part of broader PMQR surveillance programs.