Effects of Bacillus subtilis supplementation in soybean meal-based diets on growth performance, diet digestibility and gut health in bullfrog Lithobates catesbeianus

Abstract

An 8-week feeding experiment was carried out to evaluate the supplemental effects of Bacillus subtilis in all-plant protein diets on growth, nutrients digestibility, and intestinal histomorphology and microbial composition in bullfrog Lithobates catesbeianus. Totally four diets were prepared: a non-supplemented control diet and three probiotic diets containing 1 × 105, 1 × 107 or 1 × 109 CFU g−1 of B. subtilis (Con, BS5, BS7 and BS9 diets). The experiment was done in triplicates and bullfrogs averaging 42.5 ± 0.2 g were fed to visual satiety twice daily. No significant changes in growth, feed utilization, body composition and survival rate were found by probiotic application (P > 0.05). However, significant improvements in apparent digestibility coefficients of protein, calcium and phosphorus were found by probiotic supplementation. Moreover, whole-body phosphorous and calcium contents were enhanced by probiotic administration. The highest villus height and thickness were recorded in the BS5 group. Operational taxonomic unit (OTU) diversity and richness significantly increased by application of 1 × 107 CFU g-1 of B. subtilis. Furthermore, B. subtilis abundance in the gut was enhanced with increasing the probiotic inclusion level indicating the successful establishment of the probiotic. The findings in this research showed that supplementing 1 × 107 CFU g−1 of B. subtilis in soybean meal-based diets for bullfrog beneficially influences nutrients digestibility and uptake, and intestinal structure and microbial composition.