In Vitro Selection of Ceftazidime-Avibactam Resistance in Enterobacteriaceae with KPC-3 Carbapenemase

Ceftazidime-avibactam is active against most Enterobacteriaceae isolates with KPC carbapenemases. We investigated whether this activity could be compromised by mutation. Single-step and multistep selections were attempted using ceftazidime-avibactam (avibactam fixed at 1 or 4 μg/ml) versus two strains each of Enterobacter cloacae and Klebsiella pneumoniae, all with the KPC-3 enzyme. Mutant bla KPC alleles were sequenced, and their parentage was confirmed by typing. Ceftazidime-avibactam selected mutants at up to 16× MIC, with frequencies of ca. 10(-9). This contrasted with previous experience for ceftaroline-avibactam, where mutant frequencies under similar conditions were 256 μg/ml and those of ceftazidime with 4 μg/ml avibactam from 0.25 to 1 μg/ml to 4 to 128 μg/ml; ceftaroline-avibactam MICs rose less, typically from 0.5 to 1 μg/ml to 1 to 8 μg/ml. The MICs of carbapenems and cephalosporins except ceftazidime and piperacillin-tazobactam were reduced for many mutants. Sequencing of blaKPC revealed point and insertion changes in 12/13 mutants investigated, representing all four parents; one mutant lacked bla KPC changes and possibly had reduced permeability. Amino acid changes commonly involved Ω loop alterations or 1 to 6 amino acid insertions immediately C-terminal to this loop. The most frequent change, seen in four mutants from three strains, was Asp179Tyr, replacing a residue that ordinarily forms a salt bridge to stabilize the Ω loop. Since ceftaroline-avibactam was less affected than ceftazidime-avibactam, we postulate that these mutations increase ceftazidimase specificity rather than conferring avibactam resistance. The clinical relevance remains uncertain.