Selection of Vibrio crassostreae relies on a plasmid expressing a type 6 secretion system cytotoxic for host immune cells

Adele James

Abstract:

Pacific oyster mortality syndrome affects juveniles of

Crassostrea gigas oysters and threatens the sustain-
ability of commercial and natural stocks of this spe-
cies. Vibrio crassostreae (V. crassostreae) has been

repeatedly isolated from diseased animals, and the
majority of the strains have been demonstrated to be

virulent for oysters. In this study, we showed that oys-
ter farms exhibited a high prevalence of a virulence

plasmid carried by V. crassostreae, while oysters, at

an adult stage, were reservoirs of this virulent popula-
tion. The pathogenicity of V. crassostreae depends on

a novel transcriptional regulator, which activates the
bidirectional promoter of a type 6 secretion system
(T6SS) genes cluster. Both the T6SS and a second
chromosomal virulence factor, r5.7, are necessary for
virulence but act independently to cause haemocyte
(oyster immune cell) cytotoxicity. A phylogenetically
closely related T6SS was identified in V. aestuarianus
and V. tapetis, which infect adult oysters and clams
respectively. We propose that haemocyte cytotoxicity
is a lethality trait shared by a broad range of mollusc
pathogens, and we speculate that T6SS was involved
in parallel evolution of pathogen for molluscs.