Variable susceptibility to piperacillin/tazobactam amongst Klebsiella spp. with extended-spectrum beta-lactamases

Gioia S. Babini, Meifang Yuan, Lucinda M. C. Hall, David M. Livermore

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MICs of piperacillin/tazobactam are conventionally determined by varying the concentration of piperacillin in the presence of a fixed 4 mg/L tazobactam. When tested in this way, the MIC distribution for Klebsiella isolates with extended-spectrum beta-lactamases (ESBLs) is strongly bimodal, such that many producers are inhibited at 16 + 4 mg/L whilst others require MICs of > or =512 + 4 mg/L. When, however, piperacillin/tazobactam was tested as a fixed 8:1 ratio, the MIC distribution became unimodal. If clavulanate 4 mg/L was combined with piperacillin, a unimodal MIC distribution was seen for ESBL-producing Klebsiella spp. but a bimodal distribution arose if the clavulanate concentration was reduced to 0.25 mg/L. These data for alternative combinations suggested that the bimodal MIC distribution seen for piperacillin + tazobactam 4 mg/L was a titration effect, not a reflection of some ESBLs being resistant to tazobactam. Even within single strains, as defined by serotype and DNA fingerprints, there was considerable variation in susceptibility to piperacillin + tazobactam 4 mg/L, with some representatives highly susceptible and others highly resistant. Some of the more resistant representatives produced more of their ESBL, or had a greater number of beta-lactamase types, but these associations were not universal. Elevated resistance to piperacillin + tazobactam was not associated with porin change in any ESBL producer examined, but has been found by others.
Original languageEnglish
Pages (from-to)605-612
Number of pages8
JournalJournal of Antimicrobial Chemotherapy
Issue number3
Publication statusPublished - Mar 2003


  • Bacterial Outer Membrane Proteins
  • Drug Resistance, Multiple, Bacterial
  • Humans
  • Klebsiella
  • Microbial Sensitivity Tests
  • Penicillanic Acid
  • Piperacillin
  • beta-Lactamases

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