Improved growth rate and disease resistance in olive flounder, Paralichthys olivaceus, by probiotic Lactococcus lactis WFLU12 isolated from wild marine fish

The use of probiotics is a strategy employed to improve host health status and to prevent infectious diseases. The current study was aimed at investigating the diversity of lactic acid bacteria (LAB) and Bacillus species in the gastrointestinal tracts of wild marine fishes, as well as the beneficial effects of Lactococcus lactis WFLU12 as a host-derived probiotics in olive flounder. Inmarine fishes, wild olive flounder and rock bream were shown to be good sources of LAB and Bacillus isolation, respectively. Some isolates, including the strain WFLU12, have shown stronger inhibitory activity against various aquatic bacterial pathogens and more tolerance to low pH and bile acids compared to some strains isolated from sources other than marine. Lc. lactis WFLU12 was found to confer to olive flounder protection against streptococcosis caused by Streptococcus parauberis through competitive exclusion and increased innate immune responses. Interestingly, the natural infection rate in the probiotic fed group (33%= 10/30) was significantly lower than that in the control group (60%= 18/30). None of the nisin Z and colicin V-producing probiotic-fed fish were naturally infected by S. parauberis during the feeding period. In addition, more importantly, this promising probiotic strain significantly promoted fish growth along with better feed conversion and specific growth rate. This study demonstrates that the use of host-derived probiotics can offer a significant advantage in terms of optimum survival and function in the gastrointestinal tract of the intended host. Statement of relevance: In this study, host-derived probiotic strain outperforms elimination of pathogen through competitive exclusion in the gastrointestinal tract and increased innate immune responses. More importantly, this promising probiotic strain significantly promoted fish growth along with better feed conversion. This study will provide insight into how optimal probiotics should be selected and developed. It might facilitate the replacement of commercial fish probiotic products originated from terrestrial sources with host-derived probiotics in the near future. (C) 2017 Elsevier B.V. All rights reserved.