Isolation and characterization of lactic acid bacteria from Heteropneustes fossilis for probiotic applications in aquaculture.

Probiotics, notably lactic acid bacteria (LAB), provide an alternative to chemical treatments and therapeutics by improving microbial balance and supporting overall health. This study aimed to investigate the probiotic potential of LAB from Heteropneustes fossilis as feed additives in aquaculture. The experiments were focused on morphological, physiological, and molecular characterization of LAB isolates from the intestines of H. fossilis. Four strains of LAB belonging to the Lactococcus genus were identified: L. lactis HFL8, L. garvieae HFL1, L. garvieae HFL7, and L. formosensis HFL6. Among these, L. lactis HFL8 emerged as a promising probiotic candidate due to its strong tolerance to diverse environmental and physiological conditions, including fluctuations in temperature, pH, and NaCl concentration, as well as exposure to gut bile. L. lactis HFL8 exhibited growth at temperatures up to 45 °C and survived at 4 °C without proliferation. The strain tolerated acidic conditions as low as pH 2.0 and withstood gut bile concentrations up to 7.5%. In safety tests, the isolate exhibited no hemolytic activity and showed susceptibility to several commonly used antibiotics, being sensitive to ampicillin, vancomycin, erythromycin, tetracycline, chloramphenicol, and ciprofloxacin, while exhibiting resistance only to cefotaxime, suggesting that this strain appears safe for the host organisms. In addition, L. lactis HFL8 demonstrated strong probiotic traits, including high hydrophobicity (69.38 ± 0.54% in xylene and 67.83 ± 1.29% in toluene), robust autoaggregation (67.20 ± 0.18%), and substantial coaggregation with Bacillus pumilus (54.51 ± 0.87%) and Staphylococcus petrasii (51.37 ± 1.19%). Furthermore, L. lactis HFL8 demonstrated strong in vitro inhibitory activity against the fish pathogens Lactococcus garvieae and Aeromonas hydrophila. Collectively, these results suggest that L. lactis HFL8, isolated from H. fossilis, emerges as a potential probiotic candidate for aquaculture applications. Therefore, L. lactis HFL8 holds potential for future incorporation into functional feed formulations to enhance fish health, disease resistance, and overall sustainability in aquaculture systems. Collectively, the use of LAB-based indigenous probiotics could contribute to reduce antibiotic dependence, strengthen biosecurity, and promote more sustainable growth across the aquaculture industry.