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.
