title: AmpC cephalosporinase hyperproduction in Acinetobacter baumannii clinical strains.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Laboratoire de Bactériologie-Virologie, Hygiène hospitalière, CHU, Nantes, France. stephane.corvec@chu-nantes.fr
authors.forename: Stéphane
authors.initials: S
authors.lastname: Corvec
authors.forename: Nathalie
authors.initials: N
authors.lastname: Caroff
authors.forename: Eric
authors.initials: E
authors.lastname: Espaze
authors.forename: Cécile
authors.initials: C
authors.lastname: Giraudeau
authors.forename: Henri
authors.initials: H
authors.lastname: Drugeon
authors.forename: Alain
authors.initials: A
authors.lastname: Reynaud
mesh_terms: D020121:5' Untranslated Regions / Q000302:isolation & purification; D040981:Acinetobacter baumannii / Q000201:enzymology* / Q000235:genetics*; D001483:Base Sequence; D002510:Cephalosporinase / Q000096:biosynthesis* / Q000235:genetics*; D015964:Gene Expression Regulation, Bacterial / Q000235:genetics; D005798:Genes, Bacterial / Q000502:physiology*; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D008969:Molecular Sequence Data
chemical_list: D020121:5' Untranslated Regions; D002510:Cephalosporinase