Isolation And Genetic Characterization Of Flavobacterium Columnare From Grass Carp, Ctenopharyngodon Idellus, In China

Flavobacterium columnare is the causal agent of columnaris disease and has severe impacts on the grass carp, Ctenopharyngodon idellus, aquaculture industry. The genetic characteristics of F. columnare strains isolated from grass carp in China are not clear. Therefore, in this study, we obtained 45 bacterial isolates from grass carp with columnaris disease from 11 provinces of China. The 45 bacterial isolates were identified as F. columnare according to phenotypic traits, biochemical characteristics, and molecular identification with 16S rDNA gene sequencing. Furthermore, based on the analysis of restriction fragment length polymorphism of the 16S rDNA gene (16S-RFLP), 37 isolates belonged to genomovar I, 5 isolates were ascribed to genomovar II, and 3 isolates were identified as genomovar III. Phylogenetic analysis further confirmed the genetic differences between the three genomovar isolates and revealed a closer relationship between genomovar I and II isolates. The colony pseudo-root morphology and the degree of bacterial aggregation of the three genomovar isolates were significantly different when they were cultured under the same conditions. We concluded that there are three genotypes of F. columnare currently identified in grass carp, and genomovar I was the main type. These findings provide a basis for the genetic diversity of F. columnare in grass carp in China.