Detection of chromosomal and plasmid mediated fluoroquinolone resistance among clinical isolates of Salmonella species from urban and peri-urban settings of South India

Background: Enteric fever remains a major public health problem. Fluoroquinolones have became the drug of choice for the treatment of multidrug resistant Salmonella species. However, increased use of fluoroquinolone antibiotics leads to the emergence of Salmonella strains with reduced susceptibility to fluoroquinolones. This study was performed to detect chromosomal and plasmid mediated fluoroquinolone resistance among clinical isolates of Salmonella species from urban and periurban settings of South India.
Method: A total of 250 blood culture isolates of Salmonella spp. from patients with suspected enteric fever attending tertiary care hospitals in urban and peri-urban settings of South India were included. Antibiotic susceptibility testing was performed using disc diffusion and agar dilution methods. Genes encoding Qnr, QepA and AAC(6′)-Ib-cr proteins and mutations in quinolone resistance determining regions (QRDRs) of genes encoding DNA gyrase (gyrA and gyrB) and toposiomerase IV (parC and parE) were detected using PCR and nucleotide sequencing analysis.
Results: Most of the Salmonella isolates were intermediate resistant to ciprofloxacin. The majority of the isolates were sensitive to a third generation fluoroquinolone, levofloxacin. No mutation was detected in Salmonella isolates with ciprofloxacin MIC of 0.032 µg/ml whereas isolates with ciprofloxacin MIC of ≥4 µg/ml had triple mutation in QRDR. All the isolates were negative for plasmid encoded qnr genes, efflux pump (qepA) and fluoroquinolone acetylating aminoglycoside-(6)-N-acetyltransferase (aac(6')-Ib-cr) genes.
Conclusion: We have observed Salmonella strains with reduced susceptibility to fluoroquinolone. A lower proportion of MDR Salmonella isolates was detected in this study. Fluoroquinolone resistant isolates were found to have chromosomal mutations in gyrA and parC genes and all the isolates were negative for plasmid encoded fluoroquinolone resistance genes.