Department of Bacteriology, Nagoya University Graduate School of Medicine, Showa-ku, Nagoya, Japan.
J Infect Chemother. 2010 Feb;16(1):19-24. doi: 10.1007/s10156-009-0009-0.
Recently, hospital-associated as well as community-acquired methicillin-resistant Staphylococcus aureus (MRSA) strains showing a low level of resistance to oxacillin have emerged worldwide, and as a result, a highly sensitive method to detect MRSA has become more important. To prevent MRSA being overlooked, some selection agar media have recently been developed. We evaluated six commercially available selection agar media in regard to the detection of 35 borderline MRSA (BOMRSA) strains which were mecA-positive but showed low resistance to oxacillin. The MIC values of oxacillin differed between the broth dilution method and the agar dilution method, and 11 of the 35 BOMRSA strains were judged as sensitive by the broth dilution method and 14 of the 35 strains were judged as sensitive by the agar dilution method. Thirty-two of the 35 strains were also judged as sensitive by an oxacillin disk diffusion test. Moreover, there was no consistent pattern of resistance to the tested beta-lactams among the BOMRSA strains. Some commercially available selection media designed for the detection of MRSA contain a beta-lactam antibiotic; oxacillin, ceftizoxime, or cefoxitin, and we evaluated media containing each of these agents. The detection sensitivities of cefoxitin-based agar media, such as CHROMagar MRSA and MRSA ID, for BOMRSA were 100% at 24-h culture. On the other hand, the media containing oxacillin or ceftizoxime gave lower results at 24 h, suggesting that, possibly, BOMRSA strains may not to be able to grow on these media. These results suggest that cefoxitin-based agar media should be recommended for the first-round screening of BOMRSA.
最近，世界范围内出现了一些对苯唑西林低度耐药的医院相关性和社区获得性耐甲氧西林金黄色葡萄球菌（MRSA）菌株，因此，一种更敏感的检测 MRSA 的方法变得更加重要。为了防止漏检 MRSA，最近开发了一些选择性琼脂培养基。我们评估了六种市售的选择性琼脂培养基对 35 株边缘型耐甲氧西林金黄色葡萄球菌（BOMRSA）菌株的检测能力，这些菌株 mecA 阳性但对苯唑西林的耐药性较低。苯唑西林的 MIC 值在肉汤稀释法和琼脂稀释法之间存在差异，35 株 BOMRSA 菌株中有 11 株通过肉汤稀释法被判断为敏感，14 株通过琼脂稀释法被判断为敏感。35 株菌株中有 32 株通过苯唑西林纸片扩散试验被判断为敏感。此外，BOMRSA 菌株对测试的β-内酰胺类抗生素没有一致的耐药模式。一些为检测 MRSA 而设计的市售选择性培养基含有一种β-内酰胺抗生素；苯唑西林、头孢唑肟或头孢西丁，我们评估了含有这些药物的培养基。头孢西丁为基础的琼脂培养基，如 CHROMagar MRSA 和 MRSA ID，对 BOMRSA 的检测灵敏度在 24 小时培养时为 100%。另一方面，含有苯唑西林或头孢唑肟的培养基在 24 小时时的检测结果较低，这表明 BOMRSA 菌株可能无法在这些培养基上生长。这些结果表明，头孢西丁为基础的琼脂培养基应该被推荐用于 BOMRSA 的第一轮筛选。
