Comparative Assessment of Disk Diffusion, E-test and Broth Dilution Methods for Determining Colistin Susceptibility in Acinetobacter Species

Abstract

Introduction: Colistin is the ultimate reserve drug for gram- negative bacilli especially non fermenting bacilli like Acinetobacter species and only therapeutic option available against carbapenem resistant bacteria. Clinical and Laboratory Standards Institute (CLSI) recommended broth dilution method for colistin susceptibility testing but this method require expertise and very time consuming and is also not feasible to routinely perform in clinical laboratories. In the background of ever expanding problems of drug resistance coupled with increasing use of colistin, it becomes essential to find a testing method which is faster, accurate and can be used as an alternative to broth dilution method. Aim: To assess disk diffusion, E-test and broth dilution method to determine colistin susceptibility in Acinetobacter species. Materials and Methods: This cross-sectional observational study was done in the Department of Microbiology, ABVIMS and Dr RML Hospital, New Delhi, India, from November 2016 to March 2018. Various clinical specimens were received from wards and Intensive Care Unit (ICU) and Acinetobacter species were isolated and identified following standard protocol. All strains were screened by disc diffusion test for various antibacterial drugs and among them 50 MDR strains were selected. Colistin susceptibility by disc diffusion and Epsilometer test (E-test) were done and compared against the reference broth dilution method following. Minimum Inhibitory Concentration (MIC) of colistin for the isolates were determined using macro broth dilution method using colistin sulphate. Chi-square test was used to investigate distributions of categorical variables. Results: All 50 strains were tested for colistin susceptibility by disc diffusion method. Among the 50 isolates, 10 isolates (20%) showed resistance to colistin by disc diffusion method while none of the isolate was found resistant by E-test or broth dilution method. High error rate and poor concordance of disc diffusion with reference to broth dilution method rules out the utility of disk diffusion method in clinical laboratories. Conclusion: A complete agreement between the results of E-test and broth dilution method makes E-test a suitable alternative to broth dilution test in clinical laboratories.