The antagonistic effect of Bacillus pumilus on pathogenic Vibrio

Vibriospecies are the causative agents for vibriosis,the most serious aquaculture disease worldwide.Probiotics are well known to have the potential for broad use in aquaculture environments.In this study,the aim is to screen the probiotic candidate strain from the intestinal tract of healthy,aquaculture-raised large yellow croaker(Pseudosciaena crocea)for the aquaculture.We identified the strain by rapid identification system and 16S rDNA gene sequence analysis.Based on the both identification results,strain X93 was identified as Bacillus pumilus(GenBank accession No.HM137033).We also checked its antagonistic activities against pathogenic Vibrio parahaemolyticus (Vp-1.2164)and Vibrio alginolyticus (Va-Y)by the dot-inoculating method and tested the effects of temperature,salinity,pH,proteinase K and FeCl₃ on the antibacterial activity of the extracellular products.The results indicated that strain X93 could strongly inhibit indicator bacterial strains,such as Vibrio parahemolyticus,Vibrio alginolyticus,Edwardsiella tarda and Aeromonas hydrophila,exhibiting a very broad antibacterial spectrum.The cell-free antibacterial extracellular products secreted by strain X93 most strongly inhibited Vibrio parahemolyticus(Vp1.2164)and Vibrio alginolyticus(Va-Y)when X93 was cultured at 28 ℃ and 5 salinity for 48 h.Our data suggest that strain X93′s antibacterial extracellular products exhibit a fairly broad tolerance to pH,temperature,salinity,and FeCl₃.The antibacterial activity of the extracellular products,however,is proteinase K-sensitive,indicating that the active components are proteinaceous.In summary,the results presented here demonstrate that Bacillus pumilus strain X93 can be considered a probiotic since its secreted extracellular products are able to inhibit pathogenic Vibrio,which could have useful antibacterial applications.On a broader level,these results point to the potential for isolating bacteria with novel antibacterial activity from cultivated large yellow croaker.Most importantly,these results provide the necessary data for further development of probiotic products that inhibit pathogenic bacteria in aquatic environments.