BackgroundIn tropical environments, boar semen is prepared either from a boar on the same farm as the sow herd or collected in semen collection centers and then transported to other farms. Thus, the semen doses can be used for artificial insemination either immediately or preserved for 2–3 days. The present study investigated the bacteriospermia and its antimicrobial resistance in relation to boar sperm quality during short-term storage in semen extender with or without antibiotics in Thailand.M&MIn total, 20 Duroc ejaculates were collected. Each ejaculate was diluted in Beltsville Thawing Solution extender either with 0.25 g of gentamicin per liter (ANTIBIOTIC) or without gentamicin (NO-ANITIBIOTIC) to create semen doses containing 3,000 × 106sperm/100 mL. These were stored at 17 °C for 4 days. Semen characteristics and total bacterial count (CFU per mL, log10) were measured after collection and during storage.ResultsSperm viability was decreased by 6.4% for every 1.0 log10increase in total bacterial count (p= 0.026) andStaphylococcus spp.were the most frequently isolated across ejaculates. Throughout the 4 days of storage, sperm motility, viability and acrosome integrity in the ANTIBIOTIC group were higher than those in the NO-ANTIBIOTIC group (p< 0.05), while the total bacterial count was lower (1.9 ± 0.1 versus 3.9 ± 0.1 log10, respectively;p< 0.001). Without antibiotic supplementation, the total numbers of bacteria counted on days 2 and 3 of storage were higher than those determined on days 0 and 1 (p< 0.001). Differences in semen quality were detected on days 2 and 3 between the NO-ANTIBIOTIC and ANTIBIOTIC groups in high-viability semen (p< 0.05). However, no differences in sperm quality between the NO-ANTIBIOTIC and ANTIBIOTIC groups were detected in the low-viability semen on each storage day (p> 0.05). On the last day of preservation,Globicatella sanguinis (57.2%),Delftia acidovorans (18.9%)andMicrococcus spp.(5.9%) remained as the top three most abundant contaminants in the semen with antibiotic.ConclusionOur findings contribute new insights toward reducing antibiotics as well as rational antibiotic use in the boar AI industry. The growth of bacteria was significantly greater only after 2 days of preservation in the semen without antibiotic. For semen doses diluted from highly viable ejaculates, it is possible to store for 2 days without any antibiotic supplementation. Moreover, bacterial counts increased at the end of storage in the presence of gentamycin, suggesting the loss of bacteriostatic properties of gentamicin to the growth of bacteria during storage.