The bull reproductive microbiome: a comparative analysis of microbial communities within semen and organs of the bull reproductive system

Semen from mature, healthy bulls contains commensal microbes. Potential internal sources of seminal microbes have not been extensively evaluated. Objectives were to (1) assess whether the testes of the bull contain commensal microbes and to compare the composition of the putative testicular microbiome to that of semen and rumen fluid and (2) determine whether other organs of the bull reproductive system contain microbial populations. Here, we demonstrate that the testes of the bull contain a low biomass, yet diverse, microbiome. Interestingly, the microbial composition of rumen fluid, semen and testicular tissue were each dissimilar from one another, indicating that each source contains a unique microbiota. Only four core amplicon sequence variants, Acinetobacter, Enterobacteriaceae (Escherichia coli), Jeotgalicoccus, and Kiritimatiellae WCHB1-41, were shared between semen and the testes. Along with the testes, microbial populations were also present within the penile and pelvic urethra, seminal vesicles and epididymis of mature bulls. Of these anatomic niches, the microbial populations within the penile and pelvic urethra were the most diverse and shared the greatest number of core taxa (n = 72). The microbiota of the seminal vesicles, epididymis and the testes were significantly dissimilar from each other. Only one core taxa, Mycoplasma, was shared between the testes and epididymis. Collectively, our results demonstrate that semen and organs of the bull reproductive tract contain unique microbial populations. Further research is necessary to determine whether the microbial composition of organs of the bull reproductive system, such as the testes and epididymis, influence sperm viability and bull fertility.