Virulence of Vibrios Isolated from Diseased Black Tiger Shrimp, Penaeus monodon, Fabricius

Vibrios are prominently opportunistic, but they may act as primary and secondary pathogenic invaders of shrimps in farms worldwide. However, little is known about the virulence of diverse vibrios harbored in shrimp farms. The isolation of secondary/opportunistic Vibrio pathogens was carried out from the shrimp farms located in peninsular India, reported to have mass mortalities because of White-spot syndrome virus and shell disease outbreaks. The isolates were identified and grouped into six species including luminescent Vibrio harveyi, Vibrio alginolyticus, Vibrio vulnificus, Vibrio fischeri, Vibrio parahaemolyticus, and Photobacterium damsela. To make a detailed study on their virulence, the isolates were screened in vitro for the production of extracellular factors. In the present study, 68.9% of the shrimp Vibrio isolates were found to be positive for both phospholipase and hemolysin production. The results showed that various factors including hydrophobicity, presence of capsule, and production of extracellular enzymes such as lipase, protease, phospholipase, and hemolysin were responsible for pathogenesis. Using the bacterial adherence to hydrocarbons, most of the strains were classified as highly hydrophobic. In vivo challenge experiments using the Vibrio species and the extracellular products demonstrated the pathogenicity of vibrios. In the present study, we established the shrimp farm water isolates V. vulnificus, V. fischeri, and P. damsela as moderately opportunistic shrimp pathogens.