| The addition of probiotic bacteria to aquaculture system water or feed has proven effective at decreasing pathogen-induced mortality of fish and shellfish. The mechanisms by which host protection is achieved are being studied extensively, as the implications of probiosis to the aquaculture industry are substantial. The present study begins to assess the mechanisms used by an oyster probiotic Vibrio sp., OY15, in protecting its larval host, Crassostrea virginica, from a pathogenic Vibrio sp., B183. Plasmids harboring gfp and rfp were introduced into OY15 (named OY15g) and B183 (named B183r), respectively, forming fluorescent derivatives, which yielded unique genetic identifiers for each strain. Under non-selective conditions, plasmids were maintained faithfully for at least 72 hours. qPCR amplification assays were developed to measure gfp, rfp, and genes for 16S and 18S rRNA at 24, 48 and 72 hours during the course of an oyster challenge experiment. These experiments demonstrated temporal relationships between probiotic, pathogen, and total bacteria with their larval host and revealed that larval survival and mortality could be correlated to the presence or absence of probiotic and pathogen. Experiments examining the processes by which OY15g inhibits B183r activity included the growth of both bacteria in liquid media, which indicated that OY15g is capable of outcompeting B183r. In addition, OY15g supernatants were found to possess a factor(s) that destabilized B183r biofilm over time, suggesting a potential mechanism for OY15g probiosis. Furthermore, while quorum sensing N-acyl homoserine lactone (AHL) production could not be detected for OY15g by an Agrobacterium tumefaciens reporter assay, B183r was found to produce AHLs, suggesting that OY15g may influence B183r pathogenicity by interfering with AHL production. |
