Genomic and phenotypic analysis of probiotic properties of GABA-producing Enterococcus lactis A1 in Micropterus salmoides aquaculture.

Aquaculture contributes substantially to global food and nutrition security. Dysfunction of the fish intestinal mucosal barrier increases susceptibility to pathogens and affects aquaculture production. Indigenous probiotics, specifically those targeting intestinal health, are promising agents for disease control and growth promotion. Gamma-aminobutyric acid (GABA), derived from microorganisms, is a postbiotic with multiple beneficial functions. However, there is a lack of reports on GABA-producing probiotic strains in aquatic animals. Herein, we evaluated the probiotic properties of Enterococcus lactis A1 isolated from the gut of Micropterus salmoides. Whole-genome sequencing revealed its potential to synthesize GABA, which was confirmed in MRS broth and commercial feed. The strain contained multiple bacteriocin genes and showed antibacterial activity against bacterial pathogens prevalent in aquaculture. Genomic and phenotypic analyses revealed its biosafety, stress tolerance, and adhesion ability. In vitro, it exhibited immunomodulatory properties and displayed resistance against Aeromonas hydrophila in M. salmoides head kidney lymphocytes. An eight-week in vivo study showed that orally supplemented E. lactis A1 significantly promoted growth, enhanced intestinal trypsin and lipase activities, increased the activities of superoxide dismutase and catalase, and reduced the malondialdehyde levels. Diversity and composition of the intestinal microbiota were positively modulated by E. lactis A1. Moreover, it effectively improved intestinal morphology by increasing villus height and improving resistance against A. hydrophila infection, as indicated by improved intestinal integrity, reduced blood pathogen counts, and increased fish survival rates. This study highlights the potential of GABA-producing E. lactis A1 as a probiotic for growth promotion and disease resistance in M. salmoides.