Ono Seiji, Muratani Tetsuro, Matsumoto Tetsuro
Department of Urology, School of Medicine, University of Occupational and Environmental Health (UOEH), 1-1 Iseigaoka, Yahatanisi-Ku, Kitakyusyu 807-8555, Japan.
Antimicrob Agents Chemother. 2005 Jul;49(7):2954-8. doi: 10.1128/AAC.49.7.2954-2958.2005.
We found ampicillin- and imipenem-resistant isolates of vanA-possessing Enterococcus faecalis with MICs of 8 to 16 microg/ml and 4 to 32 microg/ml, respectively. There have been few reports about penicillin- and imipenem-resistant E. faecalis. Two mechanisms of beta-lactam resistance in E. faecalis, the production of beta-lactamase and the overproduction of penicillin-binding proteins (PBPs), have been reported. The resistant isolates in the current study did not produce any beta-lactamases and analysis of the PBPs showed no overproduction. However, the affinities of PBP4 for beta-lactams in the resistant strains were lower than those of susceptible strains but the affinities of other PBPs for beta-lactams did not change. Accordingly, whole pbp4 fragments from these resistant isolates were sequenced. Two amino acid substitutions at positions 520 and 605 were observed in the highly resistant strains compared to the susceptible ones, Pro520Ser and Tyr605His, and a single Tyr605His amino acid substitution was found in the low-resistance strains. These two point mutations exist in the region between the active-site-defining motifs SDN and KTG of the penicillin-binding domain, the main target of beta-lactams. A strong correlation was seen between these substitutions and decreasing affinities of PBP4 to beta-lactams. In E. faecalis, resistance due to mutations in PBPs has not been reported, though it has in Enterococcus faecium. Our results suggest that development of high-level resistance to penicillins and imipenem depends on point mutations of PBP4 at positions 520 and 605.
我们发现了携带vanA的耐氨苄西林和亚胺培南的粪肠球菌分离株，其对氨苄西林和亚胺培南的最低抑菌浓度（MIC）分别为8至16微克/毫升和4至32微克/毫升。关于耐青霉素和亚胺培南的粪肠球菌的报道很少。粪肠球菌对β-内酰胺类抗生素耐药的两种机制，即β-内酰胺酶的产生和青霉素结合蛋白（PBPs）的过量表达，已有报道。本研究中的耐药分离株未产生任何β-内酰胺酶，对PBPs的分析也未显示过量表达。然而，耐药菌株中PBP4对β-内酰胺类抗生素的亲和力低于敏感菌株，但其他PBPs对β-内酰胺类抗生素的亲和力没有变化。因此，对这些耐药分离株的整个pbp4片段进行了测序。与敏感菌株相比，在高耐药菌株中观察到520位和605位有两个氨基酸替换，即Pro520Ser和Tyr605His，在低耐药菌株中发现了单个Tyr605His氨基酸替换。这两个点突变存在于青霉素结合域的活性位点定义基序SDN和KTG之间的区域，该区域是β-内酰胺类抗生素的主要作用靶点。这些替换与PBP4对β-内酰胺类抗生素亲和力的降低之间存在很强的相关性。在粪肠球菌中，尚未报道因PBPs突变导致的耐药性，尽管在屎肠球菌中有相关报道。我们的结果表明，对青霉素和亚胺培南的高水平耐药性的产生取决于PBP4在520位和605位的点突变。
