In vitro-obtained meropenem-vaborbactam resistance mechanisms among clinical Klebsiella pneumoniaecarbapenemase-producing K. pneumoniaeisolates

•MEM-VAB resistant mutants were difficult to select for most strains.•MEM-VAB resistant mutants required multiple rounds of selection.•MEM-VAB resistant mutants resulted from a combination of permeability changes.•MEM-VAB resistant mutants increased blaKPCcopy number.•Cross-resistance was observed between MEM-VAB and other BLBI combinations.•Cross-resistance was observed to CAZ-AVI, IPM-REL, and to a lesser extent ATM-AVI.