Emergence of Imipenem Resistance and Presence of Metallo--Lactamases Enzymes in Multi Drug Resistant Gram Negative Bacilli Isolated from Clinical Samples in Kerman, 2007-2008

Background & Aims: Imipenem is a member of Carbapenem with stability against most -lactamases .It is of particular use in the treatment of infections associated with drug resistant gram negative bacteria harboring ESBL and AmpC genes. The aim of this study was to determine the imipenem resistance in gram negative bacteria causing nosocomial infections and the presence of metallo--lactamases (MBLS) in resistant isolates. Methods: Minimum inhibitory concentration (MIC) of imipenem was determined for 276 multiple drug resistant gram negative bacteria by agar dilution method. Escherichia coli ATCC 25922, Klebsiella pneumoniae ATCC 700603 and Pseudomonas aeruginosa ATCC 27853 were used as standard strains. Disk diffusion method with disks containing imipenem and imipenem+10µl EDTA (0.5 M) was used for determination of the presence of metallo--lactamases. Zone diameter 7mm of imipenem tested in combination with EDTA versus imipenem alone was considered as MBL positive. Results: From a total of 276 -lactamase producing isolates including K. pneumoniae (n=38), E. coli (n=169) and P.aeruginosa (n=69), 3 isolates with MIC=32µg/ml were found to be resistant to Imipenem. From these isolates, one strain of K. pneumoniae and one strain of P.aeruginosa isolates were determined to be MBL producers by phenotypic method. Conclusion: According to the presence of metallo - lactamases in bacterial strain in the region, resistance to this valuable therapeutic agent is not unexpected.