COMPARISON OF DISC DIFFUSION, AGAR DILUTION, AND BROTH MICRODILUTION METHODS FOR DETECTION OF COLISTIN RESISTANT ENTEROBACTERIACEAE AT MINIA UNIVERSITY HOSPITALS, EGYPT

Abstract

Background: With widespread increasing in multidrug resistant Gram-negative bacteria, the use of colistin
has increased. Colistin use is associated with high rates of neurotoxicity and nephrotoxicity even at optimal
doses. As such, determination of MIC of an infecting organism and categorical interpretation is of
significant clinical value. In this study, we compared three different methods for colistin susceptibility
testing using a set of Enterobacteriaceae isolates that included colistin-resistant strains.
Methods: The colistin resistance of 372 Enterobacteriaceae isolates collected from different clinical cases
were detected by disc diffusion method with determination the MICs by both agar dilution (AD) and broth
microdilution (BMD) as the gold standard using the new clinical breakpoints for colistin approved by the
Clinical and Laboratory Standards Institute (CLSI). The performance of disc diffusion and AD were
evaluated versus reference BMD. Essential agreement (EA), category agreement (CA), very major error
(VME), and major error (ME) were calculated for comparison.
Results: Thirty-six of 372 (9.7%) included isolates were found to be resistant to colistin by the reference
BMD. The rates of very major errors for AD and disc diffusion were 0.0% and 2.78%, respectively. For the
336 isolates found sensitive by reference BMD, the rates of major errors by AD and disc diffusion were
1.5% and 2.38%, respectively. By AD Escherichia and Klebsiella spp. showed the highest performance
characteristics that met the required standard, but Citrobacter spp. met the required standard in EA and
VME and Enterobacter spp. met the required standard in VME only.
Conclusion: Agar dilution method showed good concordance with BMD especially for Escherichia and
Klebsiella spp. The disk diffusion method can be useful for initial screening in diagnostic laboratories