Screening and Evaluation of Host-Derived Intestinal Probiotic Candidates Lactococcus sp. and Bacillus sp. from Largemouth Bass for Bacterial Disease Biocontrol

Bacterial infections represent a major constraint to largemouth bass (Micropterus salmoides) aquaculture; safe and sustainable alternatives to conventional antibiotics are needed. This study aimed to screen, characterize, and evaluate host-derived intestinal probiotics from healthy largemouth bass for bacterial disease biocontrol. A total of 68 intestinal bacterial strains were isolated and screened for antagonistic activity against 21 bacterial pathogens previously isolated from diseased largemouth bass. Five isolates with stable inhibitory activity were identified as Plesiomonas shigelloides (MS-24), Aeromonas dhakensis (MS-26), Lactococcus lactis (MS-54), Bacillus subtilis (MS-67), and Bacillus paranthracis (MS-68) by 16S rRNA and gyrB gene sequencing. Morphological characteristics, physiological and biochemical profiles, and growth kinetics of the 5 isolates were assessed. Among them, only MS-54 and MS-67 exhibited non-hemolytic phenotypes and low in vivo toxicity, with host mortality rates of 0.00% and 1.11%, respectively. In challenge trials, dietary supplementation with MS-54, MS-67, or their combination reduced cumulative mortality to varying degrees. Significant protection was observed against V. alginolyticus and A. jandaei, and the highest relative percent survival (RPS) was 22.62% in the MS-54 + MS-67 group after A. jandaei challenge. Results so far indicated that Lactococcus lactis MS-54 and Bacillus subtilis MS-67 are promising host-derived probiotic candidates with good antagonistic activities, providing potential strategies for bacterial disease biocontrol and sustainable management in largemouth bass aquaculture.