REASONS FOR PERFORMING THE STUDY: The microbiota plays a key role in health and disease. Probiotics are a potential way to therapeutically modify the intestinal microbiota and prevent disease.

OBJECTIVES: The aim of this study was to investigate the effects of probiotics on the bacterial microbiota of foals during and after administration.

STUDY DESIGN: Randomised placebo controlled field trial.

METHODS: Thirty-eight healthy neonatal foals enrolled in a prior study were selected. The foals had received a multi-strain probiotic (four Lactobacillus spp 3-4x10(3) cfu/g each, Bifidobacterium animalis spp. lactis, 1x 10(3-4) cfu/g) or placebo once daily for 3 weeks. A total of 3 faecal samples were collected from each foal at 2-week intervals and assessed via metagenomic sequencing. The Wilcoxon test was used to compare data between treatment groups.

RESULTS: There were no changes on the phylum, order or class level between treatment groups at any age (all p>0.08) but some significant changes in relative abundance of families. Probiotic administration did not result in an increased relative abundance of lactobacilli or bifidobacteria at any age (Lactobacillus: p = 0.95, p = 0.1 and p = 0.2, Bifidobacterium: p = 0.26, p = 0.62 and p = 0.12 for week 2, week 4 and week 6 respectively). Lactobacillus was enriched in the probiotic group at week 6 on LEfSe analysis (LDA0.34, p = 0.016). There was no effect on alpha diversity (all p>0.24) or community structure when parsimony and unifrac analysis were applied (all p>0.65).

CONCLUSIONS: There were limited effects of probiotic treatment on the bacterial microbiota of foals. The studied probiotic based on lactobacilli and bifidobacteria has a limited potential for therapeutic modification of the gastrointestinal microbiota. This article is protected by copyright. All rights reserved.