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Background  The probiotic bacterium  Phaeobacter inhibens  strain S4Sm, isolated from the inner shell surface of a healthy oyster, secretes the antibiotic tropodithietic acid (TDA), is an excellent biofilm former, and increases oyster larvae survival when challenged with bacterial pathogens. In this study, we investigated the specific roles of TDA secretion and biofilm formation in the probiotic activity of S4Sm.    Results  Mutations in  clpX  (ATP-dependent ATPase) and  exoP  (an exopolysaccharide biosynthesis gene) were created by insertional mutagenesis using homologous recombination. Mutation of  clpX  resulted in the loss of TDA production, no decline in biofilm formation, and loss of the ability to inhibit the growth of  Vibrio tubiashii  and  Vibrio anguillarum  in co-colonization experiments. Mutation of  exoP  resulted in a ~60 % decline in biofilm formation, no decline in TDA production, and delayed inhibitory activity towards  Vibrio  pathogens in co-colonization experiments. Both  clpX  and  exoP  mutants exhibited reduced ability to protect oyster larvae from death when challenged by  Vibrio tubiashii . Complementation of the  clpX  and  exoP  mutations restored the wild type phenotype. We also found that pre-colonization of surfaces by S4Sm was critical for this bacterium to inhibit pathogen colonization and growth.    Conclusions  Our observations demonstrate that probiotic activity by  P. inhibens  S4Sm involves contributions from both biofilm formation and the production of the antibiotic TDA. Further, probiotic activity also requires colonization of surfaces by S4Sm prior to the introduction of the pathogen.    Electronic supplementary material  The online version of this article (doi:10.1186/s12866-015-0617-z) contains supplementary material, which is available to authorized users.

Contributions of tropodithietic acid and biofilm formation to the probiotic activity of Phaeobacter inhibens