INTRODUCTION: Imipenem-resistant Pseudomonas aeruginosa resulting from metallo-β-lactamases has been reported to be an important cause of nosocomial infection and is a critical therapeutic problem worldwide, especially in the case of bacteremia. OBJECTIVES: To determine the frequency of metallo-β-lactamases among imipenem-resistant Pseudomonas aeruginosa isolates and to compare methods of phenotypic and molecular detection. METHODS: During 2006, 69 imipenem-resistant Pseudomonas aeruginosa samples were isolated from blood and tested for metallo-β-lactamase production using both phenotypic methods. Minimal Inhibitory Concentratrions (MIC) (μg/mL) was determined with commercial microdilution panels. Pulsed Field Gel Electrophoresis (PFGE) was performed among metallo-β-lactamase producers. RESULTS: Of all the blood isolates, 34.5% were found to be imipenem-resistant Pseudomonas aeruginosa. Positive phenotypic tests for metallo-β-lactamases ranged from 28%-77%, and Polymerase Chain Reaction (PCR) were positive in 30% (of note, 81% of those samples were blaSPM-1 and 19% were blaVIM-2). Ethylenediamine tetracetic acid (EDTA) combinations for the detected enzymes had low kappa values; thus, care should be taken when use it as a phenotypic indicator of MBL. Despite a very resistant antibiogram, four isolates demonstrated the worrisome finding of a colistin MIC in the resistant range. PFGE showed a clonal pattern. CONCLUSION: Metallo-β-lactamases among imipenem-resistant Pseudomonas aeruginosa were detected in 30.4% of imipenem-resistant Pseudomonas aeruginosa isolates. This number might have been higher if other genes were included. SPM-1 was the predominant enzyme found. Phenotypic tests with low kappa values could be misleading when testing for metallo-β-lactamases. Polymerase Chain Reaction detection remains the gold standard.
Pseudomonas aeruginosa; imipenem resistance; MBL detection; SPM-1
