title: Prevalence of AmpC over-expression in bloodstream isolates of Pseudomonas aeruginosa.
abstract: This study examined the contribution of AmpC over-expression to beta-lactam resistance in clinical isolates of Pseudomonas aeruginosa obtained from a hospital in Houston, TX, USA. Seventy-six non-repeat bloodstream isolates obtained during 2003 were screened for ceftazidime resistance in the presence and absence of clavulanic acid 4 mg/L. AmpC was identified by isoelectric focusing (with and without cloxacillin inhibition); stable derepression was ascertained phenotypically by a spectrophotometric assay (with and without preceding induction by imipenem) using nitrocefin as the substrate, and was confirmed subsequently by quantitative RT-PCR of the ampC gene. The clonal relatedness of the AmpC-over-expressing isolates was assessed by pulsed-field gel electrophoresis. In addition, the ampC and ampR gene sequences were determined by PCR and sequencing. For comparison, two standard wild-type strains (PAO1 and ATCC 27853) and three multidrug-susceptible isolates were used as controls. AmpC over-expression was confirmed in 14 ceftazidime-resistant isolates (overall prevalence rate, 18.4%), belonging to seven distinct clones. The most prevalent point mutations in ampC were G27D, V205L and G391A. Point mutations in ampR were also detected in eight ceftazidime-resistant isolates. AmpC over-expression appears to be a significant mechanism of beta-lactam resistance in P. aeruginosa. Understanding the prevalence and mechanisms of beta-lactam resistance in P. aeruginosa may guide the choice of empirical therapy for nosocomial infections in hospitals.
authors.affiliation: University of Houston College of Pharmacy, Houston, TX 77030, USA. vtam@uh.edu
authors.forename: V H
authors.initials: VH
authors.lastname: Tam
authors.forename: A N
authors.initials: AN
authors.lastname: Schilling
authors.forename: M T
authors.initials: MT
authors.lastname: LaRocco
authors.forename: L O
authors.initials: LO
authors.lastname: Gentry
authors.forename: K
authors.initials: K
authors.lastname: Lolans
authors.forename: J P
authors.initials: JP
authors.lastname: Quinn
authors.forename: K W
authors.initials: KW
authors.lastname: Garey
mesh_terms: D016470:Bacteremia / Q000382:microbiology*; D001426:Bacterial Proteins / Q000235:genetics*; D016521:Electrophoresis, Gel, Pulsed-Field; D006801:Humans; D007525:Isoelectric Focusing; D017354:Point Mutation; D011550:Pseudomonas aeruginosa / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013053:Spectrophotometry; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000235:genetics*
chemical_list: D001426:Bacterial Proteins; C048795:AmpC beta-lactamases; D001618:beta-Lactamases