Abstract
Flavobacterium columnare is the aetiological agent of columnaris disease and severely affects various freshwater aquaculture fish species worldwide. The objectives of this study were to determine the phenotypic characteristics and genetic variability among F. columnare isolates isolated from red tilapia in Thailand. Forty‐four F. columnare isolates were recovered from diseased fish in different geographical locations. The isolates exhibited homologous phenotypic characteristics but exhibited genetic diversity. One isolate was assigned to genomovar I , and the remainder were assigned to genomovar II , indicating the coexistence of these genomovars but predominance of genomovar II . Phylogenetic analysis of the 16 S ‐23 S ISR sequences revealed that a subset of the Thai isolates ( n = 25) contained a smaller intergenic spacer region ( ISR ) (523–537 bp) and formed a unique ISR phylogenetic group. Phylogenetic analysis of the 16 S r RNA gene supported the unique cluster of Thai isolates. This is the first description of the phenotypic and molecular characteristics of F. columnare isolated from red tilapia in Thailand as well as five isolates of F. columnare derived from other fish species including Nile tilapia, koi carp and striped catfish.