Abstract
Poor utilization efficiency of plant protein diets always leads to intestinal barrier dysfunction and growth inhibition in animals. Probiotics have shown promise in improving growth performance and gut health of the host. However, obtaining the host-beneficial probiotic from thousands of bacterial phylotypes is challenging. Here, four intestinal autochthonous bacteria were isolated from fast-growing bullfrog after a 60-day feeding on a soybean meal (SM)-based diet. Another feeding trial was conducted to evaluate the effects of supplementing these strains in an SM-based diet on growth, nutrient digestibility, immunity, and gut health of bullfrog. A high-SM basal diet was used as a non-supplemented control group (NC), and four other diets were prepared by supplementing the basal diet with 1 × 107 CFU/g of Bacillus siamensis, Bacillus tequilensis (BT), Bacillus velezensis, and Lactococcus lactis (LL). Results showed that weight gain, feed efficiency, nitrogen retention, and apparent digestibility coefficients of dry matter and protein were significantly higher in the LL group compared with the NC group (p < 0.05). Furthermore, compared with the NC group, both BT and LL groups showed markedly higher jejunal protease and amylase activities, serum complement 4 and immunoglobulin M levels, jejunal muscularis thickness (p < 0.05), and up-regulated expression of il-10 and zo-1 genes (p < 0.05). High-throughput sequencing revealed higher abundances of Bacillus and Cetobacterium in BT and LL groups, respectively, accompanied with decreased abundances of Enterobacter and Escherichia–Shigella. Besides, KEGG pathways related to metabolisms were significantly enhanced by the LL diet relative to the NC diet (p < 0.05). Overall, the beneficial effects of two frog-derived probiotics were determined: supplementation of L. lactis in SM-based diet promoted growth and nutrient digestibility; both B. tequilensis and L. lactis supplementation improved immune response and intestinal barrier function of bullfrogs.
Highlights
Plant proteins are widely used as substitutes for expensive animal-derived protein sources in aquaculture and livestock sectors (Li Z.C. et al, 2017; Naylor et al, 2021)
The bacterial colonies were divided into different types based on the colony characteristics of shape, structure, size, opacity, and color, and the colonies of each recognizable type was counted for determining number and occurrence rate [(the number of plates containing the strain)/(the number of all plates) × 100]
Compared with the non-supplemented control group (NC) group, WG was significantly increased in bullfrogs fed the Lactococcus lactis (LL) diet (p < 0.05), and intermediary WG values were observed in BS and Bacillus tequilensis (BT) groups (p > 0.05; Table 2)
Summary
Plant proteins are widely used as substitutes for expensive animal-derived protein sources in aquaculture and livestock sectors (Li Z.C. et al, 2017; Naylor et al, 2021). Increasing studies showed that gut microbiota is deeply involved in various host physiological processes (Gao et al, 2018) It can be regarded as an “internal factor,” which confers multifaceted effects on the host. A research on sea cucumber (Apostichopus japonicus) revealed that the differences in Actinobacteria abundance might be associated with the remarkable difference in body weight (Sha et al, 2016) These studies indicated that the growth of animals is closely linked to the gut microbiota. A 58-day feeding trial was conducted to investigate the effects of supplementing these bacterial strains in a high-SM diet on growth performance, feed utilization, nutrient digestibility, immune function, and gut health of bullfrog
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