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Objective: To compare the in vitro efficacy of meropenem, colistin and tigecycline against extended spectrum Betalactamase producing Gram negative bacilli by minimal inhibitory concentration., Study Design: Cross-sectional descriptive study., Place and Duration of Study: Department of Microbiology, Army Medical College, National University of Sciences and Technology, Rawalpindi, from June to December 2010., Methodology: Routine clinical specimens were subjected to standard microbiological procedures and the isolates were identified to species level. Extended spectrum β-lactamase producing Gram negative bacilli were detected by Jarlier disc synergy method and confirmed by ceftazidime and ceftazidime-clavulanate Etest. Minimum Inhibitory Concentration (MIC(90)) of meropenem, colistin and tigecycline was determined by Etest (AB BIOMERIUX) and the results were interpreted according to the manufacturer's instructions and Clinical and Laboratory Standards Institute guidelines and Food and Drug Authority recommendations. Results were analyzed by using Statistical Package for the Social Sciences version 20., Results: A total of 52 non-duplicate extended spectrum Beta-lactamase-producing Gram negative bacilli were included in the study. The MIC(90) of tigecycline (0.75 µg/ml) was lowest as compared to the meropenem (2 µg/ml) and colistin (3 µg/ml)., Conclusion: Tigecycline is superior in efficacy against the extended spectrum Beta-lactamase producing Gram negative bacilli as compared to colistin and meropenem.

Multi-drug resistant bacteria are an important cause of mortality and morbidity. In the management of various infections, timely detection and appropriate treatment, in accordance with the culture and sensitivity reports can help improve the treatment outcome. Colistin is a bactericidal antibiotic which is emerging as a reliable solution for treating infections with multi-drug resistant Gram negative bacilli. The aim of this study was to find out the in-vitro efficacy of colistin against multidrug resistant Pseudomonas aeruginosa isolates by minimum inhibitory concentration. This cross sectional, descriptive study was conducted in the Department of Microbiology, Army Medical College, National University of Sciences and Technology, Islamabad from February 2010 to January 2011. Antimicrobial sensitivity testing was done on Pseudomonas aeruginosa isolated from routine clinical specimens received and the strains which appeared resistant to at least one antimicrobial agent in three or more anti-pseudomonal antimicrobial categories were subjected to the Colistin Etest. The MIC endpoint of colistin was read, as per manufacturers instructions (AB Biodisk, Solna, Sweden). The isolates showing MIC of 2μg/ml or less were considered sensitive, those with 4-6μg/ml as intermediate and >μg/ml as resistant. MIC(50) and MIC(90) of colistin against MDR Pseudomonas aeruginosa was determined. A total of 52 MDR Pseudomonas aeruginosa strains were isolated during the period of the study. The highest percentage was isolated from urine (36%) followed by respiratory tract infections (18%) and pus specimens (20%). The highest percentage of these isolates was found to be susceptible to colistin followed by piperacillin-tazobactam and cefoperazone-sulbactam. A total of 36(69%) isolates were sensitive, 10(20%) were intermediate and 6(11%) were resistant to colistin by Kirby Bauer disc diffusion method. MIC(50) was found to be 1.0μg/ml while MIC(90) was 3.0μg/ml. Colistin is a reliable solution in cases of infections with MDR, XDR or PDR Pseudomonas aeruginosa.