Based on antimicrobial susceptibility testing (AST), correct classifications as susceptible, intermediate or resistant are challenging for some antimicrobial agent-bacterial species combinations. In this study, we investigated 19 equine Staphylococcus aureus isolates for their susceptibility to the combination sulfamethoxazole/trimethoprim (SXT) by using broth microdilution (BMD), agar disk diffusion (DD) and automated test systems. To elucidate the presence of the corresponding genetic resistance properties among the isolates, whole genome sequence analysis was performed and the genomes were screened for trimethoprim (TMP) resistance genes and mutations in the deduced FolP amino acid (aa) sequences, known to confer sulfonamide resistance. To check for hetero-resistance, zone diameters in DD were screened after 18 and 42 h of incubation. All 19 isolates harboured one of the TMP resistance genes dfrG or dfrS1. Three isolates had an aa exchange in their FolP aa sequence (F17L), which has previously been described to result in sulfonamide resistance. These isolates were classified as SXT-resistant by all methods. The remaining 16 isolates were classified as SXT-susceptible or -intermediate (BMD and/or DD) or SXT-resistant (mainly automated test systems). None of the isolates had relevant aa variations in their FolP aa sequences. All 19 isolates showed slight growth within their SXT inhibition zone by DD, pointing towards hetero-resistance. Overall, automated test systems classified isolates lacking genetic resistance determinants more frequently as SXT-resistant than DD and BMD. Therefore, further studies are needed to define a reliable method for SXT susceptibility testing.

中文翻译：

---

从临床马金黄色葡萄球菌分离物中使用不同方法获得的磺胺甲恶唑/甲氧苄氨嘧啶的抗菌药敏试验结果异质性。

基于抗菌药敏试验（AST），对于某些抗菌剂-细菌种类组合，正确的分类为易感，中等或耐药是具有挑战性的。在这项研究中，我们通过肉汤微稀释（BMD），琼脂圆盘扩散（DD）和自动化测试系统研究了19种马金黄色葡萄球菌分离株对磺胺甲恶唑/甲氧苄氨嘧啶（SXT）组合的敏感性。为了阐明分离株之间存在相应的遗传抗性特性，进行了全基因组序列分析，并筛选了基因组中的甲氧苄啶（TMP）抗性基因和推导的已知具有磺酰胺抗性的FolP氨基酸（aa）序列中的突变。 。为了检查异质性，在孵育18和42小时后筛选了DD中的区域直径。所有19个分离株均带有TMP抗性基因dfrG或dfrS1之一。三个分离株在其FolP氨基酸序列（F17L）中具有一个aa交换，先前已经描述了其导致的磺酰胺抗性。通过所有方法将这些分离物归类为抗SXT。其余的16个分离株被分类为对SXT敏感或中等（BMD和/或DD）或对SXT耐药（主要是自动化测试系统）。没有一个分离株的FolP氨基酸序列具有相关的氨基酸变异。所有19个分离株在DD的SXT抑制区内均显示出轻微的生长，表明具有异抗性。总体而言，自动化测试系统将缺乏遗传抗性决定因素的分离株比DD和BMD更频繁地归类为SXT抗性。因此，需要进一步的研究来定义一种可靠的SXT敏感性测试方法。