Background: Bacterial contamination remains an unavoidable consequence of boar semen collection and can compromise sperm function during chilled storage. However, despite Australia's geographically isolated and closed pig herd, the influence of bacterial proliferation on sperm quality during commercial semen storage has not been investigated under Australian production conditions. As pressure to reduce antibiotic use in livestock production increases, a better understanding of bacteria-sperm interactions is required to support antimicrobial stewardship. This study investigated the effects of antimicrobial-free storage on boar semen quality under Australian commercial conditions and considered the potential role of bacterial contaminants in sperm deterioration. 

Methods: Semen was collected from 27 boars across three Australian commercial studs. Each ejaculate was divided into antimicrobial-treated and antimicrobial-free aliquots and stored at 16°C for seven days. Sperm quality was assessed on Days 1 and 7 using computer-assisted sperm analysis (CASA), flow cytometry, pH measurement, and bacterial culture. Bacterial isolates were identified by MALDI-TOF mass spectrometry. 

Results: Antimicrobial-free storage resulted in significantly greater bacterial proliferation than antimicrobial-treated semen. Increased bacterial growth was associated with significant decline (p ≤ 0.05) in sperm quality, including a reduction in total motility, decreased viability and acrosome integrity, and marked acidification of the storage environment by Day 7. Potentially pathogenic bacteria, including Proteus mirabilis, Pseudomonas aeruginosa, Citrobacter koseri and Enterococcus faecalis, were detected throughout the study and possess virulence mechanisms capable of impairing sperm function, including adhesion, toxin production, oxidative stress and membrane damage.  

Conclusions: Bacterial proliferation destabilises the semen storage environment and is associated with reduced sperm function during extended storage. These findings provide the first baseline assessment of bacterial proliferation and associated changes in sperm quality in Australian commercial boar semen, estabilishing a foundation for future mechanistic studies investigating bacteria–sperm interactions and the development of antimicrobial-reduced semen preservation strategies.