Characterization of isolates of Flavobacterium psychrophilum associated with coldwater disease or rainbow trout fry syndrome:phenotypic and genomic studies

Abstract

Isolates of Flavobacterium psychrophiluln (formerly Cytophaga psychrophila and Flexlbacter psychrophilus) mainly originating from clinical outbreaks of either coldwater disease (CWD) or rainbow trout fry syndrome (RTFS) were studied for selected biochemical, physiological, rnorphological and genomic characteristics, and compared with previously characterized French and American strains. DNA hybridization studies showed that the Danish isolates were highly related to the type strain, F. psychrophilum NCIMB 1 9 4 7 ~ Plasmid profiling of Danish isolates and those from other European countries revealed differences, which might be related to differences in pathogenicity. European ~solates originating from clinical outbreaks of either RTFS or CWD usually harboured one plasmid of 3.2 kb, whereas isolates originating from fish with different or no disease slgns had other profiles. Phenotypically, the Danish isolates appeared very homogeneous and shared most characteristics with the type strain, and with French and American strains studied by other authors. Further studies on the importance of the plasmids and the proteolytic activities of the bacterium might help in elucidating possible virulence factors.