Effects of <i>Limosilactobacillus reuteri</i> on gastrointestinal microbiota, muscle fiber formation and meat quality of Sunit sheep

<i>Limosilactobacillus reuteri</i> is a species of lactobacillus present in the gastrointestinal tract of animals and may contribute to the balance of intestinal flora. This study aimed to investigate the effects and correlations of <i>L. reuteri</i> on both intestinal flora and muscle fiber characteristics of Sunit sheep, with the objective of enhancing meat quality. Twelve 8-month-old Sunit sheep were randomly divided into two groups: control (fed without <i>L. reuteri</i>) and <i>L. reuteri</i> group. The results showed that <i>L. reuteri</i> had a significant positive impact on meat quality, particularly improving the tenderness of sheep meat compared with the control group, with values of 69.8 and 76.8 N, respectively (<i>P</i> &lt; 0.05). <i>L. reuteri</i> group showed significantly more type I muscle fibers (23% vs. 13%) and higher succinate dehydrogenase activity than the control group, while lactate dehydrogenase activity was significantly lower (<i>P</i> &lt; 0.05). The <i>L. reuteri</i> group showed significantly higher expression of genes encoding colonic epithelial nutrient transport (<i>GPR43</i>) and immune barrier function (<i>Zonulin</i>) than the control group. At the genus level, <i>Alistipes</i> and <i>Methanobacterium</i> were significantly different between <i>L. reuteri</i> and control groups (<i>P</i> &lt; 0.05), with <i>L. reuteri</i> group showing an increase of 2.9 times and a decrease of 8.0 times compared to the control group, respectively. At the species level, <i>Succiniclasticum ruminis</i> and <i>Bacteroides</i> sp. 43_108 were significantly (<i>P</i> &lt; 0.05) lower and higher, respectively in <i>L. reuteri</i> group than those in the control group<i>.</i> Butyric acid exhibited the highest concentration in <i>L. reuteri</i> group, reaching 7.52 mmol/L. In conclusion, supplementation with <i>L. reuteri</i> may affect muscle fiber characteristics, enhance intestinal health, and facilitate carbohydrate digestion of Sunit sheep. This study offers novel insights into the use of feed additives for improving meat quality.