Mastitis in dairy animals affect milk quality and quantity, animal health and welfare, antimicrobial use and economics of dairy farm, and antimicrobial residues in milk. Staphylococcus aureus is most common mastitis pathogen with ability to cause infections which are difficult to treat. The present study aimed to characterize the S. aureus strains associated with dairy animal with reference to its virulence, biofilm formation, antimicrobial resistance including methicillin and penicillin G resistance. A total of 100 bovine milk samples were screened by bacterial culture method, out of which 18 S. aureus and 6 methicillin-resistant S. aureus (MRSA) isolates were identified and characterized for virulence determinants. The strains were uniformly positive for the virulence determinants. However, the hemolysis in blood agar was found to be specific but not a sensitive criterion for virulence. The biofilm formation ability of the isolates showed 61.66% of S. aureus and 83.33% of MRSA strains were positive by Microtiter plate method. The biofilm formation genes (icaA and icaD) were detected in all the strains. The multi-drug resistance profile of the strains was studied by disk diffusion assay where over 70% of S. aureus strains were sensitive to all the anti-microbial agents (except penicillin) and only 33.33% of the strains had the MAR index above 0.2. All the MRSA strains (100%) had a MAR index of ≥ 0.2. All the strains showed resistance to penicillin which is considered as a prognostic marker for mastitis. The presence of penicillin and/or methicillin resistant, biofilm forming S. aureus mastitis strains can severely affect the treatment outcomes and economics of small dairy farmers of the region. Further studies to understand the population structure of the strains, by whole genome or traditional sequence-based methods and MIC values of antibiotics are required.

奶牛乳腺炎影响牛奶的质量和数量、动物健康和福利、奶牛场抗菌药物的使用和经济以及牛奶中的抗菌药物残留。金黄色葡萄球菌是最常见的乳腺炎病原体，能够引起难以治疗的感染。本研究旨在描述与奶牛动物相关的金黄色葡萄球菌菌株的毒力、生物膜形成、抗菌素耐药性（包括甲氧西林和青霉素 G 耐药性）。采用细菌培养法对100份牛乳样品进行筛选，鉴定出18株金黄色葡萄球菌和6株耐甲氧西林金黄色葡萄球菌（MRSA），并对其毒力决定子进行了鉴定。这些菌株的毒力决定子均呈阳性。然而，血琼脂中的溶血被发现是特异性的，但不是毒力的敏感标准。微量滴定板法检测分离菌的生物膜形成能力，金黄色葡萄球菌和MRSA菌株的阳性率分别为61.66%和83.33%。所有菌株均检测到生物膜形成基因（ica A 和ica D）。通过纸片扩散法研究了菌株的多重耐药性，超过70%的金黄色葡萄球菌菌株对所有抗菌药物（青霉素除外）敏感，只有33.33%的菌株的MAR指数高于0.2 。所有 MRSA 菌株（100%）的 MAR 指数≥ 0.2。所有菌株都显示出对青霉素的耐药性，青霉素被认为是乳腺炎的预后标志。青霉素和/或甲氧西林耐药、形成生物膜的金黄色葡萄球菌乳腺炎菌株的存在可能会严重影响该地区小型奶农的治疗结果和经济。需要通过全基因组或传统的基于序列的方法和抗生素的 MIC 值进行进一步研究，以了解菌株的群体结构。