Transmission of Staphylococcus aureus along the dairy production chain is an emerging public health problem with human, veterinary, and food safety issues. The prevalence of multidrug-resistant, particularly methicillin-resistant S. aureus (MRSA), has steadily increased in several European countries. In this study, the prevalence of S. aureus in raw cow milk and farm workers was investigated, and the trajectories of MRSA transmission at the primary stage of the dairy chain were assessed. To this purpose, a longitudinal survey was conducted in 618 dairy farms in two contiguous regions with high livestock density in North-eastern Italy. S. aureus contamination of bulk tank milk (BTM) was observed in more than 80% of farms, while MRSA prevalence was 3.6% and 15.9% in BTM and farm workers, respectively. The majority of MRSA isolates from both BTM and farm workers were assigned to ST398, and showed a worrisome multidrug-resistant phenotype. Enterotoxin and Panton-Valentine leukocidin genes were detected in 11.5% and 4.9% of MRSA isolates from both sources. Nearly all MRSA isolates from workers belonged to the same epidemiological type as BTM isolates from the corresponding farm, denoting a bidirectional MRSA transmission pattern. A focus on the ST398 spa type t899 MRSA lineage in the Italian livestock system highlighted the presence of two major clusters whose dissemination was likely facilitated by the selective pressure imposed by antimicrobial use in animal farming. Our findings emphasize the need for continuous monitoring of MRSA along the dairy production chain, not only to avoid transmission between animals and exposed workers, but also to contain the risk of raw milk and dairy product contamination by multidrug resistant and toxigenic strain.

金黄色葡萄球菌沿乳制品生产链的传播是一个新出现的公共卫生问题，涉及人类，兽医和食品安全问题。在多个欧洲国家，耐多药，特别是耐甲氧西林的金黄色葡萄球菌（MRSA）的患病率稳步上升。在这项研究中，调查了生牛奶和农场工人中金黄色葡萄球菌的流行情况，并评估了乳品链初级阶段MRSA传播的轨迹。为此，在意大利东北部两个畜牧密度高的连续地区的618个奶牛场进行了纵向调查。金黄色葡萄球菌在超过80％的农场中发现了散装罐装牛奶（BTM）的污染，而BTM和农场工人的MRSA患病率分别为3.6％和15.9％。来自BTM和农场工人的大多数MRSA分离株都被分配到ST398，并表现出令人担忧的多重耐药表型。在两种来源的MRSA分离株的11.5％和4.9％中检测到肠毒素和Panton-Valentine白细胞抑制素基因。几乎所有来自工人的MRSA分离株都属于与来自相应农场的BTM分离株相同的流行病学类型，这表示双向MRSA传播模式。专注于ST398水疗中心意大利牲畜系统中的t899型MRSA谱系突出显示了两个主要簇的存在，其传播很可能是由于在畜牧业中使用抗菌剂施加的选择性压力所致。我们的研究结果强调，需要持续监控整个乳制品生产链中的MRSA，不仅避免在动物和裸露的工人之间传播，而且要遏制多药耐药和产毒菌株污染原奶和乳制品的风险。