AmpC Cephalosporinase Hyperproduction in Acinetobacter Baumannii Clinical Strains

J Antimicrob Chemother. 2003 Oct;52(4):629-35. doi: 10.1093/jac/dkg407. Epub 2003 Sep 1.


Objective: To compare the genetic environments of ampC genes in different Acinetobacter baumannii isolates showing different levels of beta-lactam resistance.

Methods: The patterns of beta-lactam resistance and beta-lactamase production were investigated for 42 A. baumannii clinical strains. The MICs of various beta-lactams were determined in the presence or absence of the class C cephalosporinase inhibitor, cloxacillin (500 mg/L). The ampC gene and its 5' adjacent sequence were analysed by PCR and DNA sequencing. An RT-PCR method was developed to evaluate ampC transcript levels.

Results: Strains fell into three resistance groups: first, strains with a ceftazidime MIC < or =8 mg/L (20 strains, 47.6%); secondly, strains with a ceftazidime MIC 32 mg/L, which was reduced four-fold in the presence of cloxacillin (eight strains, 19%); and thirdly, strains with a ceftazidime MIC > or =256 mg/L, which did not decrease in the presence of cloxacillin (14 strains, 33.4%). In all of the resistant isolates (groups II and III), but not in any of the ceftazidime-susceptible isolates (group I), a 1180 bp insert showing all the characteristics of an insertion sequence was detected upstream from the ampC gene. Isolates having this insert overexpress ampC, according to RT-PCR experiments.

Conclusion: Presence of an insertion sequence upstream of ampC in A. baumannii clinical isolates, possibly including a strong promoter, has the potential to cause over-expression of AmpC, resulting in high-level ceftazidime resistance.

MeSH terms

  • 5' Untranslated Regions / isolation & purification
  • Acinetobacter baumannii / enzymology*
  • Acinetobacter baumannii / genetics*
  • Base Sequence
  • Cephalosporinase / biosynthesis*
  • Cephalosporinase / genetics*
  • Gene Expression Regulation, Bacterial / genetics
  • Genes, Bacterial / physiology*
  • Humans
  • Microbial Sensitivity Tests / methods
  • Molecular Sequence Data


  • 5' Untranslated Regions
  • Cephalosporinase