A Lactococcus lactis Strain Isolated from the Intestine of Grass Carp (Ctenopharyngodon idella) Exhibiting Antagonistic Activity and Intestinal Colonization Characteristics
LU Hong-Yu1, WANG Ya-Jun1, REN Yan1, MO Xu-Bing1, ZHAN Yu-Tian1, WANG Qing1, LI Qing-Yong2, ZHANG De-Feng1,*
1 Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences/Key Laboratory of Fishery Drug Development, Ministry of Agriculture and Rural Affairs/Guangdong Provincial Key Laboratory of Aquatic Animal Immunology and Sustainable Aquaculture, Guangzhou 510380, China; 2 Fisheries Research and Extension Center of Huizhou, Huizhou 516001, China
Abstract：Grass carp (Ctenopharyngodon idella) aquaculture in China faces serious challenges from various bacterial diseases. Conventional prevention and control strategies mainly rely on antibiotics and chemical disinfectants, the excessive use of which has led to increasing bacterial resistance and potential threats to the quality and safety of aquatic products. This study aimed to screen aquatic animal-derived probiotics with potential for the prevention and control of fish bacterial diseases. Intestinal and mucus samples were collected from healthy grass carp, homogenized, and used for the isolation and purification of lactic acid bacteria using MRS medium. Strain GCC10 was selected based on its antagonistic activity and hemolytic activity from intestinal. The morphological characteristics and taxonomic classification of strain GCC10 were determined, and its extracellular enzyme activity, antibiotic susceptibility, bile salt tolerance, intestinal colonization capasity, and biosafety were evaluated. Whole-genome sequencing was performed to evaluate the genomic safety and potential probiotic functions of strain GCC10. Strain GCC10 was identified as a Gram-positive Lactococcus lactis with no hemolytic activity. It exhibited extracellular protease activity, strong bile salt tolerance, and the ability to colonize the intestinal tract. The antibiotic susceptibility test showed that strain GCC10 was sensitive to several antimicrobial agents, including enrofloxacin, florfenicol, doxycycline, sulfamonomethoxine, but resistant to gentamicin. Antagonistic assays indicated that GCC10 displayed strong inhibitory activity against several common fish pathogens, such as Flavobacterium psychrophilum, Photobacterium damselae, Aeromonas hydrophila, Aeromonas sobria, and Aeromonas veronii.Whole-genome analysis revealed that GCC10 lacked major virulence genes. However, the genome contained genes related to heat and cold stress responses, bile salt and acid tolerance, immune modulation, proteolysis, and adhesion, as well as gene clusters involved in the production of lactococcal bacteriocins. This study screened out a Lactococcus lactis strain GCC10, which possessed good biosafety, strong antagonistic antibacterial activity, and effective intestinal colonization capability. This study provides a promising biological resource for controlling bacterial diseases in fish and has great potential for development and application as an antibiotic-reducing or antibiotic-replacing agent in aquaculture.
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