Comparison of disc diffusion, Etest and agar dilution for susceptibility testing of colistin against Enterobacteriaceae

Aims: In this study, we compared different methods of colistin susceptibility testing, disc diffusion, agar dilution and Etest using a set of Enterobacteriaceae isolates that included colistin-resistant strains. Methods and results: Susceptibility of 200 clinical isolates of Enterobacteriaceae to colistin was tested to compare agar dilution (reference method), disc diffusion (50 and 10 mu g) and Etest. MICs (minimum inhibitory concentrations) were interpreted using the criteria established by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). Colistin exhibited excellent activity against Escherichia coli and E. cloacae (MIC90 = 0 5 mg l(-1)). In contrast, colistin was less active against Klebsiella pneumoniae (MIC90 = 16 mg l(-1)). Resistance rates varied from 0% in E. coli to 1.8% in E. cloacae and 13% in K. pneumoniae. High rates of very major errors were observed in the disc diffusion test using either the criteria of the Comite de l'antibiogramme de la Societe Francaise de Microbiologie (CA-SFM) or the criteria of the Clinical and Laboratory Standards Institute (CLSI), respectively, 3.5 and 2.5%. When the criteria of Gales et al. were applied, the number of very major errors was reduced to one (0.5%). The Etest showed good concordance with agar dilution method. Conclusion: Disc susceptibility testing methods are unreliable on detecting colistin resistance. MIC should be determined to confirm the susceptibility results by disc diffusion. Significance and Impact of the study: We recommend the determination of MIC by Etest for all multidrug-resistant Enterobacteriaceae when colistin is required for the treatment.