Abstract:
Flavobacterium columnare is Gram negative bacteria which caused columnaris disease in various kinds of freshwater fishes. This study is the first report of F. columnare infection in freshwater culturing Asian sea bass in Thailand. Phenotypic characterization, antimicrobial susceptibility test and quinolone resistant-associated genes of F. columnare were performed. A total 15 of F. columnare were isolated from diseased fishes and phylogenetic analysis revealed that these isolates were belonged to genetic group 2 and 4. From disk diffusion test results, 5 and 6 isolates were resistant to oxolinic (OA) and nalidixic acid (NA) respectively. The highest MIC values to OA and NA were 16 and 64 μg/mL respectively. The complete genome analysis revealed that most resistant genes were responsible for efflux pump activity. Moreover, the mutations within quinolone resistance-determining regions (QRDRs) of gyrA, gyrB, parC, and parE were detected. The novel mutations at position 370 in gyrB and 389 in parE were founded in all quinolone resistant (QR) isolates. Moreover, mutation at position 83 in gyrA was responsible for OA resistant mechanism in QR isolates whereas mutations within gyrB, and parE were play role in both NA and OA resistant. On the other hands, the QS isolates were carried double mutations in parC at position 88 and 183. Thus, these mutations may not play important role in resistant against quinolones.
