Phaeobacter inhibens as probiotic bacteria in non-axenic Artemia and algae cultures

The growing aquaculture industry is in need for non-antibiotic based disease control strategies to reduce risk of bacteria developing and spreading antibiotic resistance. We have previously, in axenic model systems of live larval feed, demonstrated that bacteria from the Roseobacter clade can antagonize fish pathogens such as Vibrio anguillarum and Vibrio harveyi and that they can reduce larval mortality in challenge trials. However, in the aquaculture production, a natural microbiota is present at all stages and may affect the efficacy of the probiotic bacteria. The purpose of the present study was to determine if marine roseobacters in non-axenic systems were capable of antagonizing fish pathogenic vibrios. We added a controlled background microbiota of four bacterial strains to axenic Artemia and algae (Duniella) and these bacteria had a marginal but significant reducing effect on inoculated Vibrio anguillarum that grew to 10⁷ in control samples but to a level 1–2 log lower in samples with background microbiota. The addition of the Roseobacter-clade bacteria, Phaeobacter inhibens, caused a significant reduction in growth of the pathogen that reached levels 3–4 log lower than in the control. In non-axenic natural Artemia and algae (Tetraselmis) received from an aquaculture unit, Vibrio anguillarum grew to 10⁷ CFU/ml but only reached 10⁴ CFU/ml when P. inhibens was also added. P. inhibens was added at a concentration 10⁶ CFU/ml in all systems and remained at this concentration at the end of the study, irrespective of the background microbiota. We therefore conclude that P. inhibens are indeed promising as probiotic bacteria in marine larvi-culture where it in natural live feed can suppress fish larval pathogens.