Composition and diversity of the seminal microbiota in bulls and its association with semen parameters

The male bovine microbiota is a relatively unexplored area even though it has been shown to impact the fertility of not only the male but also the female. With this study, our aim was to evaluate the bacterial composition of semen from bulls with either satisfactory or unsatisfactory semen quality as represented by poor sperm motility and/or morphology. We hypothesized based on findings in human and other domestic animal studies, that the microbiota diversity and composition may be associated with decreased sperm quality. Semen samples from 45 beef bulls were collected, 32 of those bulls were classified as having satisfactory spermiograms according to Society for Theriogenology bull breeding soundness exam standards with the remaining 13 bulls being classified as unsatisfactory. Microbiota profiling was performed using 16S rRNA gene amplicon sequencing of the V4 region. The most abundant genera in the seminal microbiome include Bacteroides, Corynebacterium 1, Escherichia, Gemella, and S5-A14a (an uncultured bacterium from the Clostridiales order). Bulls with satisfactory spermiograms had a higher abundance of sequence types belonging to many genera including Bacteroides, S5-A14a, Trueperella, and two methanogenic archaea genera Methanosphaera and Methanobrevibacter. Comparatively, samples from bulls with unsatisfactory speriograms had greater abundance of sequence types belonging to Veillonellaceae, Campylobacter, and Methanobacterium. Network analysis was also performed for satisfactory and unsatisfactory samples and indicated that cooccurrence of microbial taxa was less common in the S group than in the U group. In the satisfactory group, Methanobrevibacter cooccurred with twelve amplicon sequence variants (ASVs), indicating that it may be important for maintenance of the microbiota in satisfactory samples, and members of this genus were enriched in the satisfactory group. Opportunistic pathogens such as Campylobacter, and Fusobacterium co-occurred with four and ten ASVs, respectively, in the unsatisfactory group, potentially indicating they are acting in synergy with other member of the microbial community, but only in the unsatisfactory group.