Villar H E, Jugo M B, Visser M, Hidalgo M, Hidalgo G, Maccallini G C
Hugo Edgardo Villar, Departamento de Bacteriología Clínica. Laboratorio Hidalgo, Ladislao Martínez 43 B1640EYA Martínez, Buenos Aires, Argentina.
Rev Esp Quimioter. 2014 Mar;27(1):51-5.
The occurrence of community-associated infections due to extended-spectrum β-lactamase (ESBL)-producing Escherichia coli is increasing worldwide. These organisms are frequently resistant to many of the antimicrobial agents but remain susceptible to carbapenems. We investigated the in vitro emergence of carbapenem resistance in a collection of clinical isolates of ESBL -producing E. coli.
First and second-step resistant mutants were obtained from E. coli with ESBL. Aliquots of 50 μl containing > 109 CFU were applied to Mueller-Hinton plates containing meropenem, imipenem or ertapenem. MICs for native strains and mutants were determined using the epsilometric test (E-test).
Resistant mutants were not selected with imipenem or meropenem. E. coli growth was observed on ertapenem (0.5 mg/L)-containing plates in 13 of 57 clinical isolates (22.8 %).The ertapenem MIC for these first-step mutants were ≥ 1 mg/L, remaining susceptible to imipenem and meropenem. The first-step mutants were used as native strains. Six second-step resistant mutants were selected with ertapenem. All were fully resistant (CMI ≥ 8 mg/L) to ertapenem, three were resistant to meropenem and one to imipenem. Four second-step resistant mutants were selected with meropenem. All were resistant to ertapenem, meropenem, and two of them were resistant to imipenem.
Stable resistant mutants were easy to select with ertapenem among ESBL-producing E. coli. Two steps were necessary to select resistant mutants to meropenem or imipenem. The use of ertapenem in high-inoculum infections or in undrained focus of infection should be monitored to reduce the risk on selection of resistance.
产超广谱β-内酰胺酶（ESBL）的大肠埃希菌引起的社区获得性感染在全球范围内呈上升趋势。这些细菌通常对多种抗菌药物耐药，但对碳青霉烯类仍敏感。我们研究了产ESBL的大肠埃希菌临床分离株中碳青霉烯类耐药性的体外出现情况。
从产ESBL的大肠埃希菌中获得第一步和第二步耐药突变株。将含有>109 CFU的50 μl等分试样接种到含有美罗培南、亚胺培南或厄他培南的穆勒-欣顿平板上。使用E试验（epsilometric test）测定原始菌株和突变株的最低抑菌浓度（MIC）。
未用亚胺培南或美罗培南筛选出耐药突变株。在57株临床分离株中的13株（22.8%）中，观察到在含厄他培南（0.5 mg/L）的平板上大肠埃希菌生长。这些第一步突变株的厄他培南MIC≥1 mg/L，对亚胺培南和美罗培南仍敏感。第一步突变株用作原始菌株。用厄他培南筛选出6株第二步耐药突变株。所有突变株对厄他培南均完全耐药（CMI≥8 mg/L），3株对美罗培南耐药，1株对亚胺培南耐药。用美罗培南筛选出4株第二步耐药突变株。所有突变株对厄他培南、美罗培南均耐药，其中2株对亚胺培南耐药。
在产ESBL的大肠埃希菌中，用厄他培南很容易筛选出稳定的耐药突变株。筛选对美罗培南或亚胺培南耐药的突变株需要两步。在高接种量感染或未引流的感染灶中使用厄他培南时应进行监测，以降低耐药性选择的风险。
