fa تحقيقي Original Paper <div><strong>ABSTRACT</strong><strong><span dir="RTL"></span></strong><br> <strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Background and Objectives</strong>: Mastitis caused by <em>Staphylococcus aureus</em> is a widely distributed disease in cattle, goats and sheep. The infection is often subclinical in cattle, leading to reduced milk production and quality, but acute catarrhal or even gangrenous inflammation may also occur. The aim of this study was to investigate resistance of <em>S. aureus</em> isolates from bovine subclinical mastitis to a number of antibiotics.<span dir="RTL"></span><br> <strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Methods</strong>: Milk samples were collected from 120 cows in different regions of Tabriz, Iran. Milk samples were cultured and bacteriological identification was performed. Antimicrobial susceptibility of the isolates was assessed by determining minimum inhibitory concentration. Plasmid DNA was extracted by an alkaline lysis method.&nbsp; <span dir="RTL"></span><br> <strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Results:</strong> The highest frequency of resistance was observed against gentamicin (100%) and &beta;- lactam antibiotics including amoxicillin (96%), ampicillin (40%) and penicillin (96%). The isolates mostly contained large plasmids, which might harbor acquired antibiotic resistance.<br> <strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Conclusion:</strong> The results confirm the high frequency of antibiotic resistance among staphylococci isolated from bovine subclinical mastitis.<br> <strong>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Keywords: </strong>Anti-infective Agents, Cattle,<em> Staphylococcus Aureus</em>, Mastitis, Plasmids.</div> Anti-infective Agents, Cattle, Staphylococcus Aureus, Mastitis, Plasmids 34 38 http://mlj.goums.ac.ir/browse.php?a_code=A-10-1-469&slc_lang=fa&sid=1 Katayoon Nofouzi 100319475328460013921 100319475328460013921 Yes Department of Pathobiology, Faculty of Veterinary Medicine, University of Tabriz, Tabriz, Iran Alaleh Rakhshanpoor 100319475328460013922 100319475328460013922 No Residency student of Small Animal Internal Medicine, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran Reza Asadpour 100319475328460013923 100319475328460013923 No Department of Clinical Science, Faculty of Veterinary Medicine, University of Tabriz, Tabriz, Iran