Detection of metallo betalactamase producing Pseudomonas aeruginosa in hospitalized patients.

Background & Objective: Metallo beta-lactamase (MBL)-mediated resistance to carbapenems is an emerging threat in hospital isolates of Pseudomonas aeruginosa. Though there are several screening methods to detect this enzyme production, the National Committee for Clinical Laboratory Standards (NCCLS) does not have performance standards documented so far. There is not enough information from the Indian subcontinent regarding the prevalence and the screening methods for these enzymes. The present study was undertaken to detect MBL in nosocomial isolates of P. aeruginosa by two screening methods.

Methods: Fifty consecutive P. aeruginosa isolates obtained from hospitalized patients were subjected to susceptibility testing to antipseudomonal drugs by disc diffusion, and minimum inhibitory concentration (MIC) of imipenem was determined. The production of MBL was detected by 4-fold reduction in MIC with imipenem-ethylene diamine tetraacetic acid (EDTA) and the zone size enhancement with EDTA impregnated imipenem and ceftazidime discs.

Results: Sixteen per cent of the isolates tested were resistant to imipenem by disc diffusion method of which 87.5 per cent exhibited a zone size enhancement with EDTA impregnated imipenem and ceftazidime discs as well as a 4-fold reduction in MIC with imipenem EDTA. The imipenem susceptible isolates (84%) had normal MIC values and exhibited no zone diameter enhancement with EDTA impregnated antibiotic discs.

Interpretation & Conclusion: MBL-mediated imipenem resistance in P. aeruginosa is a cause for concern in the therapy of critically ill patients. The two confirmatory methods i.e., zone diameter enhancement with EDTA impregnated imipenem and ceftazidime discs and 4-fold reduction in MIC with imipenem EDTA combination are equally effective for their detection.