Carbapenem resistance mechanisms in Norwegian clinical isolates of Pseudomonas aeruginosa

Abstract number: 1733_365

Samuelsen Ø., Buarø L., Aasnæs B., Giske C.G., Haldorsen B., Simonsen G.S., Leegaard T.M., Sundsfjord A.

Objectives: The objective of the study was to investigate resistance mechanisms to carbapenems and evaluate phenotypic selection criteria for metallo-b-lactamase (MBL) testing in carbapenem resistant clinical isolates of P. aeruginosa in a low-prevalence country like Norway.

Methods: 31/138 imipenem resistant P. aeruginosa clinical isolates from Norwegian hospitals (year 2004 to 2006) with a positive MBL Etest (IP/IPI ratio of ≥8 and/or ellipse/phantom zone) were retested at the Reference Centre. A broad spectrum b-lactam MIC screen by Etest was also done. Verification of MBL presence was done by PCR (blaIMP and blaVIM) and by spectrophotometric analysis of imipenem hydrolysis by crude cell extracts. As part of an ongoing study a subset of the strains (n = 10) was evaluated for changes in the transcription of oprD and mexB by quantitative RT-PCR.

Results: Of the 31 MBL Etest positive isolates 7 were positive upon retesting. Interestingly, the IP MIC was reproducible while the IPI MIC was consistently increased, often by more than 2-fold dilutions. MBL production was verified in two isolates by PCR and hydrolysis of imipenem, resulting in 29 and 5 false-positive isolates in the initial and retesting respectively. Analysis of the resistance profile of the isolates showed that additional selection criteria for MBL testing such as resistance to meropenem and/or ceftazidime using EUCAST breakpoints would reduce the number of false positives further. qRT-PCR on the subset of the isolates (n = 10) revealed a significant downregulation of oprD (n = 6) and/or upregulation of mexB (n = 4) in the tested isolates.

Conclusions: (i) In this study MBL Etest results were difficult to reproduce and overestimated the presence of MBL in a low prevalence country like Norway. (ii) Additional selection criteria such as meropenem and/or ceftazidime resistance will reduce false-positive results and should be considered before more labour intensive analysis are performed. (iii) Gene transcription analyses indicate that decreased permeability and efflux are more prevalent mechanisms than MBL production in carbapenem resistant Norwegian P. aeruginosa isolates. (iv) False positive test results are probably due to the permeabilising effect of EDTA.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
Back to top