Effect of Bacillus subtilis mutants on growth performance of piglets fed tryptophan- and valine-deficient diets

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The objective was to determine the concentration of L-Trp and L-Val to be substituted by feeding piglets Bacillus subtilis strains developed to overproduce Trp (B. subtilis Trp mutant [BsTrp]) and Val (B. subtilis Val mutant [BsVal]) and by using equations obtained in 3 dose-response studies with Leu, Trp, and Val. A Leu-deficient diet with BsVal was included to distinguish between a general probiotic effect and in situ AA production. Eight diets were formulated for each of the 3 studies: 6 diets with 6 levels of AA and 2 diets with BsTrp or BsVal at 10x or 100x standard doses added to the second lowest level of Leu, Trp, and Val. Pigs (n = 16; 7-9 kg BW) were fed ad libitum from 1 wk after weaning. Blood samples were taken on d 8 and 15. The effect of BsTrp and BsVal was evaluated by expressing the improvement in daily gain and feed intake in equiv-alents of grams L-Leu, L-Trp, or L-Val per kilogram feed using curvilinear plateau and broken-line equations obtained by modeling the 6 AA levels. Bacillus subtilis Val mutant increased animal performance corresponding to 0.88 and 0.39 g L-Leu and 0.17 and 0.44 g L-Val per kilogram feed for 10x and 100x doses, respectively. Bacillus subtilis Trp mutant was equivalent to 0.02 and 0.11 g L-Trp/kg feed for 10x and 100x doses, respectively. Bacillus subtilis Val mutant (10x dose) increased (P < 0.05) plasma Val concentration in the Val study. Bacillus subtilis Trp mutant tended (P = 0.06) to increase Trp plasma concentrations. In conclusion, dietary inclusion of B. subtilis mutants indicated some improvements in the performance of pigs fed AA-deficient diets, which could be related to intestinal synthesis and release of L-Trp and L-Val, to protease activity, or to a general probiotic effect.