Genomic and Functional Insights into a Novel Lactococcus sp. AK05 from Cirrhinus mrigala with Potent Antagonism Against Aeromonas hydrophila

Resistance in pathogens has increased as a result of the extensive usage of antibiotics in aquaculture. This emphasizes how urgently safe and efficient probiotic substitutes are needed. In our study, we isolated and characterized a new strain of Lactococcus sp., called AK05, from the gut of Cirrhinus mrigala . The phenotypic tests showed that it can survive harsh conditions, including a pH of 4, 0.8% bile salts, and even up to 8% NaCl. It also showed strong autoaggregation at 95.65%, co-aggregation with Aeromonas hydrophila subsp. hydrophila MTCC 1739 at 82.2%, and antimicrobial activity, with a 15.66 mm inhibition zone. A safety evaluation revealed γ-hemolysis, no gelatinase or DNase activity, and vulnerability to the majority of antibiotics, with the exception of trimethoprim. With 97.61% ANI and 78.9% dDDH, whole-genome sequencing showed that AK05 and Lactococcus lactis subsp. hordniae are closely related, yet only have 21% phenotypic similarity. This supports its classification as a unique strain. Our genomic analysis found genes that produce bacteriocins, like lactococcin, as well as genes that provide acid and bile tolerance, adhesion, and carbohydrate use. These traits, along with its ability to contain A. hydrophila , make AK05 a promising option for managing diseases without antibiotics and improving gut health in aquaculture.