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•Phenotypic and genotypic characterisation of fosA-positive Escherichia coli.•Complete fosAplasmid sequences were determined for 12 isolates.•fosAgenes were often found encoded on MDR plasmids, highlighting the potential for co-selection.

•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.

•SUL-DUR exhibited excellent in-vitroantibacterial activity against MDR A. baumannii•Mutations in penicillin binding protein 3 were associated with resistance to SUL-DUR•Efflux and permeability played no role in SUL-DUR resistance in these isolates