Storage of boar semen at 17°C without conventional antibiotics in an extender containing an organic bactericidal substance.

Introduction: Facing the global threat of antimicrobial resistance, the reduction of antibiotic use in semen extenders is a main goal in artificial insemination (AI) of pigs. The aim of this study was to investigate the potential of a commercial extender containing an organic bactericidal supplement in the absence of conventional antibiotics to control bacterial growth and to maintain the quality of boar spermatozoa during long-term semen storage for up to 144 h at 17°C.

Methods: Semen from 233 boars housed at 16 European AI centers was split and diluted in the long-term extender "Androstar Plus without antibiotics + organic bactericidal supplement" (APlus) and in the control extender Beltsville Thawing Solution (BTS) with gentamicin, which is routinely used in many AI centers. Sperm motility was assessed with computer-assisted semen analysis (CASA) and membrane integrity was evaluated with flow cytometry. The number of bacteria was determined by counting colonies on agar plates.

Results: At the end of storage, bacterial counts were ≥ 10 CFU/mL in 10.7% of the APlus and in 0.4% of the BTS samples. At the same time, bacterial counts were only weakly correlated with sperm motility ( = -0.23,  < 0.05), and there was no correlation with sperm membrane integrity ( > 0.05). Among the 12 identified bacterial species in APlus samples, loss of sperm quality was exclusively observed in the presence of >10 CFU/mL and . Both these bacterial species, despite their known multi-drug resistance and the continuous use of gentamicin in Europe, proved sensitive to this antibiotic, thus indicating an efficient quality assurance program and responsible antibiotic use.

Conclusion: Long-term storage of boar semen at 17°C without conventional antibiotics in an extender containing an organic bactericidal supplement is an option if semen samples are regularly tested for the presence of and , and the source of contamination is eliminated.