Molecular typing by pulsed-field gel electrophoresis of Flavobacterium psychrophilum isolates derived from Japanese fish

Sixty-four isolates of Flavobacterium psychrophilum from ayu, Plecoglossus altivelis altivelis (Temminck & Schlegel), and other fish (n=16) in Japan and the type strain (NCIMB 1947(T)) were typed using pulsed-field gel electrophoresis (PFGE) with endonuclease BlnI and XhoI. These isolates were classified into 20 clusters and 42 genotypes by PFGE analysis. The most predominant cluster of isolates from ayu was cluster XII (n=20), followed by clusters XVII, XVI, XX, XI, IX, X, XIII and XV; the remaining 17 isolates from other fish were divided into clusters I, II, III, IV, V, VI, VII, VIII, XIV, XVIII and XIX. The PFGE genotype of isolates from ayu clearly differed from those of other fish. The isolates from ayu in Gunma Prefecture belonged to clusters XII, XVI, XVII and XX, and the strains of three of these clusters (XII, XVII and XX) were isolated from ayu in 15 of 19 prefectures. PFGE typing enabled more accurate classification of isolates into clusters than previously achieved by analysing the restriction fragment length polymorphism of PCR products. These results suggest that F. psychrophilum isolated from ayu and other fish are genetically different and strains with several PFGE types have spread within Japan.