Reprod Dom Anim. 2020;00:1–8. wileyonlinelibrary.com/journal/rda | 1 © 2020 Blackwell Verlag GmbH 1 | INTRODUCTION The routine use of artificial insemination (AI) in cattle production systems allows the acceleration of genetic improvement by using high-quality sperm of selected bulls, also reducing the likelihood of disease transmission through breeding (reviewed by Moore & Hasler, 2017). Nonetheless, the collection of sterile ejaculates from bulls is physiologically unfeasible. Ejaculates of clinically healthy bulls com- monly include several microorganisms, such as Mycoplasma spp., Corynebacterium spp., Proteus spp., Pseudomonas spp., Staphylococcus spp., Streptococcus spp., Bacillus spp. and Escherichia coli (Wentink, Frankena, Bosch, Vandehoek, & Berg, 2000; Wierzbowski, 1981), which may be common inhabitants of their preputial microflora or of their external genitalia (Thibier & Guerin, 2000). Furthermore, microorganisms may be subsequently added to the ejaculates during the procedures of sperm collection and processing (Kaproth & Krick, 2004). During sperm cryopreservation, cooling itself and the inclu- sion of antimicrobials in extenders may help to minimize bacterial growth (Eaglesome, Garcia, & Bielanski, 1995; Wentink et al., 2000). However, in the presence of large contamination levels (Stringfellow & Givens, 2000) or of bacterial resistance to commonly used anti- microbials (Becher et al., 2013; Gloria et al., 2014; Visser, Laak, & Jansen, 1999), such processes cannot fully assure microbiological safety to the final product. Thus, extenders avoid bacterial growth until a given threshold that may induce negative effects on sperm quality (Althouse, 2008). Additionally, contamination may also occur during storage of frozen sperm samples (Bielanski & Vajta, 2009). Received: 13 December 2019 | Accepted: 29 December 2019 DOI: 10.1111/rda.13621 ORIGINAL ARTICLE Antibiotic resistance in microorganisms isolated in a bull semen stud Karina L. Goularte 1,2 | Flávia L. S. Voloski 2,3 | Josiara F. M. Redú 2,3 | Carlos E. R. Ferreira 1,2 | Arnaldo D. Vieira 1,2 | Eduarda H. Duval 2,3 | Rafael G. Mondadori 1,4 | Thomaz Lucia Jr 1,2 1 ReproPel, Universidade Federal de Pelotas, Pelotas-RS, Brazil 2 Faculdade de Veterinária, Universidade Federal de Pelotas, Pelotas-RS, Brazil 3 Lipoa, Universidade Federal de Pelotas, Pelotas-RS, Brazil 4 Instituto de Biologia, Universidade Federal de Pelotas, Pelotas-RS, Brazil Correspondence Thomaz Lucia Jr, ReproPel, Universidade Federal de Pelotas, Pelotas-RS, Brazil. Emails: thomaz@ufpel.edu.br; thomaz@ pq.cnpq.br Funding information Conselho Nacional de Desenvolvimento Científico e Tecnológico, Grant/Award Number: 578597/2008-0 Abstract Many microorganisms from various sources may be present in ejaculates of bulls. This study identified and isolated bacteria from bull sperm samples in a commercial stud and evaluated their resistance to antibiotics. The number of colony-forming units was determined in semen samples collected at distinct steps during freezing and thaw- ing. The minimum inhibitory concentration and the minimum bactericidal concen- tration were determined for four antibiotics commonly used in commercial studs. A total of 135 microorganisms from 25 genera were isolated. After a sensitivity test, all evaluated microorganisms (n = 55) were resistant to penicillin and most of them were resistant to tylosin and lincomycin (n = 54). Resistance to all tested antibiotics was observed in 22% of all isolates, whereas only 3.9% of the isolates were inhibited by the tested antibiotics at the concentrations recommended by the international legislation. As the isolated microorganisms presented high resistance to frequently used antibiotics, sensitivity tests should be periodically conducted in commercial bull semen studs to prevent the use of contaminated semen in artificial insemination. KEYWORDS antimicrobials, bacteria, bull semen, sensitivity