Supplementation of the probiotic Lactobacillus rhamnosus GG to pre-weaning calves decreases diarrhea incidence by modulating gut bacteria and associated metabolites

The aim of the present study was to investigate whether <i>Lactobacillus rhamnosus</i> GG (LGG) could be utilized as an effective strategy to promote growth performance of calves and to further study its impact on decreasing diarrhea. A total of 18 healthy Holstein calves (age 8 ± 3 days, weight 42.48 ± 1.04 kg) were raised in a centralized manner to minimize differences and then divided into two groups: one group received LGG supplementation (1 × 10¹⁰ CFU per calf per day), while the other served as the control (CON). Each group consisted of 9 replicates, with 1 calf per replicate. The experimental period spanned 6 weeks. The experimental results demonstrated an increase of the average daily growth (<i>P</i> = 0.094) and a highly significant reduction in diarrhea (<i>P</i> = 0.001) for the LGG group compared with the CON group. The supplementation of LGG resulted in an increased abundance of endogenous beneficial bacteria in the gut including <i>Lysinbacillus</i> (<i>P</i> = 0.012) and <i>Rikenellaceae_RC9_gut_group</i> (<i>P</i> = 0.041) while reducing levels of pathogenic bacteria in the <i>Escherichia-Shigella</i> (<i>P</i> = 0.066). These particular groups could serve as biomarker bacteria for calf diarrhea and gut health. The alteration of metabolite production and metabolic pathways were also closely associated with the variation of bacterial composition in the gut microbiome of calves, of which included enrichment of biosynthesis of unsaturated fatty acids, α-linolenic acid and steroid biosynthesis. Overall, LGG administration had a positive impact on improving the intestinal homeostasis, alleviating diarrhea, and enhancing growth performance by modulating gut bacteria and metabolites in pre-weaning calves. Thus, LGG could be a potential alternative to prophylactic antibiotic treatment that would lower associated costs and provide new insights into preventing and treating calf diarrhea.