DAO 58:223-230 (2004)  -  doi:10.3354/dao058223

Pathogenicity testing of shellfish hatchery bacterial isolates on Pacific oyster Crassostrea gigas larvae

Robyn M. Estes1, Carolyn S. Friedman1, Ralph A. Elston2, Russell P. Herwig1,*

1School of Aquatic and Fishery Sciences, University of Washington, PO Box 355020, Seattle, Washington 98195, USA
2AquaTechnics and Pacific Shellfish Institute, PO Box 687, Carlsborg, Washington 98324, USA
*Corresponding author. Email:

ABSTRACT: Bacterial diseases are a major cause of larval mortality in shellfish hatcheries. Even with proper sanitation measures, bacterial pathogens cannot be eliminated in all cases. The pathogenicity of bacteria isolated from Pacific Northwest shellfish hatcheries to Pacific oyster Crassostrea gigas larvae was investigated. We found 3 highly pathogenic strains and 1 mildly pathogenic strain among 33 isolates tested. These strains appear to be members of the genus Vibrio. Although there have been many studies of bivalve bacterial pathogens, a standard method to assess bacterial pathogenicity in bivalve larvae is needed. Thus, we developed 2 methods using either 15 ml conical tubes or tissue culture plates that were employed for rapidly screening bacterial strains for pathogenicity to Pacific oyster larvae. The tissue culture plates worked well for screening both mildly pathogenic strains and LD50 (lethal dose) assays. This method allowed for non-intrusive and non-destructive observation of the oyster larvae with a dissecting microscope. The LD50 for the 3 highly pathogenic strains ranged between 1.6 and 3.6 × 104 colony forming units (CFU) ml-1 after 24 h and between 3.2 × 102 and 1.9 × 103 CFU ml-1 after 48 h.

KEY WORDS: Pacific oyster larvae · Vibrio · Bacteria · Pathogenicity testing · Shellfish hatchery

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