AmpC cephalosporinase hyperproduction in Acinetobacter baumannii clinical strains.
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.
Corvec
Stéphane
S
Laboratoire de Bactériologie-Virologie, Hygiène hospitalière, CHU, Nantes, France. stephane.corvec@chu-nantes.fr
Caroff
Nathalie
N
Espaze
Eric
E
Giraudeau
Cécile
C
Drugeon
Henri
H
Reynaud
Alain
A
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
D020121:5' Untranslated Regions; D002510:Cephalosporinase