Virulence assay of rhizoid and non-rhizoid morphotypes of Flavobacterium columnare in red tilapia, Oreochromis sp., fry.

Numerous isolates of Flavobacterium columnare were previously recovered from red tilapia, Oreochromis sp., exhibiting columnaris-like disease in Thai farms, and the phenotypic and genetic characteristics were described. The objective of this study was to determine the virulence of two morphotypes (rhizoid and non-rhizoid colonies) of F. columnare and to determine their ability to adhere to and persist in red tilapia fry. The results showed that the typical rhizoid isolate (CUVET1214) was a highly virulent isolate and caused 100% mortality within 24 h following bath challenge of red tilapia with three different doses. The non-rhizoid isolate (CUVET1201) was avirulent to red tilapia fry. Both morphotypes adhered to and persisted in tilapia similarly at 0.5 and 6 h post-challenge as determined by whole fish bacterial loads. At 24 and 48 h post-challenge, fry challenged with the rhizoid morphotype exhibited significantly higher bacterial loads than the non-rhizoid morphotype. The results suggested that an inability of the non-rhizoid morphotype to persist in tilapia fry may explain lack of virulence.