DNA gyrase and topoisomerase IV mutations in quinolone-resistant Flavobacterium psychrophilum isolated from diseased salmonids in Norway.

Flavobacterium psychrophilum is the causative agent of the recognized diseases 'bacterial coldwater disease' and 'rainbow trout fry syndrome' and is found in many farmed freshwater and marine fish species. In Norway, the bacterium has mainly been isolated from Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.). In the present study, 26 isolates from Norwegian farmed salmonids were examined. All isolates were tested for susceptibility towards various antibacterial drugs by the disk diffusion method, and minimum inhibitory concentration values for oxolinic acid and flumequine were established for selected isolates. All isolates from rainbow trout displayed reduced susceptibility towards quinolones, while brown trout and Atlantic salmon isolates were susceptible. The quinolone resistance determining regions (QRDRs) of the gyrA, gyrB, parC, and parE genes were sequenced. Sequence analysis of the QRDR of gyrA in quinolone resistant isolates revealed a threonine:arginine amino acid substitution at position 82 in all 16 isolates from Norwegian rainbow trout and a single reference strain isolated from rainbow trout in Sweden. No evidence for plasmid-mediated quinolone resistance was found in any of the isolates. Pulsed-field gel electrophoresis and phylogenetic analysis of parC and gyrB sequences indicate a clonal relationship between rainbow trout isolates.