JAC Advance Access originally published online on December 8, 2007
Journal of Antimicrobial Chemotherapy 2008 61(2):301-308; doi:10.1093/jac/dkm461
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Original research |
False extended-spectrum β-lactamase detection in Acinetobacter spp. due to intrinsic susceptibility to clavulanic acid
1 Servicio de Microbiología, Complejo Hospitalario Universitario Juan Canalejo, Xubias de Arriba s/n,15006 La Coruña, Spain 2 Servicios de Microbiología, Hospital Virgen Macarena, Avda. Sánchez Pizjuan,s/n 41071 Sevilla, Spain 3 Servei de Microbiología, Hospital Clinic, Villarroel 170, 08036 Barcelona, Spain 4 Enfermedades Infecciosas, Hospital Virgen Macarena, Avda. Sánchez Pizjuan, s/n 41071 Sevilla, Spain 5 Servicio de Enfermedades Infecciosas, Hospital Universitario Virgen del Rocío, Avda. Manuel Siurot,s/n 41013 Sevilla, Spain
Received 7 June 2007; returned 10 September 2007; revised 11 October 2007; accepted 29 October 2007
* Corresponding author. Tel: +34-981-176087; Fax: +34-981-176097; E-mail: germanbou{at}canalejo.org
Background: There are some reports showing the susceptibility of some strains of Acinetobacter baumannii to the β-lactamase inhibitor clavulanic acid. To address this issue, we determined the MIC of clavulanic acid for a broad collection of Acinetobacter spp. isolates collected in a multicentre study. In addition, we showed the consequences of this susceptibility to yield false extended-spectrumβ-lactamase (ESBL) detection in this genus.
Methods: The strains used were 244 isolates of Acinetobacter (226 A. baumannii, 15 Acinetobacter genomic species 3 and 3 unidentified Acinetobacter spp.) and several A. baumannii as positive controls. The isolates were subjected to molecular typing. One isolate of each genotype was subjected to clavulanic acid MIC analysis. As no breakpoints for clavulanic acid are available, we arbitrarily established three categories of susceptibility: 
16, 32–128 and 
256 mg/L. The presence of ESBL in Acinetobacter spp. was analysed by using microdilution, double disc diffusion, combined discs, Etest and isoelectric focusing.
Results: A total of 100 different genotypes were detected. Among them, 44, 26 and 30 genotypes were inhibited by 16, 32–128 and 256 mg/L clavulanic acid, respectively. Representative isolates of each group were tested for ESBL production. Only those with the lower clavulanic acid MICs yielded a false-positive ESBL test with all methods tested with the exception of the double disc diffusion assay.
Conclusions: Forty-four per cent of the genotypes tested were inhibited by 16 mg/L clavulanic acid and these Acinetobacter isolates yielded a false ESBL-positive test. These results may have implications for susceptibility testing in routine microbiology laboratories.
Keywords: β-lactamase inhibitors , PBP alterations , disc diffusion
Present address. Servicio de Microbiología, Hospital Universitario Marqués de Valdecilla, Avda. Valdecilla, s/n 39008 Santander, Spain.