An economically cost‐effective production medium optimization of <i>Bacillus subtilis</i> Ö‐4‐68: A potential probiotic for aquaculture

Probiotic use has become more important in aquaculture for healthy and sustainable output. In particular, Bacillus spp. have emerged as effective probiotic agents, improving gut health, enhancing the immune system, promoting growth, and providing protection against pathogens in fish. Therefore, the application of Bacillus in aquaculture offers a strategic approach to increasing productivity while reducing the reliance on antibiotics. In this study, the antibacterial activities of Bacillus isolates, whose probiotic properties will be determined, against test bacteria that are fish pathogens such as Aeromonas hydrophila, Vibrio anguillarum, Lactococcus garvieae, and Yersinia ruckeri were determined by using cross-streak method and agar well diffusion methods. Then, antibiotic resistances of 75 isolates determined to have antibacterial activity were screened against 9 different antibiotics by the agar disc diffusion method. Gastric juice (pH 2.5) tolerance of 55 isolates determined to be sensitive to antibiotics was examined, and the tolerance of 13 isolates to gastric juice was determined. Optimum growth characteristics at acidic pH, surface hydrophobicity, bile tolerance, and protease, amylase, lipase, and cellulase activities, hemolytic activities, coagulase activities, bacterial adhesion abilities, and biofilm production properties of these isolates were determined. As a result, Bacillus subtilis Ö-4-68, with the best probiotic properties, was selected from the examined isolates, and production medium optimization was carried out with laboratory scale statistical experiment design (Response Surface Methodology, RSM) for high amount of biomass production. As a result of the trials, an economical cost-effective production medium content with high biomass production was determined.