Pathogenicity of Vibrio harveyi to cultured marine fishes in Korea
Introduction
V. harveyi has been widely recognized as a primary pathogen of many commercially cultured invertebrate species the world over. In the black tiger shrimp, Penaeus monodon, for example, the mortality of larval stages (protozoea to postlarvae) in the hatcheries often reaches 100 % (Lavilla-Pitogo et al., 1990). An apparently new organism was independently isolated from a sandbar shark, Carcharhinus plumbeus, which had died at the National Aquarium in Baltimore, USA (Grimes et al., 1984). This organism was classified as a new species, Vibrio carchariae. A similar organism has been recovered from many marine fishes (Yii et al., 1997, Soffientino et al., 1999, Zhang and Austin, 2000). Subsequent phenotypic and genotypic studies, including 16S rDNA sequencing, have established that V. harveyi is the senior synonym of V. carchariae (Pedersen et al., 1998).
Despite being a serious pathogen of marine fish, the pathogenic mechanisms of V. harveyi have yet to be fully elucidated. Extracellular products (ECPs) (Liu et al., 1996), including proteases, hemolysins, and cytotoxins (Liu et al., 1996) are considered to be important determinants of virulence in V. harveyi. Other virulent factors associated with the pathogenicity of this organism were identified by Soto-Rodriguez et al. (2003) and Owens et al. (1996), who demonstrated the role of cell surface hydrophobicity and siderophore production in V. harveyi virulence, respectively. However, these results were obtained by different researchers using various bacterial strains isolated from several different fish species; consequently, a clear correlation between virulence factors and pathogenicity is yet to emerge.
In the present study, live cells and extracellular products (ECPs) from various V. harveyi strains were examined for pathogenicity, using olive flounder and black rockfish, and putative virulence mechanisms were identified.
Section snippets
Bacterial culture
Disease outbreaks occurred during the summer of recent several years in marine fish farms, Korea. The causative bacteria isolated from olive flounder, Paralichthys olivaceus, black rockfish, Sebastes schlegeli and turbot, Scophthalmus maximus were identified as V. harveyi based on morphological and biochemical characteristics and the sequence of 16S rDNA (Won et al., 2006b). Five V. harveyi isolates, with diverse range of hosts and two type strains (V. harveyi ATCC14126 and V. carchariae
Fish pathogenicity experiments
Table 2 shows the pathogenicity of V. harveyi, live cells and ECPs to olive flounder and black rockfish. The LD50 values of V. harveyi live cells for olive flounder and black rockfish were 2.48 × 105 to > 108 cfu g− 1 fish and 2.0 × 104 to 6.6 × 107 cfu g− 1 fish, respectively. Particularly, the FR2 strain was the most pathogenic to olive flounder and black rockfish, with LD50 values of 2.48 × 105 and 2.0 × 104 cfu g− 1 fish, respectively. In contrast, the type strains, ATCC35084 and ATCC14126, exhibited
Discussion
Epizootic outbreaks caused by V. harveyi have been reported in many marine fishes (Soffientino et al., 1999, Zhang and Austin, 2000) and over a wide geographical range including Korea (Sunaryanto and Mariam, 1986, Lavilla-Pitogo et al., 1990, Won et al., 2006b). We also confirmed the pathogenicity of the isolates to marine fish using a challenge test. All V. harveyi strains were considerably more pathogenic to black rockfish than to olive flounder in both live bacteria and ECPs. Particularly,
Acknowledgments
A part of this study was supported by grants from the National Fisheries Research and Development Institute, South Korea.
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