Sheep-Derived Lactobacillus johnsonii M5 Enhances Immunity and Antioxidant Capacity, Alleviates Diarrhea, and Improves Intestinal Health in Early-Weaned Lambs

The early weaning of lambs frequently leads to weakened immunity, impaired intestinal function, and increased susceptibility to intestinal disease. <i>Lactobacillus</i> plays a role in regulating immunity, enhancing antioxidant capacity, and maintaining intestinal health. This study aims to isolate a strain of <i>Lactobacillus</i> with favorable probiotic properties from sheep feces and investigate its effects on the intestinal health of early-weaned lambs. In this study, the growth characteristics, acid production capacity, bacteriostatic capacity, bile salt tolerance, gastrointestinal fluid tolerance, self-coagulation capacity, and surface hydrophobicity of <i>Lactobacillus</i> isolated from sheep feces were analyzed for in vitro probiotic properties. <i>Lactobacilli</i> with strong probiotic properties were used for in vivo validation. A total of 72 Hu lambs were allocated into four groups: a ewe-reared group (ER), early-weaning group (EW), low-dose <i>Lactobacillus</i> group (LL), and high-dose <i>Lactobacillus</i> group (HL). Early weaning was performed in the EW, LL, and HL groups at the age of 28 days. <i>Lactobacillus johnsonii</i> M5 (<i>L. johnsonii</i> M5), isolated from sheep feces, exhibited strong probiotic properties in vitro. Feeding EW lambs with a low dose of <i>L. johnsonii</i> M5 significantly reduced their diarrhea rate (<i>p</i> < 0.05). Its supplementation increased the levels of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) and total antioxidant capacity (T-AOC) in serum and jejunal mucosa and decreased levels of malondialdehyde (MDA) (<i>p</i> < 0.05). Compared to the EW group, serum immunoglobulin G (IgG) levels were significantly increased in the LL group (<i>p</i> < 0.05). Compared to the EW group, feeding with <i>L. johnsonii</i> M5 increased the content of anti-inflammatory cytokines, while reducing the content of pro-inflammatory cytokines in serum and jejunal mucosa (<i>p</i> < 0.05). Feeding early-weaned lambs with <i>L. johnsonii</i> M5 also decreased jejunal crypt depth and increased occludin and claudin-1 in jejunal mucosa (<i>p</i> < 0.05). These findings indicate that feeding early-weaned lambs with <i>L</i>. <i>johnsonii</i> M5 enhances their immunity and antioxidant capacity, improving intestinal health, and mitigates diarrhea in early-weaned lambs.