Genomic and functional insights into a novel Lactococcus sp. AK05 from Cirrhinus mrigala with potent antagonism against Aeromonas hydrophila.

In aquaculture, addressing antibiotic-resistant fish pathogens underscores the immediate necessity for the identification of candidate probiotic strains with favorable safety and functional attributes as alternatives to antibiotics. 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 and grow in harsh conditions, including a pH of 4, and 0.8% bile salts. At 8% NaCl, it survived and exhibited merely a one-log rise in CFU·mL⁻¹, while NaCl levels ≤ 5% facilitated over a 4-log increase in 24 h of incubation. 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. Genome analysis indicated a lack of virulence and antibiotic resistance genes, except presence of two accessory genes, vanY and vanW, which in absence of other core van genes do not confer vancomycin resistance. In addition, the genome of AK05 displayed genes responsible for producing lactococcin, alongside genes that confer tolerance to acid and bile, adhesion capabilities, and carbohydrate utilization. These traits, along with its ability to contain A. hydrophila, make AK05 a further evaluation as a non-antibiotic strategy for disease management in aquaculture.