Antimicrobial susceptibility testing (AST) is routinely used to establish predictive antibiotic resistance metrics to guide the treatment of bacterial pathogens. Recently, a novel phenotype termed “bicarbonate (NaHCO₃)-responsiveness” was identified in a relatively high frequency of clinical MRSA strains, wherein isolates demonstrate in vitro “susceptibility” to standard β-lactams (oxacillin [OXA]; cefazolin [CFZ]) in the presence of NaHCO₃, and in vivo susceptibility to these β-lactams in experimental endocarditis models. We investigated whether a targeted phenotypic-genotypic screening of MRSA could rule in or rule out NaHCO₃ susceptibility upfront. We studied 30 well-characterized clinical MRSA bloodstream isolates, including 15 MIC-susceptible to CFZ and OXA in NaHCO₃-supplemented Mueller–Hinton Broth (MHB); and 15 MIC-resistant to both β-lactams in this media. Using a two-tiered strategy, isolates were first screened by standard disk diffusion for susceptibility to a combination of amoxicillin-clavulanate [AMC]. Isolates then underwent genomic sequence typing: MLST (clonal complex [CC]); agr; SCCmec; and mecA promoter and coding region. The combination of AMC disk susceptibility testing plus mecA and spa genotyping was able to predict MRSA strains that were more or less likely to be NaHCO₃-responsive in vitro, with a high degree of sensitivity and specificity. Validation of this screening algorithm was performed in six strains from the overall cohort using an ex vivo model of endocarditis. This ex vivo model recapitulated the in vitro predictions of NaHCO₃-responsiveness vs. nonresponsiveness above in five of the six strains.

中文翻译：

---

用于体外检测碳酸氢盐的组合表型-基因型预测算法：耐甲氧西林金黄色葡萄球菌 (MRSA) 中的 β-内酰胺致敏

抗生素药敏试验 (AST) 通常用于建立预测性抗生素耐药性指标，以指导细菌病原体的治疗。最近，在相对高频率的临床 MRSA 菌株中发现了一种称为“碳酸氢盐 (NaHCO ₃ ) 反应性”的新表型，其中分离株在体外表现出对标准 β-内酰胺类药物（苯唑西林 [OXA]；头孢唑林 [CFZ] ) 在存在 NaHCO ₃的情况下，以及在实验性心内膜炎模型中对这些 β-内酰胺的体内敏感性。我们调查了 MRSA 的靶向表型-基因型筛查是否可以排除或排除 NaHCO ₃易感性前期。我们研究了 30 种充分表征的临床 MRSA 血流分离株，包括在 NaHCO ₃补充的 Mueller-Hinton Broth (MHB) 中对 CFZ 和 OXA 敏感的 15 种 MIC；和 15 MIC 对这种培养基中的两种 β-内酰胺均具有抗性。使用两层策略，首先通过标准纸片扩散筛选分离株对阿莫西林-克拉维酸 [AMC] 组合的敏感性。然后对分离物进行基因组序列分型：MLST（克隆复合物 [CC]）；农业; SCC机械；和mecA启动子和编码区。AMC 磁盘敏感性测试加上mecA和spa基因分型的组合能够预测或多或少可能是 NaHCO 的 MRSA 菌株_3-体外反应，具有高度的敏感性和特异性。使用心内膜炎的离体模型在来自整个队列的六个菌株中验证了该筛选算法。该离体模型概括了六种菌株中的五种中NaHCO ₃反应性与上述无反应性的体外预测。