322 Correlation Between Etest and Broth Microdilution for Colistin Antimicrobial Susceptibility Testing of Enterobacteriaceae

Abstract

Colistin is increasingly being used to treat serious infections caused by multidrug-resistant Gram-negative bacteria. With the recent emergence of plasmid-mediated colistin resistance (mcr genes), screening for isolates with phenotypic colistin resistance has become clinically and epidemiologically relevant. However, antimicrobial susceptibility testing (AST) of colistin is associated with several technical challenges. Disk diffusion does not work because of poor diffusion of the large colistin molecule, Etest has been reported to underestimate minimum inhibitory concentration (MIC) values, and most clinical microbiology laboratories lack the capacity to perform broth microdilution (BMD). In this study, we compared the Etest method with the reference BMD method described by the Clinical and Laboratory Standards Institute (CLSI) for determining colistin MICs among Enterobacteriaceae. A total of 114 isolates of Enterobacteriaceae (51 carbapenem-resistant [CRE] and 63 carbapenem-susceptible [CSE]) were selected for colistin AST. One CRE isolate and 13 CSE isolates harbored a mcr gene. BMD testing was performed according to CLSI guidelines using a frozen reference panel with colistin concentrations ranging from 0.25 to 8 µg/mL. The Etest (BioMérieux, Durham, NC) method was performed on Mueller-Hinton agar (BBL, Becton Dickinson, Sparks, MD) according to the manufacturer’s instructions. Interpretive criteria based on CLSI epidemiological cutoff values (non-wild-type ≥4 µg/mL; wild-type ≤2 µg/mL) were used to define resistance and susceptibility to colistin, respectively. By BMD, colistin MICs ranged from ≤0.25 to >8 µg/mL and 27% were determined to be colistin-resistant. The categorical agreement between Etest and BMD was 92%. All 14 mcr-positive isolates were categorized as colistin-resistant by BMD, while Etest classified 13/14 as resistant. In conclusion, clinical microbiology laboratories lacking capacity to perform BMD could implement the Etest method with BBL Mueller-Hinton agar for epidemiological purposes to help identify candidate isolates for mcr screening. To determine whether colistin can be used clinically, BMD would still be required.