Identification of microbiota in bull semen

Bacterial contamination of bull semen can have a negative effect on semen quality parameters as well as an impact on the transmission of opportunistic and pathogen bacteria from clinically healthy bulls to cows. Due to the fact that not all bacteria can be isolated and identified by culture-dependent methods, 16S rRNA sequencing was the method of choice based on its objectivity and reliability. The aims of this study were to identify bacteria in the semen of healthy bulls using 16S sequencing, investigate differences in the bacterial community between individual bulls, and establish if there was a relationship between bacterial flora and bull fertility. Semen from 18 bulls was used for DNA extraction and 16S sequencing; 107 bacterial genera were identified. Differences in Operational Taxonomic Units (OTUs) and numbers of genera between bulls were noted. There were negative correlations (P < 0.05) between several bacterial genera with Curvibacter and Dyella spp in majority of cases. Other bacteria which were also negatively correlated were Curvibacter, Cutibacterium, Dyella, Ruminococcaceae UCG-005, Ruminococcaceae UCG-010, and Staphylococcus all within the top 20 genera with high OTUs. Two genera, W5053 and Lawsonella were enriched in bulls in the low fertility group; this is the first time that these bacteria were described in bull semen samples. The majority of the bacterial present in bull semen samples were environmental genera or bacteria that originated from mucous membranes of animals and humans, indicating a potential need for more effective management of critical control points during semen collection. The results of this study indicate that differences in bacterial microbiota of healthy bulls occur and might affect the fertility potential of semen.