MF1

Abstract Over the past few decades, there has been increasing interest in using probiotics as an alternative to antibiotics. Researchers have conducted studies to investigate bacterial strains for their probiotic potential. Like other animals, fish also have several bacterial strains in their gut that possess probiotic properties, although this is limited to Bacillus species. Therefore, this study aimed to isolate and characterize probiotic Bacillus species from the gut of Masheer fish (Tor Puititora). Four pure bacterial isolates were selected as potential probiotic strains based on selection criteria, including survival rate in acid and bile salt. The isolates exhibited significant antimicrobial activity against pathogenic bacteria, including Escherichia coli (ATCC8739), Pseudomonas aeruginosa (ATCC9027), and Staphylococcus aureus (ATCC6538). MF2 and MF3 demonstrated clear zones, including antimicrobial activity against all three indicator pathogens. MF1 and MF4 exhibited antimicrobial activity against E. coli (ATCC8739) and P. aeruginosa (ATCC9027). Furthermore, the 16S rRNA gene sequences of all isolates exhibited a close association with Bacillus tequilensis (KCTC13622), with nucleotide similarity of 98.63%, 98.25%, 98.80%, and 98.35%, respectively. Our results demonstrate that these bacterial isolates show promise as an alternative to antibiotics in the fisheries food system. In this study, all isolates identified in the fish gut were subspecies of B. tequilensis.