False extended-spectrum β-lactamase detection in Acinetobacter spp. due to intrinsic susceptibility to clavulanic acid

Background: There are some reports showing the susceptibility of some strains of Acinetobacter baumannii to the beta-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-spectrum beta-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.