Characterization of the vaginal microbiota of dairy cows treated with probiotics

Abstract Approximately 50% of dairy cows develop postpartum uterine diseases, significantly affecting reproductive performance and economic returns. Treatment is usually based on antibiotics; however, alternative approaches have been investigated. This study evaluated the effects of intravaginal supplementation with lactic acid bacteria (LAB) on the vaginal microenvironment, focusing on microbial viability, pH, and taxonomic composition. A total of 21 non-pregnant, multiparous and non-lactating Jersey cows with synchronized estrous cycle were used. Animals were randomly assigned to two groups: Probiotic group (PRO; n = 11) received two intravaginal capsules containing Lactobacillus rhamnosus and Lactobacillus reuteri at 4.5 × 10¹⁰ CFU/capsule; and Control group (CTR; n = 10), receiving two intravaginal capsules without LAB. The recovery of Lactobacillus rhamnosus was significantly higher in the PRO group ( p < 0.0001) vs. CTR. No significant differences were observed between CTR and PRO groups in vaginal pH ( p > 0.05). Microbiota analysis based on the 16 S rRNA gene showed no significant differences in differential abundance between CTR and PRO groups at the genus level (q > 0.05). However, PERMANOVA with paired design revealed a significant effect of the group x timepoint model on microbial community composition ( p = 0.004). This is the first study characterizing the vaginal microbiota of cows treated with probiotic LAB strains. Lactobacillus rhamnosus and Lactobacillus reuteri at the tested concentration did not alter vaginal pH up to Day 14 and did not significantly change individual taxon abundances at Days 4 or 7; however, paired analysis detected a significant compositional shift associated with treatment.