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

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