A Vibrio splendidus strain is associated with summer mortality of juvenileoysters Crassostrea gigas in the Bay of Morlaix (North Brittany, France)

Juvenile oysters Crassostrea gigas cultured in the Bay of Morlaix (France) have suffered unexplained summer mortalities for over a decade. In the pres ent study, we tested the hypothesis that a bacterial pathogen could be resp onsible for this phenomenon. A first attempt failed to isolate a bacterial pathogen from moribund or weak oysters. Only non-pathogenic, probably oppor tunistic, bacteria were isolated. As an alternative approach, we focused on oysters presenting reduced stress-response capacities (determined by circu lating noradrenaline measurements), a characteristic of juvenile oysters en tering an early phase of the disease. Cultures of bacterial isolates on TCB S plates revealed that a Vibrio strain was present in diseased oysters and scarce or absent in healthy oysters, Experimental infections indicated that this Vibrio can cause mortalities of juvenile oysters when injected at con centrations ranging from 10(4) to 10(8) CFU oyster(-1). Similarly to the su mmer mortality disease, the Vibrio isolate caused higher mortalities at hig her temperatures; apparently, it could not be transmitted horizontally, it did not affect adult oysters and it induced stress-response dysfunctions in juvenile oysters. Phenotypic and genotypic characterizations identified th e pathogen as Vibrio splendidus. Taken together, the present results satisf y Koch's postulate and suggest that this bacterial strain is probably respo nsible for the juvenile oyster summer mortalities in the Bay of Morlaix.