JAC Advance Access published online on March 8, 2007
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkm060
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Extended double disc synergy testing reveals a low prevalence of extended-spectrum ß-lactamases in Enterobacter spp. in Vienna, Austria
1 Department of Clinical Microbiology, Institute of Hygiene and Medical Microbiology, Medical University Vienna, Vienna, Austria 2 Department of Hospital Hygiene, Institute of Hygiene and Medical Microbiology, Medical University Vienna, Vienna, Austria
Received 8 August 2006; returned 13 November 2006; revised 29 January 2007; accepted 5 February 2007
* Correspondence address. Department of Clinical Microbiology, Institute of Hygiene and Medical Microbiology, Vienna General Hospital, Waehringer Guertel 18-20/5P, 1090 Vienna, Austria. Tel: +43-1-40400-5151; Fax: +43-1-40400-5228; E-mail: petra.apfalter{at}meduniwien.ac.at
Objectives: The aims of this study were to determine the prevalence of extended-spectrum ß-lactamases (ESBLs) in AmpC-carrying Enterobacter spp. in a tertiary care university hospital in Vienna, Austria, and to implement a cost-effective strategy to detect ESBLs in this particular genus on a routine basis.
Methods: Clinical Enterobacter isolates (n = 208) were investigated by means of (i) an inhibitor-potentiated diffusion test using cefpodoxime, (ii) an expanded double disc diffusion synergy test (discs of cefotaxime, ceftazidime, cefpodoxime and cefepime placed around amoxicillin/clavulanic acid), (iii) the Etest ESBL screening method and (iv) the cefoxitin–cefotaxime antagonist test. Cefepime MICs were determined by separate Etests.
Results: Of 208 isolates, 76 (37%), 18 (9%) and 92 (44%) were derepressed, partially derepressed and inducible AmpC producers, respectively. Eight (4%) ESBL-producing Enterobacter strains could be detected, all of which would have been detected using disc-based tests. Six out of eight strains were genetically not related, as assessed by random amplification of polymorphic DNA. Typing results were confirmed by means of enterobacterial repetitive intergenic consensus PCR. The MIC90 of cefepime was not different in ESBL carriers (range 2–4 mg/L), and was especially low in inducible AmpC producers (0.125 mg/L). More than half of all Enterobacter isolates (n = 110; 53%) were partly derepressed or fully inducible AmpC producers. In the absence of cefoxitin, they appeared susceptible or intermediately susceptible to cefazolin (n = 8; 9%), cefuroxime (n = 75; 81.5%), ceftazidime (n = 91; 99%), cefotaxime (n = 92; 100%), cefpodoxime (n = 75; 81.5%) and cefepime (n = 91; 99%).
Conclusions: Susceptibility to third-generation cephalosporins would have been falsely assumed in more than half of all Enterobacter isolates, but ESBL in Enterobacter is currently rare in our institution. Integration of multiple double disc tests into the routine antibiogram seems a reliable approach to screen for emerging resistance mechanisms. Etests did not provide additional information in this study.
Key Words: Etest , AmpC , third-generation cephalosporins , fourth-generation cephalosporins