Screening of bacterial pathogens associated with mass summer mortality of the Pacific oyster, Crassostrea gigas, in China

The Pacific oyster (Crassostrea gigas) is an important shellfish species for aquaculture. Mass summer mortality of C. gigas has seriously hampered the development of oyster industry in recent years, while the mortality causing factors remain elusive. In order to explore the pathogenic factors associated with mass summer mortality, we performed a bacterial screening of moribund oysters collected from an oyster farm in major C. gigas aquaculture area in northern China. Fourteen strains of bacteria were isolated from hemolymph of moribund oysters, and were further identified as five Vibrio species, including Vibrio harveyi, V. alginolyticus, V. rotiferianus, V. brasiliensis, and V. owensii, based on 16S rRNA sequencing and gyrB sequencing. The virulence of bacterial strains was examined by measuring extracellular protease activity and performing experimental challenge trials. Three previously reported pathogenic Vibrio strains, V. crassostreae, V. aestuarianus and V. splendidus, were also included to better understand the virulence of the pathogens isolated in this study that can cause mortality. Experimental challenge trials revealed that V. splendidus, V. brasiliensis, V. harveyi and V. alginolyticus were all pathogenic to C. gigas, with V. alginolyticus exerting the highest level of pathogenicity. This work performed a preliminary investigation on the pathogenic factors associated with mass summer mortality of C. gigas, which should be valuable for disease surveillance