Colistin MIC Variability by Method for Contemporary Clinical Isolates of Multidrug-Resistant Gram-Negative Bacilli

In vitro evaluation of colistin susceptibility is fraught with complications, due in part to the inherent cationic properties of colistin. In addition, no reference method has been defined against which to compare the results of colistin susceptibility testing. This study systematically evaluated the available methods for colistin MIC testing in two phases. In phase I, colistin MICs were determined in 107 fresh clinical isolates of multidrug-resistant (MDR) Gram-negative bacilli (GNB) by broth microdilution with polysorbate 80 (BMD-T), broth macrodilution (TDS), and the Etest. In phase II, 50 of these isolates, 10 of which were colistin resistant, were tested in parallel using BMD-T, TDS, agar dilution, broth microdilution without polysorbate 80 (BMD), and the TREK Gram-negative extra MIC format (GNXF) Sensititre. The Etest was also performed on these 50 isolates using Mueller-Hinton agar (MHA) from three different manufacturers. Colistin MIC results obtained from the five methods were compared to the MIC results obtained using BMD-T, the method that enables the highest nominal concentration of colistin in the test medium. Essential agreement ranged from 34% (BMD) to 83% (TDS), whereas categorical agreement was >90% for all methods except for BMD, which was 88%. Very major errors (VMEs) (i.e., false susceptibility) for the Etest were found in 47 to 53% of the resistant isolates, depending on the manufacturer of the MHA that was used. In contrast, VMEs were found for 10% (n = 1) of the resistant isolates by BMD and 0% of the isolates by the TDS, agar dilution, and Sensititre methods. Based on these data, we urge clinical laboratories to be aware of the variable results that can occur when using different methods for colistin MIC testing and, in particular, to use caution with the Etest.