Impact of supplementary Lactobacillus casei K17 on growth and gut health of largemouth bass Micropterus salmoides

Micropterus salmoides is one of the most economically valuable fish species in China. Currently, information in the effect of probiotics-supplemented feed on M. salmoides is rare. Present study investigated the impact of supplementary Lactobacillus casei K17 with six treatments on growth performance, gut digestion, absorption and non-specific immune, and gut bacterial biota and metabolomics of M. salmoides. A total of 450 M. salmoides (114 day of age, average 33.0 +/- 0.5 g) were randomly allocated into six groups for a 69-day rearing experiments. M. salmoides was given commercial feed supplemented with: 1) live L. casei K17 (1 & times; 109 CFU/g), 2) live L. casei K17 protected by skim milk powder (10 %), 3) heat-killed dead L. casei K17, 4) its fermentation supernatant, 5) skim milk powder (10 %), and 6) normal saline (0.85 % NaCl), designated as LB, MB, DB, FS, MG, and SG, respectively. MB and DB diets significantly increased body weight and growth rate, but decreased feed conversion ratio of M. salmoides (p < 0.05). These two diets also significantly increased gut amylase and lipase activities and microvilli surface areas (p < 0.05). Comparative bacterial biota analysis revealed distinct bacterial community compositions in M. salmoides gut contents among the six groups, which were remarkably dominated by L. casei in LB (90.0 %), MB (95.5 %), and DB (92.4 %) groups. Occurrences of some pathogenic bacteria were significantly reduced (p < 0.05). Meanwhile, up-modulated carbohydrate and down-modulated amino acid metabolisms were observed in LB, MB, and DB groups (p < 0.05). Comparative metabolomic analysis also revealed a number of notably altered metabolites, which, coupled with increased serum LZM and SOD, improved gut non-specific immune of M. salmoides, particularly in MB and DB groups. The results provided a reference of promising probiotic component L. casei K17 to feed industry.

Automated summary

The study found that supplementing probiotics feed can significantly improve the growth performance, gut digestion and absorption, non-specific immune system, and gut bacterial community of Micropterus salmoides. Diets with Lactobacillus casei K17 as the main component can enhance the immune response of Micropterus salmoides, providing a reference for the feed industry.