Competitive exclusion as a mode of action of a novel Bacillus cereus aquaculture biological agent

Aims: To determine the contribution of potential modes of action of a Bacillus cereus aquaculture biological control agent in inhibition of the fish pathogen, Aeromonas hydrophila. Methods and Results: When B. cereus was tested in plate well inhibition studies, no production of antimicrobial compounds was detected. Bacillus cereus had a high growth rate (0 center dot 96 h-1), whereas Aer. hydrophila concentration decreased by c. 70% in co-culture experiments. In nutrient limitation studies, B. cereus had a significantly higher growth rate when cultured under glucose (P < 0 center dot 05) and iron (P < 0 center dot 01) limitation in comparison with Aer. hydrophila. Bacillus cereus glucose (0 center dot 30 g l-1 h-1) and iron (0 center dot 60 mg l-1 h-1) uptake rates were also significantly higher (P < 0 center dot 01) than the Aer. hydrophila glucose (0 center dot 14 g l-1 h-1) and iron (0 center dot 43 mg l-1 h-1) uptake rates. Iron uptake was facilitated by siderophore production shown in time profile studies where relative siderophore production was c. 60% through the late exponential and sporulation phases. Conclusions: Competitive exclusion by higher growth rate, competition for organic carbon and iron, facilitated by siderophore production, could be identified as mechanisms of pathogen growth inhibition by B. cereus. Significance and Impact of the Study: This study is the first elucidation of the mechanism of action of our novel B. cereus biological agent in growth attenuation of pathogenic Aer. hydrophila. This study enhances the application knowledge and attractiveness for adoption of B. cereus NRRL 100132 for exploitation in aquaculture.