A comparative study among different methods of detection of colistin resistant gram negative bacilli in a tertiary care hospital

Abstract

Background: Multi-drug resistant gram negative bacteria (MDR GNB) have become a major health challenge worldwide. Colistin has an excellent bactericidal activity against these MDR GNB. However, Colistin resistance has been reported globally. Material and methods: This study was conducted in the Department of Microbiology at Krishna Institute of Medical Sciences, Secunderabad from October 2017 to September 2018. Samples received to the laboratory were identified and AST was done by Vitek 2 compact system, those isolates which are colistin resistant were then later compared with disk diffusion and broth microdilution method. Results: Most common isolate which shows colistin resistance were Klebsiella pneumoniae, Pseudomonas aeruginosa, followed by Acinetobacter baumannii and Escherichia coli. Colistin resistant isolates showed by Vitek 2 compact were further compared by reference broth microdilution method, it was found 65.6 % of them were actually susceptible to colistin, whereas 34.3 % showed resistance pattern. The differences between these two methods were found to be statistically significant. On comparison disk diffusion with BMD, it was found that the difference was not statistically significant. Based on these observations, it can be inferred that disk diffusion was an unreliable method to detect colistin susceptibility. Conclusion: Since disk diffusion was the most frequently used method for drug’s susceptibility but in case of colistin it is an unreliable method. As far as automated system is concerned, Vitek 2 compact also cannot be trusted as far as colistin resistance has to be reported. For the colistin resistant isolates conventional methods like BMD should always be considered.

Keywords

• vitek 2 compact
• broth microdilution method
• kirby bauer
• disk diffusion
• multi drug resistant
• gram negative bacilli
• colistin resistance