Detection of oxacillin resistance in Staphylococcus aureus by screening tests
Detection of oxacillin resistance in Staphylococcus aureus by screening tests
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DOI：
10.1007/s100960050413
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发表时间：
1999-11-01
影响因子：
4.5
通讯作者：
Witte, W
中科院分区：
文献类型：
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作者：
Cuny, C;Pasemann, B;Witte, W
在过去的15年里，耐甲氧西林金黄色葡萄球菌(MRSA；同义词ORSA[耐苯唑西林金黄色葡萄球菌]，因为苯唑西林被用作检测物质)在世界许多地区一直有很高的流行率[1，2]。由于耐甲氧西林金黄色葡萄球菌对多种抗菌剂的耐药性，这种细菌引起的感染可能很难治疗。甲氧西林耐药主要是由于mecA基因编码的青霉素结合蛋白PBP2a的形成；由于突变而对PBP2亲和力降低的分离株更为罕见[3]。基于PBP2a的甲氧西林耐药的表型表达取决于许多其他因素[4]，并且大多数最近播散的MRSA菌株异质性地表达这种表型(异质性耐药的MRSA[5])。最近特别关注对苯唑西林和相关内酰胺类药物(苯唑西林耐药金黄色葡萄球菌，Borsa)的敏感性达到临界水平的金黄色葡萄球菌。博尔萨菌株属于广泛传播的克隆群，呈现94、96噬菌体模式[6]。已经描述了Borsa[1]中的一种内酰胺酶的高产，但这种内酰胺酶不能降解甲氧西林[7]。在Borsa中可能存在第二种β-内酰胺酶，具有一定的水解甲氧西林和苯唑西林的能力[8，9]。mecA[10，11]的聚合酶链式反应可以很容易地评估MRSA的基因型，根据常规药敏试验标准[12]，这使得在对苯唑西林具有低水平耐药性的菌株中检测出内在的甲氧西林耐药性。由于异质性耐药，检测MRSA的表型方法需要比通常用于琼脂扩散试验或MIC测定的接种量更高的接种量，例如琼脂筛选试验[13-15]。此外，目前的抗生素敏感性测试指南建议在旨在诱导mecA阳性分离株表达的条件下进行金黄色葡萄球菌测试。这些条件包括35℃孵育和添加氯化钠(例如，根据国家临床实验室标准委员会(NCCLS)指南进行的琼脂筛选试验[16])。然而，这带来了博尔萨分离株表现出抗药性的风险(接种效应，氯化钠的影响[14])。正如已经描述的，博尔萨对阿莫西林/舒巴坦的敏感性，或者更好地说，对氨苄西林/舒巴坦的敏感性被认为是区分博尔萨和真正耐药的MRSA的标准[9，16]。然而，当使用常规的药敏试验条件时，仍有一小部分博尔萨分离株被错误识别[17]。在这篇通讯中，我们报告了苯唑西林/舒巴坦在高接种量筛选试验中用于检测MRSA和区分Borsa，并比较了其与基于抗体与PBP2a反应的MRSA筛选试验的适用性。
During the last 15 years there has been a consistently high prevalence of methicillin-resistant Staphylococcus aureus (MRSA; synonym ORSA [oxacillin-resistant Staphylococcus aureus], because oxacillin is used as the test substance) in many parts of the world [1, 2]. Due to MRSA’s characteristic resistance to multiple antimicrobial agents, infections due to this organism can be problematic to treat. Methicillin resistance is mainly due to the elaboration of penicillin-binding protein PBP2a, coded by the mecA gene; isolates with reduced affinity for PBP2 due to mutations are more rare [3]. The phenotypic expression of PBP2a-based methicillin resistance is dependent upon a number of other factors [4], and most of the recently disseminated MRSA strains express this phenotype heterogeneously (heteroresistant MRSA [5]). Particular attention has recently been focussed on Staphylococcus aureus with borderline levels of susceptibility to oxacillin and related ßlactam agents (borderline oxacillin-resistant Staphylococcus aureus, BORSA). BORSA strains belong to the widely disseminated clonal group exhibiting phage pattern 94, 96 [6]. Hyperproduction of a ß-lactamase has been described for BORSA [1], but this ß-lactamase is unable to hydrolyze methicillin [7]. It is likely that there is a second ß-lactamase in BORSA, with some capacity to hydrolyze methicillin and oxacillin [8, 9].The MRSA genotype can be easily assessed by the polymerase chain reaction for mecA [10, 11], which allows the detection of intrinsic methicillin resistance in strains with low-level resistance to oxacillin, according to criteria of routine susceptibility testing [12]. Because of heteroresistance, phenotypic methods for the detection of MRSA require a higher inoculum than that generally used for the agar diffusion assay or MIC determination, eg the agar screening test [13–15]. Furthermore, current guidelines for antibiotic susceptibility testing recommend testing of Staphylococcus aureus under conditions designed to elicit expression in mecA-positive isolates. These conditions include incubation at 35 7C and supplementation of media with NaCl (eg agar screening test performed according to guidelines of the National Committee for Clinical Laboratory Standards (NCCLS)[16]). This, however, bears the risk that BORSA isolates will appear resistant (inoculum effect, effect of NaCl [14]). As already described, the susceptibility of BORSA to amoxicillin/sulbactam or, better, to ampicillin/sulbactam has been regarded as a criterion for differentiating BORSA from truly resistant MRSA [9, 16]. When, however, routine conditions for susceptibility testing were used, a fraction of BORSA isolates were still identified incorrectly [17]. In this communication we report on the use of oxacillin/sulbactam in highinoculum screening tests for the detection of MRSA and the discrimination of BORSA and compare its suitability to that of the MRSA screen test based on the reaction of antibody with PBP 2a.