Phenotypic and genotypic analysis of Flavobacterium psychrophilum isolates from Ontario salmonids with bacterial coldwater disease.

Flavobacterium psychrophilum is the causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome. BCWD has a considerable economic impact on aquaculture operations in Ontario, Canada, and our limited understanding of the population structure and epidemiology of F. psychrophilum isolates is an impediment to the development of improved management strategies. Seventy-five 16S rRNA gene and gyr polymerase chain reaction positive isolates of F. psychrophilum that had been collected over a 16-year period from farmed salmonids with tail rot, necrotic myositis, and osteochondrosis were characterized morphologically, biochemically, and genotypically. Although the isolates were homogeneous by preliminary biochemical and phenotypic characterization, two distinct biovars were found by API ZYM testing. As well, four restriction pattern types were detected by 16S rRNA polymerase chain reaction -- restriction fragment length polymorphism analysis and there was a significant (P < 0.001) correlation between biovar I and digestion with MaeIII and between biovar II and digestion with MnlI or no site (P < 0.05). Further heterogenity was detected by sequence analysis of a 194 bp stem loop 3 region of rRNA. Nine sequence types were identified; 40/46 biovar I isolates were sequence type "a", while 21/32 biovar II isolates belonged to either sequence type "c" or "d". More than one biovar and genotype was identified among the strains recovered from separate fish sampled from three groups of rainbow trout (Oncorhynchus mykiss) experiencing BCWD mortality events. No association was found between genotype or biovar and type of disease. Taken together, these data suggest that F. psychrophilum from Ontario can be grouped into two major lineages based on biovar and 16S rRNA polymorphisms, and although three major strain types were most frequently isolated in this study, it appears that the population of F. psychrophilum with pathogenic potential is quite heterogeneous.