qnrS

qnrS is a plasmid-borne gene that encodes the QnrS protein, a member of the qnr family of pentapeptide repeat proteins involved in quinolone resistance. qnrS and its variants, such as qnrS1, qnrS2, and qnrS3, have been identified in a range of Gram-negative bacteria and are commonly carried on plasmids, enabling horizontal transfer between species.

Mechanism and impact: The QnrS protein protects DNA gyrase and topoisomerase IV from inhibition by fluoroquinolones, leading to reduced susceptibility. In most isolates, qnrS confers low-level resistance, but it can promote the selection of higher-level resistance when combined with chromosomal mutations in target genes (such as gyrA and parC) or with additional plasmid-mediated resistance determinants.

Genetic context and spread: qnrS is frequently located on plasmids within diverse incompatibility groups and often resides in complex resistance regions that include other antimicrobial resistance genes. It is associated with mobile genetic elements such as insertion sequences and transposons, which facilitate its mobility and dissemination among bacterial populations and environments.

Clinical significance and detection: qnrS contributes to plasmid-mediated quinolone resistance (PMQR) and can complicate treatment by elevating baseline resistance levels and enabling the rapid emergence of higher-level resistance under drug pressure. Detection is typically performed by PCR-based methods and sequencing in diagnostic and surveillance settings, contributing to monitoring of PMQR genes and antimicrobial resistance trends.