Prevalence and Mechanisms of Low Level Quinolone Resistance among Non-Typhoidal Salmonella Isolates from Human and poultry/Livestock in Korea: Usefulness of Nalidixic Acid Resistance Test

Background: Non-typhoidal Salmonella (NTS) are important commensal microorganisms. We intended to investigate the prevalence and mechanisms of nalidixic acid resistance among NTS isolated from human and poultry/livestock. Methods: A total of 151 Salmonella isolates (36 human and 115 livestock isolates, respectively) was tested for the Minimum inhibitory concentrations (MICs) of nalidixic acid, together with serotyping. As for the nalidixic acid resistant isolates, further studies were taken: MICs of ciprofloxacin, mutation analysis of gyrA and parC genes, and organic solvent tolerance test. Results: Eighty-four isolates of 151 human and livestock isolates were resistant to nalidixic acid. The prevalence of nalidixic acid resistance and was 13.9% (5 of 36 isolates) in human isolates and 68.7% (79 of 151 isolates), in the livestock isolates respectively. Among 84 nalidixic acid-resistant isolates, the The prevalence of ciprofloxacin resistance in livestock isolates was 24.1% (1 resistant and 18 intermediate of 79 strains), but no ciprofloxacin resistance was found in 5 human isolates. Among 65 nalidixic acid resistant, ciprofloxacin-susceptible isolates, 3 (60%, of 5 human isolates) and 60 (100%, all livestock isolates) showed low level fluoroquinolone resistance (ciprofloxacin MIC, 0.125-1.0 mu g/mu L). Six types of point mutations were found in the analysis of DNA sequencing of the gyrA gene in the 84 isolates; 75 isolates showed point mutations on amino acid Ser 83 and/or Asp 87. On the other hand, no point mutation was found from the parC genes. Forty-seven nalidixic acid resistant isolates showed tolerance to organic solvents. Conclusions: Nalidixic acid resistance was a good marker of low level fluoroquinolone resistance. As for the severe NTS infection, MIC test for nalidixic acid would be required.