Comparative analysis of Vibrio splendidus related strains isolated during Crassostrea gigas mortality events

French mollusc production is based mainly on the Pacific cupped oyster, Crassostrea gigas. Since 1991, annual mass mortality of juveniles has been reported during summer months. These recurring episodes concern professionals who fear that like Portugese oyster, C. angulata, C. gigas could in turn disappear following one of these epizooties. Previously, bacteriological analysis of moribund oyster juveniles yielded an isolate of a Vibrio splendidus biovar II strain, named TNEMF6. This isolate was demonstrated to be pathogenic to Crassostrea gigas spat by experimental challenge. To study the association between summer oyster mortality and presence of TNEMF6 cluster strains, Vibrionaceae fauna were isolated from infected spat along the French Atlantic coast between 1997-1998. Strains related to V. splendidus biovar II were selected. Comparison with TNEMF6 was performed by classical biochemical tests and polymerase chain reaction - restriction fragment length polymorphism (PCR-RFLP) of SSU rDNA, rpoD, and gyrB genes. Genomic similarities were confirmed by DNA/DNA hybridization. Only one strain out of 14, TNNIII7, was found to be closely related to the pathogenic bacteria. Neither the phenotypic nor the genotypic markers used in this study were able to distinguish pathogenic from non-pathogenic strains of the widespread V. splendidus. However, future genetic comparisons of TNEMF6 and TNNIII7 is likely to reveal genes involved in pathogenicity.Original Abstract: La production francaise de mollusques est principalement basee sur la culture d'huitres creuses Crassostrea gigas. Depuis 1991, en periode estivale, de forts taux de mortalite de naissain d'huitres creuses sont observes regulierement en milieu naturel et en ecloseries. Ces episodes recurrents inquietent les professionnels qui craignent qu'a l'instar de l'huitre portugaise, C. angulata, C. gigas puisse disparaitre a son tour, suite a une epizootie. Differentes souches bacteriennes appartenant au genre Vibrio ont ete associees a des mortalites de mollusques. Recemment, nous avons isole une souche Vibrio splendidus biovar II, TNEMF6, pathogene de naissain d'huitre creuse. Dans ce travail, nous avons recherche la presence de cette souche dans d'autres episodes de mortalite estivale. La flore vibrionacee d'animaux sains et moribonds a ete isolee et les souches apparentees a Vibrio splendidus biovar II ont ete selectionnees. Une comparaison des souches a ete effectuee par phenotypage (39 tests biochimiques) et genotypage (PCR-RFLP SSU rDNA, rpoD et gyrB). Les homologies genomiques ont ete confirmees par hybridation ADN/ADN. Seule, une souche sur 14, apparait proche de ce vibrio pathogene. Cependant, les techniques phenotypiques et genotypiques utilisees dans ce travail ne permettent pas de distinguer les isolats pathogenes et non pathogenes. Cependant, les deux souches TNEMF6 et TNNIII7 vont etre utilisees dans une approche de genomique comparative pour l'etude des supports moleculaires de la virulence.