Broth microdilution.

Antibiotics were tested using frozen panels prepared at Accelerate Diagnostics, Inc. according to CLSI methods (38) using cation-adjusted Mueller-Hinton broth (Difco, BD, Sparks, MD). BMD was performed in triplicate for each inoculum of bacteria. Bacterial inocula were prepared to 0.5 McFarland from isolates subcultured twice on sheep’s blood agar plates from −70°C frozen stocks. DensiChek was used to prepare inocula with additional saline or pure colony as needed for titration. BMD was performed as outlined in CLSI M07, and panels were incubated at 35°C ± 2°C for 16 to 20 h in an ambient air incubator (38). Piperacillin-tazobactam (TZP), aztreonam (ATM), meropenem (MEM), ceftazidime (CAZ), and cefepime (FEP) were evaluated for both isolate sets. Ceftazidime-avibactam (CZA) and ceftolozane-tazobactam (TZC) were only evaluated for bacterial isolate set 1, as these drugs were not tested at the time bacterial isolate set 2 data were generated. Antibiotic concentration ranges (μg/ml) on the BMD panels were 0.25 to 256 and 1 to 256 for TZP, 0.5 to 64 for ATM, 0.5 to 64 for FEP, 0.5 to 64 for CAZ, 0.0.3125 to 128 and 0.03125 to 32 for MEM, 0.25 to 64 for TZC, and 0.25 to 64 for CZA.