Isolation and identification of Vibrio Harveyi using 16S rRNA Gene and its virulence to sea bass (Lates calcarifer)

In year 2005, sea bass (Lates calcarifer) juveniles cultured in the net cages of the Borneo Marine Research Institute were experienced high mortality. The signs of the outbreak were resembled to vibriosis. Bacterial samples were isolated from the liver and kidney of affected sea bass (Lates calcarifer) juveniles, and were subjected to both biochemical and molecular test for identification. Polymerase chain reaction and sequencing of 795bp fragment of l6S rRNA gene was selected for this study. Based on sequencing results, the isolated bacteria can be assigned to Vibrio cholerae, Vibrio corallilyticus and Vibrio harveyi. These bacteria were screened for their virulence to sea bass juveniles through challenged test, but only one isolate, Vibrio harveyi (HS7)was found pathogenic to sea bass. This bacterium was further tested for its virulence to sea bass by challenging 8 individuals of sea bass juveniles per replicate. The test was run in triplicates. The LD₅₀ of the pathogen for sea bass juveniles (1-3g) was 2.836 x 10³ CFU/ml. The pathogen was also tested for extracellular products (ECPs) and found even a minute amount of ECPs (O.lml) could result in total mortality within 2 days. This study however, did not attempt to characterize the ECPs produced by bacterium. The bacterium was found tolerant to salt (NaCl) up to 5.5%, but was sensitive to some antibiotics including Nitrofurantoin (F 300), Furazolidone (FR 50), Streptomycin (S 25), Sulphamethoxazole (RL 100), Trimethoprim (W 5), Oxytetracycline (OT 30), Novobiocin (NV 30),0129 DD14 (129/10) and 0129 DD15 (129/150). This study has ascertained that the main cause of high mortality of sea bass juveniles in the net cages of the Borneo Marine Research Institute was due to Vibrio harveyi. Study on prevention and treatment of disease due to this bacterium should be a priority for future investigations.