Vibrio crassostreae, a benign oyster colonizer turned into a pathogen after plasmid acquisition

Adele James

Abstract:

Vibrios are frequently associated with oyster mortality; however whether they are the primary
causative agent or secondary opportunistic colonizers is not well understood. Here we combine
analysis of natural infection dynamics, population genomics and molecular genetics to ask (i) to what
extent oysters are passively colonized by Vibrio population present in the surrounding water, (ii) how
populations turn over during pathogenicity events and (iii) what genetic factors are responsible for
pathogenicity. We identified several populations of Vibrio preferentially associated with oyster
tissues. Among these, Vibrio crassostreae is particularly abundant in diseased animals while nearly
absent in the surrounding water, and its pathogenicity is correlated with the presence of a large
mobilizable plasmid. We further demonstrate that the plasmid is essential for killing but not
necessary for survival in tissues of oysters. Our results suggest that V. crassostreae first
differentiated into a benign oyster colonizer that was secondarily turned into a pathogen by
introgression of a virulence plasmid into the population, possibly facilitated by elevated host density
in farming areas.