transformants expressing oxacillinases and metallo-β-lactamases that confer resistance to meropenem: new tools for anti- drug development and AMR preparedness.

Antimicrobial resistance (AMR) in is an unmet medical need. Multiple drug-resistant/extremely drug-resistant strains of do not display growth well in models, and consequently, their response to antibacterial therapy is inconsistent. We addressed this issue by engineering carbapenem resistance motifs into the highly virulent genetic background of AB5075. This strain has a chromosomally encoded that was deleted (), then plasmids expressing , , , , , and were introduced to create the mutant strains. Each transformant was used as a challenge strain in a neutropenic murine thigh infection model and assessed for the extent of growth and response to meropenem 200 mg/kg subcutaneously every 6 h (q6h). Pharmacodynamic analyses were performed by transforming drug exposure from dose (mg/kg) to the fraction of the dosing interval; free meropenem concentrations were >minimum inhibitory concentration (MIC) (T > MIC). AB5075 and the AB5075 mutant had a MICs of 32 and 4 mg/L, respectively. The transformants harboring oxacillinases and had an MIC of 64 mg/L. The metallo-β-lactamases , , and had MICs of 128, 64, and 64 mg/L, respectively. All vehicle-treated transformants displayed growth in the range of 0.75-1.4 log. The response to meropenem was consistent with the varying T > MIC of the transformants and was readily described by an inhibitory sigmoid relationship. Stasis was achieved with a T > MIC of 0.36. These transformants are invaluable new tools for the assessment of anti- compounds and provide a new pathway for AMR preparedness.