Methicillin resistance mediated by the mecA gene in Staphylococcus aureus, also known as “true MRSA”, is typically associated with high oxacillin MIC values (≥8 mg/L). Because non-mecA-mediated oxacillin resistant S. aureus phenotypes can also cause hard-to-treat diseases in humans, their misidentification as methicillin-susceptible S. aureus strains (MSSA) can compromise the efficiency of the antimicrobial therapy. These strains have been refereed as Borderline Oxacillin-Resistant S. aureus (BORSA) but their characterization and role in clinical microbiology have been neglected. Considering the increasing importance of livestock-associated methicillin-resistant S. aureus ST398 (LA-MRSA) as an emerging zoonotic pathogen worldwide, this study aimed to report the genomic context of oxacillin resistance in porcine S. aureus ST398 strains. S. aureus isolates were recovered from asymptomatic pigs from three herds. Oxacillin MIC values ranged from 4 to 32 mg/L. MALDI-TOF-confirmed isolates were screened for mecA and mecC by PCR and genotyped by means of PFGE and Rep-PCR. Seven isolates were whole genome sequenced. None of the isolates harbored the mecA gene or its variants. Although all seven sequenced isolates belonged to one sequence type (ST398), two different spa types (t571 and t1471) were identified. All isolates harbored conserved blaZ gene operon and no mutations on genes encoding for penicillin-binding-proteins were detected. Genes conferring resistance against other drugs such as aminoglycosides, chloramphenicol, macrolide, lincosamide and streptogramin (MLS), tetracycline and trimethoprim were also detected. Isolates also harbored virulence genes encoding for adhesins (icaA; icaB; icaC; icaD; icaR), toxins (hlgA; hlgB; hlgC; luk-PV) and protease (aur). Pigs can serve as reservoirs of non-mecA-mediated oxacillin-resistant ST398 strains potentially pathogenic to humans. Considering that mecA has been the main target to screen methicillin-resistant staphylococci, the occurrence of BORSA phenotypes is probably underestimated in livestock.

由金黄色葡萄球菌中的mec A 基因介导的甲氧西林耐药性，也称为“真正的 MRSA”，通常与高苯唑西林 MIC 值（≥8 mg/L）有关。由于非mec A 介导的苯唑西林抗性金黄色葡萄球菌表型也会导致人类难以治疗的疾病，因此将它们错误识别为甲氧西林敏感的金黄色葡萄球菌菌株 (MSSA) 会影响抗菌治疗的效率。这些菌株被称为边界苯唑西林耐药金黄色葡萄球菌(BORSA)，但它们的特征和在临床微生物学中的作用被忽视了。考虑到牲畜相关的耐甲氧西林的重要性日益增加金黄色葡萄球菌ST398 (LA-MRSA) 作为全球新兴的人畜共患病病原体，本研究旨在报告猪金黄色葡萄球菌ST398 菌株苯唑西林耐药性的基因组背景。从三头猪群的无症状猪中分离出金黄色葡萄球菌。苯唑西林 MIC 值范围为 4 至 32 mg/L。MALDI-TOF 确认的分离株通过 PCR筛选mec A 和mec C，并通过 PFGE 和 Rep-PCR 进行基因分型。对七个分离株进行了全基因组测序。没有一个分离株含有mec A 基因或其变体。尽管所有七个测序分离株都属于一种序列类型 (ST398)，但两种不同的spa确定了类型（t571 和 t1471）。所有分离株都含有保守的bla Z 基因操纵子，并且未检测到编码青霉素结合蛋白的基因发生突变。还检测到对氨基糖苷类、氯霉素、大环内酯、林可酰胺和链球菌素 (MLS)、四环素和甲氧苄啶等其他药物产生抗性的基因。分离株还含有编码粘附素（ica A；ica B；ica C；ica D；ica R）、毒素（hlg A；hlg B；hlg C ；luk- PV）和蛋白酶（aur）的毒力基因。猪可以作为非mec 的宿主A 介导的苯唑西林耐药 ST398 菌株可能对人类致病。考虑到mec A 一直是筛选耐甲氧西林葡萄球菌的主要目标，BORSA 表型在牲畜中的发生率可能被低估了。