Effects of tryptophan combined with Limosilactobacillus reuteri M6 on the growth performance and intestinal barrier function of heat-stressed broilers.

This study investigated the effects of Tryptophan (Trp) combined with Limosilactobacillus reuteri M6 (L. reuteri M6) on growth performance and intestinal barrier function of broilers subjected to heat stress (HS). A total of 250 21-day-old male broilers (676.24 ± 1.43 g) were randomly assigned to five groups (5 replicates of 10 birds each). The thermoneutral (TN) group was raised at 23 ± 1 °C and fed a basal diet. The other four groups were exposed to cyclic HS (34 ± 1 °C for 8 h/day, and 23 ± 1℃ for the remaining hours) for 14 consecutive days. The HS group received the basal diet, while the HS + Trp group was fed the basal diet supplemented with 0.18% Trp; the HS + Lre group was fed the basal diet supplemented with 1 × 1010 CFU/kg L. reuteri M6; and the HS + Trp + Lre group was fed the basal diet supplemented with 0.18% Trp and 1 × 1010 CFU/kg L. reuteri M6. The results exhibited that HS increased rectal temperature and serum pro-inflammatory cytokine levels, decreased growth performance, serum anti-inflammatory cytokine levels, antioxidant capacity, and Trp metabolite concentrations, and impaired intestinal barrier function in broilers (P < 0.05). Dietary supplementation with Trp, L. reuteri M6, or their combination significantly increased serum total antioxidant capacity (T-AOC) and glutathione peroxidase (GSH-Px) activity, decreased serum tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin-1β (IL-1β), endotoxin (ET), and D-lactic acid (D-LA) concentrations, increased jejunal villus height (VH) and the ileal VH/crypt depth (CD) ratio, upregulated ZO-1 and Mucin2 mRNA expression in the ileal mucosa, and increased indole and indole-3-ethanol (IE) concentrations in the ileal contents (P < 0.05). Dietary supplementation with Trp or Trp combined with L. reuteri M6 decreased serum diamine oxidase (DAO) concentration, increased ileal VH, upregulated aryl hydrocarbon receptor (AhR) and signal transducer and activator of transcription 3 (STAT3) mRNA expression in the ileal mucosa, and increased indole-3-pyruvic acid (IPA) and 3-indoleglyoxylic acid (IGA) concentrations in the ileal contents (P < 0.05). Dietary supplementation with L. reuteri M6 or Trp combined with L. reuteri M6 decreased ileal CD (P < 0.05). Notably, Trp combined with L. reuteri M6 reduced rectal temperature on day 14 of HS, increased average daily feed intake (ADFI), serum catalase (CAT) activity, and serum interleukin-22 (IL-22) level, and decreased Claudin2 mRNA expression in the ileal mucosa (P < 0.05). In conclusion, dietary supplementation with Trp, L. reuteri M6, or their combination improved intestinal barrier function in heat-stressed broilers. The combined supplementation showed better efficacy, as reflected by increased ADFI and a tendency toward increased ADG, although F/G was not significantly affected.