Differences of semen microbiota among breeding boars with different reproductive ages

In this study, we investigated 18 healthy and fertile Duroc boars, dividing them into two groups based on their reproductive age: 9 boars aged 18 mo and 9 boars aged 36 mo. Prior to semen sampling, all boars were raised together under identical management conditions for a period of 3 mo. Our findings revealed that older boars exhibited lower sperm motility and a higher proportion of abnormal sperm morphology compared to younger boars. Furthermore, older boars demonstrated lower anti-oxidant capacity in their semen, as indicated by elevated levels of malondialdehyde and decreased levels of superoxide dismutase and glutathione peroxidase. Microbiota analysis utilizing the 16S rRNA technique showed that the semen microbiota of older boars had reduced alpha-diversity and beta-diversity in comparison to younger boars. We identified the Streptococcus genus and Streptococcus gallolyticus subsp macedonicus species served as biomarkers for semen from younger breeding boars, while the Bacteroides pyogenes species as a biomarker for semen from older breeding boars. Additionally, the semen from older boars exhibited a higher abundance of Aerococcus, Gallicola, Ulvibacter, and Proteiniphilum compared to younger boars. Spearman correlation analysis showed that these four bacteria were negatively correlated with semen quality. The abundance of Gallicola and Proteiniphilum were negatively correlated with semen anti-oxidant capacity. Additionally, the reduction of semen anti-oxidant capacity was correlated to the decrease of semen quality. Based on these findings, we concluded that the semen of older boars contains a higher abundance of harmful bacteria, which contributes to the observed reduction in semen anti-oxidant capacity and overall semen quality in this group.