Isolation, Characterization, and Identification of Bacillus spp. Strains from the Digestive Tract of Mad Carp (Leptobarbus hoevenii) and Their Potential Probiotic Properties

The present study aimed to collect isolates of Bacillus spp. from the digestive tracts of mad carps (Leptobarbus hoevenii) and to screen these for probiotic properties. In this study, 85 bacterial isolates were obtained from the digestive tracts of mad carps. Of these, 73 isolates were Gram-positive, rod shaped, and catalase positive, while only 48 bacterial isolates were endospore forming. Evaluation of antagonistic effects of Bacillus spp. against pathogenic bacteria causing motile Aeromonas septicemia (MAS) in fish indicated that a total of 18 out of the 73 isolates exhibited antimicrobial activity, and especially the isolate MCSU14 expressed a significantly enlarged zone of inhibition (p<0.05). Study on hemolytic activity of these 18 antimicrobial isolates revealed that 6 isolates were γ-hemolytic. Furthermore, the 6 γ-hemolytic isolates survived acidic and bile salt exposures, and produced extracellular digestive enzymes. Among these six Bacillus spp., the isolate MCSU14 exhibited the most potent probiotic properties. Based on its biochemical characteristics and molecular analyses, the Bacillus sp. isolate MCSU14 is closely related to Bacillus velezensis. Our findings indicate that Bacillus spp. isolates from the digestive tracts of mad carp can be screened to find potent probiotics against MAS in aquaculture.