Giovanetti Eleonora, Brenciani Andrea, Burioni Roberto, Varaldo Pietro Emanuele
Department of Microbiology and Biomedical Sciences, University of Ancona Medical School, 60131 Ancona, Italy.
Antimicrob Agents Chemother. 2002 Dec;46(12):3750-5. doi: 10.1128/AAC.46.12.3750-3755.2002.
Streptococcus pyogenes strains inducibly resistant (iMLS phenotype) to macrolide, lincosamide, and streptogramin B (MLS) antibiotics can be subdivided into three phenotypes: iMLS-A, iMLS-B, and iMLS-C. This study focused on inducibly erythromycin-resistant S. pyogenes strains of the iMLS-B and iMLS-C types, which are very similar and virtually indistinguishable in a number of phenotypic and genotypic features but differ clearly in their degree of resistance to MLS antibiotics (high in the iMLS-B type and low in the iMLS-C type). As expected, the iMLS-B and iMLS-C test strains had the erm(A) methylase gene; the iMLS-A and the constitutively resistant (cMLS) isolates had the erm(B) methylase gene; and a control M isolate had the mef(A) efflux gene. mre(A) and msr(A), i.e., other macrolide efflux genes described in gram-positive cocci, were not detected in any test strain. With a radiolabeled erythromycin method for determination of the intracellular accumulation of the drug in the absence or presence of an efflux pump inhibitor, active efflux of erythromycin was observed in the iMLS-B isolates as well as in the M isolate, whereas no efflux was demonstrated in the iMLS-C isolates. By the triple-disk (erythromycin plus clindamycin and josamycin) test, performed both in normal test medium and in the same medium supplemented with the efflux pump inhibitor, under the latter conditions iMLS-B and iMLS-C strains were no longer distinguishable, all exhibiting an iMLS-C phenotype. In conjugation experiments with an iMLS-B isolate as the donor and a Rif(r) Fus(r) derivative of an iMLS-C isolate as the recipient, transconjugants which shared the iMLS-B type of the donor under all respects, including the presence of an efflux pump, were obtained. These results indicate the existence of a novel, transferable efflux system, not associated with mef(A) or with other known macrolide efflux genes, that is peculiar to iMLS-B strains. Whereas the low-level resistance of iMLS-C strains to MLS antibiotics is apparently due to erm(A)-encoded methylase activity, the high-level resistance of iMLS-B strains appears to depend on the same methylase activity plus the new efflux system.
对大环内酯类、林可酰胺类和链阳菌素B（MLS）抗生素具有诱导性耐药（iMLS表型）的化脓性链球菌菌株可细分为三种表型：iMLS-A、iMLS-B和iMLS-C。本研究聚焦于iMLS-B型和iMLS-C型的诱导性耐红霉素化脓性链球菌菌株，它们在许多表型和基因型特征上非常相似，几乎难以区分，但在对MLS抗生素的耐药程度上有明显差异（iMLS-B型高，iMLS-C型低）。正如预期的那样，iMLS-B和iMLS-C测试菌株具有erm(A)甲基化酶基因；iMLS-A和组成型耐药（cMLS）分离株具有erm(B)甲基化酶基因；而对照M分离株具有mef(A)外排基因。在任何测试菌株中均未检测到mre(A)和msr(A)，即革兰氏阳性球菌中描述的其他大环内酯外排基因。采用放射性标记的红霉素方法测定在有无外排泵抑制剂的情况下药物在细胞内的蓄积，在iMLS-B分离株以及M分离株中均观察到红霉素的主动外排，而在iMLS-C分离株中未显示有外排。通过在正常测试培养基以及添加了外排泵抑制剂的相同培养基中进行的三联纸片（红霉素加克林霉素和交沙霉素）试验，在后一种条件下，iMLS-B和iMLS-C菌株不再可区分，均表现出iMLS-C表型。在以iMLS-B分离株为供体、iMLS-C分离株的Rif(r) Fus(r)衍生物为受体的接合实验中，获得了在所有方面均与供体的iMLS-B型相同的转接合子，包括存在外排泵。这些结果表明存在一种新的、可转移的外排系统，它与mef(A)或其他已知的大环内酯外排基因无关，是iMLS-B菌株所特有的。iMLS-C菌株对MLS抗生素的低水平耐药显然是由于erm(A)编码的甲基化酶活性，而iMLS-B菌株的高水平耐药似乎取决于相同的甲基化酶活性加上新的外排系统。
