JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 41, No. 3 June, 2010 Virulence of Vibrios Isolated from Diseased Black Tiger Shrimp, Penaeus monodon, Fabricius Aseer Manilal, S. Sujith, Joseph Selvin 1 , Chippu Shakir, and R. Gandhimathi Department of Microbiology, Bharathidasan University, Tiruchirappalli, 620 024 India G. Seghal Kiran Department of Biotechnology, Bharathidasan University, Tiruchirappalli, 620 024 India Abstract 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. Members of the genus Vibrio are autochtho- nous bacterial flora in the aquatic ecosystem, and quite a few of them are associated with infections in humans and aquatic animals. They are the normal bacterial flora of shrimp and the culture environment (Jiravanichpaisal et al. 1994; Otta et al. 1999), but they often act as secondary or opportunistic pathogens that cause mortality ranging from few to 100% in affected populations under stress (Lightner 1988). Vib- riosis has been implicated as the cause of major mortality in juvenile penaeid shrimp (Light- ner and Redman 1994). Vibrio spp. are most often considered opportunistic pathogens in shrimp, but primary disease caused by highly virulent strains has also been reported (De 1 Corresponding author. La Pe˜ na et al. 1992; Ishimaru et al. 1995). Reports on the epizootic luminescent bacte- rial diseases, especially in the shrimp farms of Asian countries, have been connected to a decrease in the shrimp production and inten- sive rearing systems (Karunasagar et al. 1994). Luminescent vibriosis is mainly caused by Vib- rio harveyi and Vibrio campbellii and occa- sionally by Vibrio splendidus, which can infect larval juveniles and adult stages of penaeid shrimp (Gomez-gill et al. 1998; Lavilla-pitogo et al. 1998). In India, luminous V. harveyi and Vibrio alginolyticus were reported as pre- dominantly opportunistic and secondary shrimp pathogens of highly devastating mid-culture outbreaks (Selvin and Lipton 2003a; Selvin et al. 2005). In this report, the disease caused by vibrios because of deteriorated water quality, © Copyright by the World Aquaculture Society 2010 332