Virulence Determination among Vibrio Harveyi Hatchery Isolates through Haemolysis and Growth Constraint

Bacterial diseases mainly due to vibriosis in Penaeid shrimp culture implicating several species of Vibrios. Vibrio harveyi is considered as an important causative agent of the systemic vibriosis, which occurs in any bio-fields. The virulence of this bacterium is due to the production and expression of several virulent factors such as hemolysin, cysteine and metalloprotease, phospholipases, exotoxins, luciferase and siderophore. The present study intends the virulence determination among three strains of V. harveyi VSH3, VSH5 and VSH9 isolated from shrimp hatchery along the coastline of Tuticorin, Tamil Nadu during vibriosis outbreaks through their haemolysis and growth performance. Haemolytic activity examined against sheep blood resulted minor or no variations among the organisms in agar plate assay. However, in microtitre assay, Extra Cellular Products (ECP) of all isolates rendered increased activity with increase in dose and time of exposure. The bacterium VSH5 exhibited more haemolytic effect (at 500 μl concentration 72% haemolysis) than that of VSH9 and VSH3 reavled that the bacterium is highly virulent than the others. The bacterium VSH5 showed well growth in the optimum temperature (33°C), NaCl (2%) and pH (7.3). The maximum luminescence was expressed in 37°C, 2.5-3% NaCl and pH 7-9 in 18 to 48 hrs. Diverse colony morphology was observed in VSH5 on solid medium incubated for 3 to 5 days or longer. Growth curve experiment revealed that the bacterim VSH5 is a fast grower, completed its log phase within 7 hrs. Pathogenic strain like V. harveyi VSH5 causes high mortality and affects aquaculture production in hatchery as well as pond level. Hence, control measure against this kind of bacterium is urgently needed for maintain the sustainability of shrimp aquaculture in India and other Asian country.