Comparative Evaluation of Broth Microdilution with Polystyrene and Glass-Coated Plates, Agar Dilution, E-Test, Vitek, and Disk Diffusion for Susceptibility Testing of Colistin and Polymyxin B on Carbapenem-Resistant Clinical Isolates of Acinetobacter baumannii.

With the increasing threat of multidrug-resistant organisms, such as Acinetobacter baumannii, the polymyxin class of drugs (colistin and polymyxin B) has become popular in clinical practice. A better understanding of antimicrobial susceptibility testing methods for colistin and polymyxin B is needed for optimal patient management. Forty-two carbapenem-resistant A. baumannii isolates were subjected to susceptibility testing for colistin and polymyxin B using the following methods: broth microdilution (BMD) (glass-coated plates [BMD-Gs] and polystyrene plates [BMD-Ps]), agar dilution (AD), E-test®, Vitek®, and disk diffusion. Using BMD as the gold standard, comparative analysis between different methods was carried out. With BMD-Gs as reference, reliability was high for BMD-Ps and moderate for AD and Vitek for both the drugs. Similar results were obtained when the BMD-P was used as reference, but drug-polystyrene interaction was observed. Different susceptibility testing methods for polymyxins show great variation in their results and BMD using glass-coated plates can be considered the best candidate for gold standard.