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Abstract：
Staphylococcus aureus can produce β-lactamases capable of hydrolyzing penicillins and first-generation cephalosporins. The propensity of type A and type C β-lactamase-producing S. aureus (TAPSA and TCPSA) to hydrolyze cefazolin at a high inoculum is termed the cefazolin inoculum effect (CIE). Strains with a CIE have a theoretical risk of causing treatment failure and are unable to be detected routinely by most laboratories. We developed a high-performing yet straightforward β-lactamase disc test that identifies and differentiates both TAPSA and TCPSA and is suitable for routine diagnostic laboratory workflows. Clinical isolates of S. aureus resistant to penicillin were identified, and their blaZ genes were sequenced. MICs were determined at low and high inocula (5 × 105 CFU/mL and 5 × 107 CFU/mL), and isolates demonstrating a CIE were characterized. A semimechanistic model was established to describe differential hydrolysis patterns, and candidate models were iteratively assessed using area-under-the-curve analysis from competitor receiver operating characteristic (ROC) curves. Biomarker thresholds were derived from Youdon index-derived optimal cutoff values. Genetic analysis of 99 isolates identified 26 TAPSA isolates and 45 TCPSA isolates. The model best differentiating TAPSA from non-TAPSA utilized cefazolin-to-cephalothin ratio analysis (sensitivity, 96.2%; specificity, 98.6%). The model best differentiating TCPSA from non-TCPSA incorporated cefazolin, cephalothin, and oxacillin (sensitivity, 88.6%; specificity, 96.6%). TAPSA and TCPSA can be differentiated using three antibiotic discs on a single agar plate. The test has potential value in typing the β-lactamase type from isolates from patients that are candidates for or have failed cefazolin therapy. IMPORTANCE The key significance of this article is that it details a straightforward method of performing a disc test that can differentiate Staphylococcus aureus isolates that are likely to be associated with a cefazolin inoculum effect and theoretical risk of cefazolin treatment failure from isolates that are less likely to be associated with a cefazolin inoculum effect.
摘要：
金黄色葡萄球菌可以产生能够水解青霉素和第一代头孢菌素的β-内酰胺酶。产生A型和C型β-内酰胺酶的金黄色葡萄球菌(TAPSA和TCPSA)在高接种物下水解头孢唑啉的倾向被称为头孢唑啉接种物效应(CIE)。具有aCIE的菌株具有引起治疗失败的理论风险,并且不能被大多数实验室常规检测。我们开发了一种高性能但直接的β-内酰胺酶圆盘测试，可识别和区分TAPSA和TCPSA，适用于常规诊断实验室工作流程。对青霉素耐药的金黄色葡萄球菌临床分离株进行鉴定,并对其blaZ基因进行了测序。在低接种物和高接种物（5×105CFU/mL和5×107CFU/mL）下测定MIC，和证明aCIE的分离株进行了表征。建立了半力学模型来描述差异水解模式，和候选模型使用来自竞争者受试者工作特征(ROC)曲线的曲线下面积分析进行迭代评估.生物标记阈值来自Youdon指数得出的最佳截止值。对99个分离株的遗传分析鉴定出26个TAPSA分离株和45个TCPSA分离株。最佳区分TAPSA和非TAPSA的模型利用头孢唑啉与头孢菌素的比率分析(敏感性，96.2%；特异性，98.6%）。该模型最好区分TCPSA和非TCPSA掺入的头孢唑林，头孢菌素,和苯唑西林(敏感性，88.6%;特异性,96.6%）。TAPSA和TCPSA可以在单个琼脂平板上使用三个抗生素盘进行区分。该测试在对来自头孢唑啉治疗的候选或失败的患者的分离株的β-内酰胺酶类型进行分型方面具有潜在价值。重要性本文的关键意义在于，它详细介绍了一种直接的圆盘测试方法，该方法可以区分可能与头孢唑啉接种物效应相关的金黄色葡萄球菌分离株和不太可能与头孢唑啉接种物效应相关的头孢唑啉治疗失败的理论风险。
