Characterization of β-lactamase and porin mutants of Enterobacteriaceae selected with ceftaroline + avibactam (NXL104)

David M. Livermore, Shazad Mushtaq, Kevin Barker, Russell Hope, Marina Warner, Neil Woodford

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Objectives: Ceftaroline + avibactam (NXL104) is a novel inhibitor combination active against Enterobacteriaceae with class A and C β-lactamases. We investigated its risk of mutational resistance.

Methods: Single- and multi-step mutants were sought and characterized from Enterobacteriaceae with extended-spectrum β-lactamases (ESBLs), AmpC β-lactamases and KPC β-lactamases.

Results: Overgrowth occurred on agar with low MIC multiples of ceftaroline + avibactam, but frequencies for single-step mutants were <10−9. Most mutants were unstable, with only three remaining resistant on subculture. For one, from an CTX-M-15-positive Escherichia coli, the ceftaroline + avibactam MIC was raised, but the organism had reduced resistance to ceftaroline and lost resistance to other oxyimino-cephalosporins, with this profile retained when the mutant blaCTX-M-15 was cloned into E. coli DH5α. Sequencing identified a Lys237Gln substitution in the CTX-M-15 variant. The other two stable single-step mutants were from an AmpC-derepressed Enterobacter cloacae strain; these had unaltered or slightly reduced resistance to other β-lactams. Both had amino acids 213–226 deleted from the Ω loop of AmpC. Further stable mutants were obtained from AmpC-inducible and -derepressed E. cloacae in multi-step selection, and these variously had reduced expression of OmpC and OmpF, and/or Asn366His/Ile substitutions in AmpC.

Conclusions: Stable resistant mutants were difficult to select. Those from AmpC-derepressed E. cloacae had porin loss or AmpC changes, including Ω loop deletions. A Lys237Gln substitution in CTX-M-15 conferred resistance, but largely abolished ESBL activity.
Original languageEnglish
Pages (from-to)1354-1358
Number of pages5
JournalJournal of Antimicrobial Chemotherapy
Issue number6
Publication statusPublished - Jun 2012


  • Anti-Bacterial Agents
  • Azabicyclo Compounds
  • Cephalosporins
  • Drug Resistance, Bacterial
  • Enterobacteriaceae
  • Genomic Instability
  • Microbial Sensitivity Tests
  • Mutant Proteins
  • Phenotype
  • Porins
  • Selection, Genetic
  • beta-Lactamases

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