A Novel Lactococcus Lactis LZK-02 Strain Antagonistic to Staphylococcus Aureus: In Vitro Probiotic Profiling and in Vivo Benefits as Feed Additive in Micropterus Salmoides.

The emergence of antibiotic-resistant pathogens has raised growing concern in aquaculture, prompting the development of probiotic-based strategies for disease control. In this study, a novel strain of Lactococcus lactis (LZK-02) was isolated from the intestine of healthy largemouth bass (Micropterus salmoides) and evaluated for its probiotic properties and immunomodulatory potential. The strain was identified through polyphasic taxonomy, including morphological characterization, 16 S rRNA gene sequencing, and whole-genome analysis. In vitro assays showed that LZK-02 exhibited strong antagonistic activity against Staphylococcus aureus, along with acid and bile tolerance, auto-aggregation, surface hydrophobicity, and digestive enzyme production. Genome annotation revealed genes related to stress resistance, adhesion, and bacteriocin biosynthesis. A feeding trial was conducted using diets supplemented with 10⁷-10⁹ CFU/g of LZK-02 for eight weeks. Results showed significant improvements in growth performance, intestinal histomorphology, antioxidant enzyme activities (SOD, CAT), and immune parameters (ACP, AKP, lysozyme) in treated groups compared to the control. Following intraperitoneal challenge with S. aureus, LZK-02-fed fish exhibited higher survival rates and lower bacterial loads in the liver. These findings suggest that L. lactis LZK-02 is a safe and effective probiotic candidate capable of enhancing immune responses and disease resistance in largemouth bass, and may serve as a potential alternative to antibiotics in intensive aquaculture.