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JAC Advance Access published online on October 27, 2004
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkh470
© 2004 by The British Society for Antimicrobial Chemotherapy
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Received June 16, 2004
Revised September 16, 2004
Accepted September 17, 2004

Original article

Pitfalls of polymyxin antimicrobial susceptibility testing of Pseudomonas aeruginosa isolated from cystic fibrosis patients

Michael Hogardt 1*, Sabine Schmoldt 1, Monika Götzfried 1, Kristin Adler 1, and Jürgen Heesemann 1

1 Max von Pettenkofer-Institut für Hygiene und Medizinische Mikrobiologie, National Consulting Laboratory Southern Germany on Cystic Fibrosis Bacteriology, Ludwig-Maximilians University Munich, Munich, Germany

* To whom correspondence should be addressed.
Michael Hogardt, E-mail: hogardt{at}m3401.mpk.med.uni-muenchen.de


  Abstract

Objectives and methods: With their potent activity against Gram-negative bacteria, the polymyxins are important alternative antibiotics for cystic fibrosis (CF) patients. A retrospective evaluation of polymyxin activity against 6001 Pseudomonas aeruginosa, 150 Achromobacter xylosoxidans and 506 Stenotrophomonas maltophilia CF isolates was initiated. In addition, we looked at how polymyxin susceptibility testing was affected by the testing method (agar dilution versus microdilution), the agent (polymyxin E versus polymyxin B), incubation time (24 h versus 48 h) and by different interpretative criteria (German DIN, French FSM, British BSAC).

Results: Polymyxin B exhibited reasonable activity against P. aeruginosa (MIC90≤2 mg/L), whereas it was less active against A. xylosoxidans (MIC90≤16 mg/L) and S. maltophilia (MIC90≤16 mg/L). During 2000-2002, polymyxin B resistance in P. aeruginosa, S. maltophilia and A. xylosoxidans was found to be 6.7%, 17.0% and 29.9% (corresponding to 12.4%, 20.7% and 35.4% of infected patients), respectively. When the agar dilution method was used, polymyxin E exhibited higher MICs than polymyxin B. The microdilution method produced lower polymyxin MICs than the agar dilution method. Therefore, the microdilution MICs after prolonged incubation (48 h) and the agar dilution MICs of polymyxin B correlated best (AUC of 0.93, r2 of 0.44 and s of 0.83).

Conclusions: Polymyxin resistance among common CF pathogens is not rare, thus underlining the necessity of accurate susceptibility testing. When compared with the agar dilution method, it was found that the microdilution method is a valid, rapid and cost effective alternative for the determination of polymyxin activity. The performance of the microdilution method was most reliable after prolonged incubation (48 h) at a susceptibility breakpoint of ≤4 mg/L according to the BSAC guidelines (specificity 91%, sensitivity 89%, 1.5% very major errors).

Keywords: polymyxin resistance; susceptibility testing methodology; interpretative criteria.
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