Variable Susceptibility to piperacillin/tazobactam Amongst Klebsiella Spp. With Extended-Spectrum Beta-Lactamases

J Antimicrob Chemother. 2003 Mar;51(3):605-12. doi: 10.1093/jac/dkg114.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Outer Membrane Proteins / genetics
  • Bacterial Outer Membrane Proteins / metabolism
  • Drug Resistance, Multiple, Bacterial / physiology*
  • Humans
  • Klebsiella / drug effects*
  • Klebsiella / enzymology
  • Klebsiella / genetics
  • Microbial Sensitivity Tests / statistics & numerical data
  • Penicillanic Acid / analogs & derivatives
  • Penicillanic Acid / pharmacology*
  • Piperacillin / pharmacology*
  • Piperacillin, Tazobactam Drug Combination
  • beta-Lactamases / genetics
  • beta-Lactamases / metabolism*


  • Bacterial Outer Membrane Proteins
  • Piperacillin, Tazobactam Drug Combination
  • Penicillanic Acid
  • beta-Lactamases
  • Piperacillin