Comparison of Various Phenotypic Methods for the Detection of Metallo-beta-lactamases in Pseudomonas aeruginosa

Aims: To evaluate the screening antibiotic and phenotypic test that can be used to confirm metallo-beta-lactamase (MBL) production in clinical isolates of Pseudomonas aeruginosa and to find out the prevalent MBL gene in them.

Materials and Methods: Three hundred and six isolates of P. aeruginosa were screened for resistance to meropenem (MEM), ceftazidime (CAZ) and imipenem (IMP). Isolates resistant to any of these were taken as screen test positive for MBL production and subjected to double disc synergy test (DDST) and combined disc synergy test (CDST), using MEM, CAZ and IMP with and without EDTA. Broth microdilution with MEM and IMP with and without EDTA was done to confirm MBL production (four fold reduction in minimum inhibitory concentration, MIC). Polymerase chain reaction (PCR) for blaVIM and blaIMP was done to find the prevalent gene in P. aeruginosa isolates.

Results: MEM picked up the highest number of MBL positive isolates 28.8% (n=76). CDST using MEM confirmed all the 76 screen test positive isolates to be MBL producers. Sensitivity of CDST using MEM, CAZ and IMP was 100%, 92.7% and 88.4% respectively. MIC by microbroth dilution for MEM and IMP was done for 76 MEM and 70 IMP positive isolates. For MEM maximum number of isolates had an MIC of 16 µg/ml and for IMP maximum number of isolates had an MIC of 32 µg/ml. blaVIM was the predominant MBL gene in P. aeruginosa isolates.

Conclusion: Meropenem was found to be a better screening as well as confirmatory agent for MBL detection. blaVIM was the predominant MBL gene in P. aeruginosa in our hospital.