Meropenem

Analytical reference standards of meropenem (as trihydrate; Tokyo Chemical Industry Co. Ltd.) and amikacin (PHR1654; Sigma-Aldrich) were used for in vitro susceptibility testing. For dosing simulation in the hollow-fiber infection model experiments, clinical formulations of meropenem (meropenem trihydrate powder for injection; Ranbaxy Australia Pty. Ltd.) and amikacin (DBL amikacin injection; Hospira Australia Pty. Ltd.) were used. For meropenem, a fresh 12.5-mg/mL dosing stock solution was prepared from the clinical formulation and kept as 1-mL aliquots in a −80°C freezer for preparation of the doses. For each meropenem dose, an aliquot was thawed immediately before dosing and the appropriate volume constituted with sterile broth, namely, CaMH (final volume, 20 mL) in a dosing syringe. Similarly, a 10-mg/mL amikacin dosing stock solution was prepared and stored in a refrigerator and an appropriate volume constituted with sterile broth (final volume, 20 mL) for dosing.

Meropenem (Tokyo Chemical Industry Co. Ltd., Tokyo, Japan), sulbactam (Acros Organics, Lot: A0405260, Shanghai, China) and fosfomycin (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were obtained from their respective manufacturers. Stock solutions of these antibiotics were prepared in sterile Milli-Q water, filter-sterilised with a 0.22 µm polyvinylidene difluoride (PVDF) syringe filter, aliquoted and stored at −80 °C until required. Before each susceptibility test, an aliquot of the drug was thawed and diluted to the desired concentrations with cation-adjusted Mueller-Hinton II (CA-MH) broth. Broth or agar containing fosfomycin was supplemented with 25 mg/L of glucose-6-phosphate (Sigma-Aldrich, St. Louis, MO, USA).

Meropenem and ceftazidime powder were obtained from Tokyo Chemical Industry Co., Ltd. (TCI) (Portland, OR, USA). Ceftriaxone powder was purchased from Sigma-Aldrich, Inc. (St. Louis, MO, USA). Ertapenem was provided by Merck (Rahway, NJ, USA). Ammonium acetate was purchased from Sigma Life Sciences (St. Louis, MO, USA). LCMS-grade ammonium hydroxide, water, and methanol were obtained from EMD Millipore Corporation (Billerica, MA, USA).