PCR-RFLP genotypes associated with quinolone resistance in isolates of Flavobacterium psychrophilum. 
A novel genotyping method for epizootiological studies of bacterial cold-water disease caused by Flavobacterium psychrophilum and associated with quinolone resistance was developed.  Polymerase chain reaction followed by restriction fragment length polymorphism (PCR-RFLP) was performed on 244 F. psychrophilum isolates from various fish species.  PCR was performed with primer pair GYRA-FP1F and GYRA-FP1R amplifying the A subunit of the DNA gyrase (GyrA) gene, which contained the quinolone resistance determining region.  Digestion of PCR products with the restriction enzyme Mph1103I showed two genotypes, QR and QS.  The difference between these genotypes was amino acid substitutions at position 83 of GyrA (Escherichia coli numbering).  The genotype QR indicated an alanine residue at this position associated with quinolone resistance in F. psychrophilum isolates.  Of the 244 isolates tested in this study, the number of QR genotype isolates was 153 (62.7%).  In isolates from ayu (n=177), 146 (82.5%) were genotype QR.  With combination of this technique and previously reported PCR-RFLP genotyping, eight genotypes were observed in F. psychrophilum isolates.  Using this genotyping system, the relationships between genotype and host fish species, or locality of isolation, were analysed and are discussed.