1. Laboratory for Pharmaceutical Technology and Dispensing, University of Groningen, A. Deusinglaan 2, 9713 AW, Groningen, The Netherlands
2. Intensive Care Unit, O-L-V Gasthuis, 1091 HA, Amsterdam, The Netherlands
3. Department of Medical Microbiology, University of Liverpool, PO Box 147, Liverpool, L69 3BX, UK