Colistin Susceptibility Testing by Reference Broth Microdilution Method in Clinical Isolates of Carbapenem-Resistant Enterobacterales at a Tertiary Care Hospital in Dhaka, Bangladesh

Background: Colistin has re-emerged as an important antimicrobial agent in recent times despite its adverse effects for the treatment of patients infected with CRE (carbapenem-resistant Enterobacterales) isolates that were not responding to other antibiotics. So, this study was carried out to observe the susceptibility pattern of colistin in clinical isolates of CRE. Materials and Methods: This cross-sectional study was conducted at the Department of Microbiology and Immunology of Bangladesh Medical University, Dhaka, Bangladesh from September 2018 to August 2019. One hundred and forty-five clinical isolates of CRE were subjected to test for colistin susceptibility by EUCAST and CLSI recommended broth microdilution method (BMD) and compared with the results of disc diffusion method (DDM). Results: Forty-two (29.0%) out of 145 CRE isolates were resistant to colistin by BMD. The resistance rate of colistin among Enterobacter spp, K. pneumoniae and E. coli were 36.36%, 31.19% and 17.39% respectively. The highest MIC value (32µg /ml) was observed in 2 isolates of Enterobacter spp. MIC50 and MIC90 value of colistin among CRE isolates were 2µg/ml and 8µg/ml respectively. Colistin resistant CRE isolates exhibited higher MIC 50 and MIC90 value of 8µg/ml and 16µg/ml respectively. DDM remains unreliable due to high major errors (47.62% of VME and 13.59% of ME) and minimal agreement (k value 0.40) in comparison to BMD. Conclusion: The findings of this study reflect an emerging burden of colistin resistant CRE in a tertiary care specialized hospital of Bangladesh and warrants implementation of reliable method for susceptibility testing of colistin.