|DONG, HA - Chulalongkorn University|
|SENAPIN, SAENGCHAN - Mahidol University|
|RODKHUM, CHANNARONG - Chulalongkorn University|
Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2015
Publication Date: 5/5/2016
Citation: Dong, H.T., Senapin, S., Lafrentz, B.R., Rodkhum, C. 2016. Virulence assay of rhizoid and non-rhizoid morphotypes of Flavobacterium columnare in red tilapia, Oreochromis sp., fry. Journal of Fish Diseases. 39:649-655.
Interpretive Summary: Flavobacterium columnare is a yellow pigmented bacterium and the causative agent of columnaris disease in freshwater fish. Recently, this bacterium was characterized from disease outbreaks of farmed tilapia in Thailand. It was observed that some of the F. columnare exhibited a change in bacterial colony morphology from rhizoid to non-rhizoid during maintenance of the bacterium in the lab. The objective of this study was to determine the ability of the two different colony types (rhizoid and non-rhizoid) of F. columnare to cause mortality in tilapia fry. The results demonstrated that the typical rhizoid colony type was highly infectious and resulted in 100% mortality, whereas the non-rhizoid colony type was unable to cause disease and mortality in tilapia. The results showed that the non-rhizoid isolate was not able to persist in tilapia and this finding may explain why it did not cause disease and mortality. Future research will develop strategies to prevent and/or treat columnaris disease in red tilapia.
Technical Abstract: Numerous isolates of F. 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.