In vitro susceptibility of characterized β-lactamase-producing strains tested with avibactam combinations.

Avibactam, a broad-spectrum β-lactamase inhibitor, was tested with ceftazidime, ceftaroline, or aztreonam against 57 well-characterized Gram-negative strains producing β-lactamases from all molecular classes. Most strains were nonsusceptible to the β-lactams alone. Against AmpC-, extended-spectrum β-lactamase (ESBL)-, and KPC-producing Enterobacteriaceae or Pseudomonas aeruginosa, avibactam lowered ceftazidime, ceftaroline, or aztreonam MICs up to 2,048-fold, to ≤4 μg/ml. Aztreonam-avibactam MICs against a VIM-1 metallo-β-lactamase-producing Enterobacter cloacae and a VIM-1/KPC-3-producing Escherichia coli isolate were 0.12 and 8 μg/ml, respectively.