1. Department of Microbiology, E.C. Slater Institute, BioCentrum, University of Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands, Department of Biochemistry, Physiology and Microbiology, State University of Ghent, Ledeganckstraat 35, 9000 Ghent, Belgium, and Institute of Mass Spectrometry, University of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands