Selection and characterization of intestinal Bacillus strain antagonistic against pathogenic Streptococcus agalactiae of tilapia

Abstract:

[Objective] The aim of this study was to screen a probiotic strain that antagonizes Streptococcus agalactiae isolated from the intestine of healthy Nile tilapia (Oreochromis niloticus). [Methods] The gut of healthy tilapia was collected, homogenized, and diluted by a 10-fold series. Then 0.3mL suspension was streaked on brain heart infusion plates and incubated for 1-2 d at 30℃, and then single colonies were selected for purification. Strains had antagonistic function against S. agalactiae were screened by the dot-inoculating method, and a strain named LF01 with a better antagonistic effect was selected. In this study, the strain LF01 was identified by physiological, biochemical characteristics and molecular biological analysis. In addition, some characteristics and biosafety of the strain LF01 were determined and analyzed. [Results] A total of 64 strains were isolated from the intestine of healthy tilapia and 6 strains with obvious antagonistic effects were obtained, with the antagonism of LF01 strain considered the best. Strain LF01 was identified as Bacillus velezensis. The best growth temperature, pH, and salinities of strain LF01 were 30℃, 7, and 5‰, respectively. Furthermore, the strain LF01 exhibited the ability to hydrolyze starch and casein. Strain LF01 was sensitive to most antibiotics and resistant only to bacitracin, and showed antimicrobial activity against a broad range of fish pathogens including Streptococcus agalactiae, Streptococcus iniae, Edwardsiella tarda, Edwardsiella ictaluri, Aeromonas hydrophila, Aeromonas schubertii, Aeromonas veronii, Aeromonas jandaei and Nocardia seriolae. Biosafety tests suggested that the strain LF01 was not pathogenic to Nile tilapia, Danio rerio, and Channa argus. [Conclusion] In this study, it was found that B. velezensis LF01 strain isolated from the intestine of healthy tilapia has good biosafety and could antagonize the common aquatic pathogens. It has the potential to prevent and control a variety of aquatic animal diseases, and has broad application prospects.