First description of the qnrS-like (qnrS5) gene and analysis of quinolone resistance-determining regions in motile Aeromonas spp. from diseased fish and water

Antimicrobial resistance patterns in a collection of 33 motile Aeromonas species were described in this study. Quinolone has been frequently employed for treatment of Aeromonas-related diseases, and prolonged use of antimicrobial compounds has led to development of resistant strains. In a sample of diseased fish and environmental water, we evaluated nalidixic acid (n=19) and ciprofloxacin (n=4) resistance via minimum inhibitory concentration (MIC) assays and the genetic basis was also investigated. Among the isolated Aeromonas spp., 17 strains encoded for chromosomal mutations of quinolone resistance-determining regions (QRDRs) in gyrA, 11 strains encoded for mutations of QRDRs in parC, 1 strain harbored plasmid-mediated quinolone resistance (PMQR) qnrS1-like gene and 4 strains harbored the PMQR qnrS2 gene. Inparticular, the new variant (qnrS1-like) differed from qnrS1 by 6 amino acid substitutions at positions 5 (Asn ⁵→Arg ⁵), 120 (Ser ¹²⁰→Thr ¹²⁰), 148 (Asn ¹⁴⁸→His ¹⁴⁸), 206 (Leu ²⁰⁶→Glu ²⁰⁶), 207 (Ile ²⁰⁷→ Leu ²⁰⁷), and 216 (Tyr ²¹⁶→Phe ²¹⁶), and the gene was designated qnrS5. These resistant strains may function as reservoirs of quinolone resistance.