Comparative evaluation of colistin susceptibility testing by disk diffusion and etest among clinical isolates of carbapenam resistant pseudomonas aeruginosa

Colistin is a polymyxin with bactericidal activity against most strains of Pseudomonas aeruginosa. The recent increase in multidrug-resistant strains of P. aeruginosa has prompted renewed interest in the use of colistin. Rapid and reliable colistin susceptibility testing (ST) is needed in routine clinical laboratories to allow appropriate therapeutic decision-making. The present study aimed to study the efficacy and reliability of disc diffusion method to test the susceptibility of colistin. Materials and Methods:A total of 50 clinical isolates of carbapenem non-susceptible Pseudomonas aeruginosa isolated from various clinical samples was identified using standard microbiological procedures. Antimicrobial susceptibility was tested by means of agar disc diffusion and E-test method. Results: Around 82% isolates showed the sensitivity zone between 11-15mm (sensitive). The comparison between the Colistin Disks and E- strip showed 6 isolates to be sensitive by E-Strip method but found to be resistant by colistin disk method. Discussion: In this study most cases were found to be sensitive by both methods (86%). Nevertheless, 7 cases (14%) were resistant to the Colistin disk method. These were further tested for the Etest and 6/7 were found to be sensitive by the Etest. Conclusion: The above study showed the disk diffusion method to be equally efficacious as the Etest with 86% of the isolates showing susceptibility by both the methods.