Custom frozen panels

MICs were determined by broth microdilution using custom frozen panels (ThermoFisher Scientific, Cleveland, OH) according to Clinical Laboratory Standards Institute (CLSI) methods (Clinical and Laboratory Standards Institute, 2015a ). MIC values were obtained in triplicate and modal values determined. β-Lactam agents tested (doubling dilution concentration range in mg/L) included ceftazidime (0.06–128), ceftaroline (0.03–64), aztreonam (0.06–64), meropenem (0.015–32) and piperacillin-tazobactam (0.06/4–128/4). Ceftazidime, ceftaroline and aztreonam were also tested at the same concentrations in the presence of a fixed concentration of 4 mg/L of Avibactam. MICs were interpreted according to current CLSI breakpoints for these compounds in the absence of the inhibitor (Clinical and Laboratory Standards Institute, 2015b ). Avibactam and ceftaroline were provided by AstraZeneca as kind gifts.

MICs for various bacterial isolates were determined by broth microdilution method using custom frozen panels (ThermoFisher Scientific, Cleveland, OH) according to the Clinical and Laboratory Standards Institute guidelines.^{48 (link),49 (link)} Compounds 1–3, avibactam, relebactam, meropenem, and cefepime were tested alone. Compounds 1–3 were tested at 4 and 8 mg/L, and avibactam and relebactam were tested at 4 mg/L in combination with increasing concentrations of meropenem and cefepime. MICs were performed in triplicate, and modal values are reported.