RESUMEN
OBJECTIVES: The aim of this study was to determine mutations associated with a quinolone-resistant (QR) phenotype of Flavobacterium columnare isolates. METHODS: The susceptibility of 53 F. columnare isolates to 11 antimicrobials, including 2 quinolones, was investigated by the disk diffusion method. Oxolinic acid (OXO) was subsequently chosen for minimum inhibitory concentration (MIC) assay. Sequence analysis of four genes within the quinolone resistance-determining regions (QRDRs) of OXO-resistant F. columnare compared with susceptible isolates was subsequently performed. RESULTS: The disk diffusion assay revealed that the majority of isolates were susceptible to all tested antimicrobials. However, 14 and 8 isolates were resistant to the quinolone antibiotics OXO and nalidixic acid, respectively. No multidrug resistance was observed. The MIC assay revealed five additional isolates that were resistant to OXO (≥4µg/mL), making a total of 19 OXO-resistant isolates observed in this study. DNA sequencing identified missense mutations both in parC and gyrA but not in gyrB or parE in QR F. columnare isolates. Mutation in parC resulted in the change His87âTyr. For gyrA, 15 isolates of Thai origin exhibited a change at residue Ser83 to either Phe, Tyr or Ala, whereas 3 Vietnamese isolates contained two mutation sites (Ser83âPhe and Asp87âTyr). CONCLUSION: This study is the first to reveal that QR phenotype F. columnare isolates harboured missense mutations both in parC and gyrA but not in gyrB or parE of the QRDRs.