Phenotypic ESBL Detection in Acinetobacter baumannii: A Real Challenge

Clinical Laboratory Standards Institute (CLSI) recommends two-step approach for extended-spectrum-β-lactamase (ESBL) detection which includes the screening of recommended agents and the phenotypic confirmation of ESBL using a combination of screening agent and β-lactamase inhibitor. To investigate this approach, we screened 145 β-lactamase-producing Acinetobacter baumannii isolates for ESBL from tracheal secretions using double disk synergy test (DDST) and phenotypic confirmatory disc diffusion test (PCDDT), the determination of minimum inhibitory concentration (MIC) for ceftazidime and cefotaxime (with/without clavulanic acid), and the unique disc placement scheme. Eighteen of 145 (12.4%) isolates showed ESBL production. All 18 isolates showed positive PCDDT. MIC of these isolates were extremely high (>512 µg/ml), and eight fold decrease in MIC was shown by only one isolate. The unique disc placement scheme detected 13 (72.2%) and 3 (16.7%) of ESBL producers and ampC producers, respectively. High level resistance to cefoxitin and cefotaxime among these isolates is suggestive of the derepressed mutants. The PCDDT was most effective ESBL detection method while the unique disc placement scheme showed advantage of detection of ampC β-lactamase, derepressed mutants and multiple β-lactam resistance mechanism in these isolates. This is a rare report comparing different tests for phenotypic ESBL detection in clinical isolates of A. baumannii.