The BMD method, as the standard reference method for antimicrobial susceptibility testing (AST), was performed in strict accordance with the CLSI M7-A10 document.13 The antibiotic drugs polymyxin B and colistin were obtained from the National Institutes for Food and Drug Control of China (polymyxin B lot: 130313-202111, colistin lot: 130327-200906). E. coli ATCC25922 and Pseudomonas aeruginosa ATCC27853 were used as polymyxin-susceptible control strains, while E. coli NCTC 13846 (mcr-1-positive) and K. pneumoniae CCUG59348 (colistin resistant, mcr-negative) served as the polymyxin-resistant control strains.14 (link) The performance of five commercial methods, including VITEK 2® COMPACT (BioMérieux, Marcy l’Etoile, France) with an AST-N335 card (colistin), PhoenixTM M50 (Becton Dickinson Diagnostics, Sparks, MD, USA) with a NMIC-502 card (colistin), DL-96II (Zhuhai DL Biotech Co., Ltd., Zhuhai, China) with a DL-E card (polymyxin B), MA120 (Zhuhai Meihua Medical Technology Co., Ltd., Zhuhai, China) with a MA card (polymyxin B), and Polymyxin B Susceptibility Test strip (Autobio Diagnostics Co., Ltd., Zhengzhou, China), which were performed according to the manufacturer’s instructions, were evaluated.
The possible ranges of MIC readings for each method were as follows: BMD (colistin and polymyxin B), ≤ 0.5 to ≥ 32mg/L; Vitek 2, ≤ 0.5 to ≥ 16 mg/L; Phoenix M50, ≤ 1 to ≥ 8 mg/L; DL-96II, ≤ 2 to ≥ 4 mg/L; MA120, ≤ 1 to ≥ 4 mg/L; Polymyxin B Susceptibility Test Strip (POL E-Strip), ≤ 0.06 to ≥ 256 mg/L. All POL E-Strip results were recorded up to the nearest MIC measured in the BMD category (eg, 0.75 mg/L was recorded as 1 mg/L).