A large-scale epidemiological study to identify bacteria pathogenic to Pacific oyster Crassostrea gigas and correlation between virulence and metalloprotease-like activity.

A 4-year bacteriological survey (2003-2007) of four molluscs cultivated in France and faced with mortality episodes was performed by the French shellfish pathology network. The more abundant bacteria isolated during 92 mortality episodes, occurring mainly in Pacific oyster Crassostrea gigas, were identified by genotyping methods. It allowed us both to confirm the representativeness of Vibrio splendidus and Vibrio aestuarianus bacterial strains and to identify both a large number of Vibrio harveyi-related strains mainly detected during 2007 oyster mortality outbreaks and to a lesser extent bacterial strains identified as Shewanella colwelliana. Because metalloprotease has been reported to constitute a virulence factor in a few Vibrio strains pathogenic for C. gigas, several bacterial strains isolated in this study were screened to evaluate their pathogenicity in C. gigas spat by experimental infection and their ability to produce metalloprotease-like activity in the culture supernatant fluids. A high level (84%) of concordant results between azocaseinase activities and virulence of strains was obtained in this study. Because bacterial metalloprotease activities appeared as a common feature of pathogenic bacteria strains associated with mortality events of C. gigas reared in France, this phenotypic test could be useful for the evaluation of virulence in bacterial strains associated with such mortality episodes.