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J Med Microbiol 56 (2007), 66-70; DOI: 10.1099/jmm.0.46816-0
© 2007 Society for General Microbiology
ISSN 1473-5644

Characterization of clinical isolates of Pseudomonas aeruginosa heterogeneously resistant to carbapenems

Spyros Pournaras1, Alexandros Ikonomidis1, Antonios Markogiannakis2, Nicholas Spanakis2, Antonios N. Maniatis1 and Athanassios Tsakris2

1 Department of Medical Microbiology, University of Thessalia, Mezourlo, Larissa, Greece

2 Department of Microbiology, Medical School, University of Athens, Athens, Greece

Correspondence
Athanassios Tsakris
atsakris{at}med.uoa.gr

Received 3 July 2006
Accepted 4 September 2006

Fourteen apparently carbapenem-susceptible Pseudomonas aeruginosa clinical isolates that exhibited colonies within the inhibition zone around carbapenem discs were analysed. MICs of carbapenems were determined and the isolates were genotyped by PFGE. Population analysis, one-step selection of carbapenem-resistant mutants and growth curves of progenitors and carbapenem-resistant subpopulations were performed. Agar dilution MICs of imipenem and meropenem ranged from 0.5 to 4 mg l–1 and from 0.25 to 2 mg l–1, respectively. Population analysis confirmed subpopulations that grew in concentrations of up to 18 mg l–1 and 12 mg l–1 of imipenem and meropenem, respectively, at frequencies ranging from 6.9x10–5 to 1.1x10–7, suggesting that they might not be detected by standard agar dilution MIC testing. The minority subpopulations exhibited MICs for imipenem ranging from 10 to 20 mg l–1 and for meropenem from 4 to 14 mg l–1. The one-step 8 mg l–1 selection of imipenem-resistant mutants test showed growth in all isolates at frequencies ranging from 3.8x10–4 to 5.1x10–7. Growth curves revealed a prolonged lag phase and a short exponential phase for the heterogeneous subpopulations compared with their respective native subpopulations. These findings may be indicative that the use of carbapenems can lead to selection of P. aeruginosa resistant subpopulations that subsequently cause infections and result in treatment failure.

Abbreviations: MBL, metallo-ß-lactamase.




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