NSW Health Pathology, Microbiology, John Hunter Hospital, Newcastle, Australia.
NSW Health Pathology, Microbiology, Prince of Wales Hospital, Randwick, Australia.
Microbiol Spectr. 2023 Aug 17;11(4):e0022023. doi: 10.1128/spectrum.00220-23. Epub 2023 Jul 6.
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 genes were sequenced. MICs were determined at low and high inocula (5 × 10 CFU/mL and 5 × 10 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. 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.
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。A型和 C 型β-内酰胺酶产生的金黄色葡萄球菌（TAPSA 和 TCPSA）在高接种量下水解头孢唑林的倾向称为头孢唑林接种量效应（CIE）。具有 CIE 的菌株存在导致治疗失败的理论风险，并且大多数实验室通常无法检测到。我们开发了一种高性能但简单的β-内酰胺酶圆盘试验，可识别和区分 TAPSA 和 TCPSA，适用于常规诊断实验室工作流程。鉴定了对青霉素耐药的金黄色葡萄球菌临床分离株，并对其基因进行了测序。在低接种量（5×10 CFU/mL）和高接种量（5×10 CFU/mL）下测定 MIC，并对表现出 CIE 的分离株进行了特征描述。建立了半机械模型来描述差异水解模式，并使用竞争接收者操作特征（ROC）曲线的曲线下面积分析迭代评估候选模型。生物标志物阈值源自 Youdon 指数衍生的最佳截断值。对 99 株分离株的基因分析鉴定出 26 株 TAPSA 分离株和 45 株 TCPSA 分离株。区分 TAPSA 和非 TAPSA 的最佳模型利用头孢唑林-头孢噻吩比值分析（敏感性 96.2%，特异性 98.6%）。区分 TCPSA 和非 TCPSA 的最佳模型纳入了头孢唑林、头孢噻吩和苯唑西林（敏感性 88.6%，特异性 96.6%）。可以在单个琼脂平板上使用三个抗生素圆盘来区分 TAPSA 和 TCPSA。该测试具有从接受或已接受头孢唑林治疗失败的患者的分离物中对β-内酰胺酶类型进行分型的潜在价值。本文的关键意义在于详细介绍了一种简单的圆盘试验方法，该方法可以区分金黄色葡萄球菌分离株，这些分离株可能与头孢唑林接种量效应和头孢唑林治疗失败的理论风险相关，而与头孢唑林接种量效应不太可能相关。
