Surface materials and biofilm formation affect tropodithietic acid biosynthesis gene expression in <i>Phaeobacter piscinae</i> S26 and its interaction with <i>Vibrio anguillarum</i>

Abstract Tropodithietic acid (TDA) is the antimicrobial compound mediating the probiotic activity of Phaeobacter spp. against pathogenic vibrios in marine larviculture. In the related genera Tritonibacter, TDA is mainly produced during biofilm growth. The purpose of the study was to determine if biofilm formation increases expression of TDA biosynthesis genes in Phaeobacter piscinae and whether surface properties can promote this expression for aquaculture. Using a GFP reporter under the tdaCDE promoter and flow cytometry, TDA biosynthesis gene expression was two-fold higher in biofilms than in planktonic cells. Biofilm formation on four surfaces showed that the most hydrophobic material, PDMS, supported ∼30-fold higher early attachment than PMMA, COC and PP, while mature biovolume was similar across materials. The TDA gene expression to biovolume ratio increased as biofilms developed. P. piscinae biofilms inhibited Vibrio anguillarum, reducing counts to 105 CFU/ml compared to the control that was 108 CFU/ml. In the presence of V. anguillarum, PDMS biofilms showed 1.5-fold higher biovolume and 2.5-fold higher TDA biosynthesis gene expression, suggesting an antagonistic response. Thus, Phaeobacter is a more effective Vibrio antagonist in the biofilm state, supporting its potential as a probiotic for future aquaculture applications.