Mechanisms of microbe-microbe-host interactions in a probiont-pathogen-bivalve model

Abstract

Marine mollusks are filter feeders and thus are continuously exposed to a myriad of microorganisms. While the majority of bacteria likely have no effect on the overall health of the animal, some microbes, including members of the bacteria genus Vibrio, can cause infections that lead to rapid mortality, especially in larval and juvenile bivalves. For example, the oyster pathogen Vibrio tubiashii causes sporadic crashes of larval oyster production in commercial hatcheries. Alternatively, it is also clear that certain beneficial microorganisms can colonize mollusks and provide disease resistance. Such microbes can potentially be developed as probiotic agents to combat disease outbreaks in bivalve hatcheries. To enable the rational development of such microbes as probiotics, a more comprehensive knowledge of the mechanisms involved in microbe-microbe-host interactions is required. In this study, we investigated how the bacterium Phaeobacter gallaeciensis S4Sm reduces mortality of larval oysters when challenged with V. tubiashii. A combination of analytical chemistry, genetic mutation, and both in vitro and in vivo challenge assays were used to interrogate the roles of specific bacterial metabolites and also bacterial behaviors such as biofilm formation. Our results show that the probiotic activity of S4Sm is multifactorial, involving its ability to form extensive biofilms, production of the potent antibiotic tropodithietic acid, and quorum quenching of pathogen virulence genes.