Polymyxins: Re-considering the Disk Diffusion Susceptibility Testing Methodin MALDI–TOF-identified Gram Negative Bacilli

Polymyxin, Colistin, Disk diffusion, E-test, MDR, XDR Background: The widespread multidrug-resistant Gram-negative bacilli (GNB)has necessitated the evaluation of the use of older antimicrobials such as polymyxins being the last resort drugs for the treatment of life-threatening nosocomial infections. Objectives: We conducted the present study to evaluate simple disk diffusion method (the routine method used in busy low resources hospitals)in comparison with MIC detection for polymyxin Band polymyxin E using the E-test according to the new Clinical and Laboratory Standards Institute (CLSI) guidelines and to find the level of polymyxins resistance of GNB in Kasr Alainy hospital. Methodology: A total of 50 Gram-negative bacterial isolates previously identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), and classified as 12 MDR by being resistant to three or more antimicrobial classes and38 XDR by being non-susceptible to at least one agent in all but two or fewer antimicrobial categories. Antimicrobial susceptibility testing of polymyxin B and polymyxin E (colistin) v was done by the determination of MIC by E-test, along with disk diffusion testing v. Results: Comparison of the disk diffusion and the MIC method by Etest showed100% correlation between both methods for all isolates of Pseudomonas aeruginosa, Acinetobacter baumanii and Klebsiella spp. All isolates were sensitive to both polymyxin B and colistin with MICs and disk zone diameters within the range proposed by the CLSI 2014.MIC 50 and MIC 90 of both polymyxin B and colistin were found to be 1μg/ml. Conclusion: Polymyxins are a reliable option for the treatment of infections caused by MDR isolates of Gramnegative bacilli with proper dose adjustment. There is good concordance between the Etest and the disk diffusion. The disk diffusion method can be used for initial screening in busy hospital laboratories.