Evaluation of susceptibility testing methods for polymyxin.

Background: The widespread resistance in Gram-negative bacteria has necessitated evaluation of the use of older antimicrobials such as polymyxins. In the present study we evaluated the different susceptibility testing methods for polymyxins B and E against Gram-negative bacteria using the new Clinical and Laboratory Standards Institute (CLSI) guidelines.

Methods: The susceptibility of 281 multidrug-resistant (MDR) Gram-negative bacteria (GNB) to polymyxin B was evaluated, comparing broth microdilution (BMD; reference method), agar dilution, E-test, and disk diffusion. Disk diffusion testing of polymyxin B was also performed against 723 MDR GNB.

Results: Twenty-four of 281 (8.5%) isolates were found to be resistant to polymyxin B by the reference BMD method. The rates of very major errors for agar dilution and E-test (for polymyxin B) were 0.7% and 1%, respectively, and those for disk diffusion (for polymyxin B and polymyxin E) were 1% and 0.7%, respectively. For the 257 isolates found sensitive by reference BMD, the rates of major errors by agar dilution and E-test (for polymyxin B) were 2.4% and 0%, respectively, and those for disk diffusion (polymyxin B and polymyxin E) were 0% and 0.7%, respectively. Twenty-six (3.6%) of the 723 Gram-negative isolates were resistant to polymyxin B by disk diffusion.

Conclusion: The E-test and agar dilution methods showed good concordance with BMD. The disk diffusion method can be useful for initial screening in diagnostic laboratories.