Billal Dewan S, Fedorko Daniel P, Yan S Steve, Hotomi Muneki, Fujihara Keiji, Nelson Nancy, Yamanaka Noboru
Department of Otolaryngology-Head and Neck Surgery, Wakayama Medical University, 811-1 Kimiidera, Wakayama, 641-8510, Japan.
J Antimicrob Chemother. 2007 Jan;59(1):28-34. doi: 10.1093/jac/dkl428. Epub 2006 Oct 25.
To perform a systematic analysis of point mutations in the quinolone resistance determining regions (QRDRs) of the DNA gyrase and topoisomerase genes of emm type 6 and other emm types of Streptococcus pyogenes strains after in vitro exposure to stepwise increasing concentrations of levofloxacin.
Twelve parent strains of S. pyogenes, each with a different emm type, were chosen for stepwise exposure to increasing levels of levofloxacin followed by selection of resistant mutants. The QRDRs of gyrA, gyrB, parC and parE correlating to mutants with increased MICs were analysed for point mutations.
Multiple mutants with significantly increased MICs were generated from each strain. The amino acid substitutions identified were consistent regardless of emm type and were similar to the mechanisms of resistance reported in clinical isolates of S. pyogenes. The number of induction/selection cycles required for the emergence of key point mutations in gyrA and parC was variable among strains. For each parent-mutant set, when MIC increased, serine-81 of gyrA and serine-79 of parC were the primary targets for amino acid substitutions. No point mutations were found in the QRDRs of gyrB and parE in any of the resistant mutants sequenced.
Despite its intrinsic polymorphism in the QRDR of parC, emm type 6 is not more likely to develop high-level resistance to fluoroquinolones when compared with other emm types. All emm types seem equally inducible to high-level fluoroquinolone resistance.
对化脓性链球菌6型及其他emm型菌株的DNA旋转酶和拓扑异构酶基因喹诺酮耐药决定区（QRDRs）在体外逐步增加左氧氟沙星浓度暴露后的点突变进行系统分析。
选择12株不同emm型的化脓性链球菌亲本菌株，逐步暴露于浓度递增的左氧氟沙星中，随后筛选耐药突变体。分析与最低抑菌浓度（MIC）升高的突变体相关的gyrA、gyrB、parC和parE的QRDRs中的点突变。
每株菌株均产生了多个MIC显著升高的突变体。所鉴定的氨基酸替代与emm型无关，且与化脓性链球菌临床分离株中报道的耐药机制相似。gyrA和parC中关键的点突变出现所需的诱导/选择循环次数在不同菌株间存在差异。对于每一组亲本-突变体，当MIC升高时，gyrA的丝氨酸81和parC的丝氨酸79是氨基酸替代的主要靶点。在任何测序的耐药突变体中，gyrB和parE的QRDRs中均未发现点突变。
尽管parC的QRDR存在内在多态性，但与其他emm型相比，6型emm对氟喹诺酮类药物产生高水平耐药的可能性并不更高。所有emm型似乎对高水平氟喹诺酮耐药的诱导程度相同。
