Chambers H F, Archer G, Matsuhashi M
Department of Medicine, University of California, San Francisco.
Antimicrob Agents Chemother. 1989 Apr;33(4):424-8. doi: 10.1128/AAC.33.4.424.
Two strains of Staphylococcus aureus expressing borderline or low-level methicillin resistance by one or more in vitro test methods were examined for resistance in vivo and for biochemical and genetic markers of methicillin resistance. In vivo, nafcillin was equally effective against experimental aortic valve endocarditis in rabbits, regardless of whether they were infected by a fully susceptible or a low-level-resistant strain. Resistance did not emerge during therapy. For the more resistant of the two low-level-resistant strains, methicillin was as effective as nafcillin. Nafcillin was ineffective against endocarditis caused by a truly methicillin-resistant strain, and resistance emerged on therapy. The low-level-resistant strains did not produce the low-affinity penicillin-binding protein 2a that is associated with methicillin resistance and did not contain DNA that hybridized with probes that recognized the methicillin resistance determinant. Low-level resistance in S. aureus is a phenomenon that is biochemically and genetically distinct from true methicillin resistance. These strains actually are susceptible to beta-lactam antibiotics. The clinical problem posed by these strains is not a therapeutic one but, instead, one of how to differentiate them from those that are truly methicillin resistant.
通过一种或多种体外试验方法表现出临界或低水平耐甲氧西林的两株金黄色葡萄球菌，接受了体内耐药性以及耐甲氧西林的生化和遗传标记检测。在体内，萘夫西林对兔实验性主动脉瓣心内膜炎同样有效，无论感染它们的是完全敏感菌株还是低水平耐药菌株。治疗期间未出现耐药性。对于两株低水平耐药菌株中耐药性更强的那株，甲氧西林与萘夫西林的效果相当。萘夫西林对真正耐甲氧西林菌株引起的心内膜炎无效，且治疗期间出现了耐药性。低水平耐药菌株不产生与耐甲氧西林相关的低亲和力青霉素结合蛋白2a，也不含有与识别耐甲氧西林决定簇的探针杂交的DNA。金黄色葡萄球菌中的低水平耐药是一种在生化和遗传上与真正的耐甲氧西林不同的现象。这些菌株实际上对β-内酰胺类抗生素敏感。这些菌株带来的临床问题不是治疗问题，而是如何将它们与真正耐甲氧西林的菌株区分开来的问题。
