1. Hospital for Sick Children, Program in Cell and Lung Biology, Department of Biochemistry and Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, M5G 1X8 Canada.
2. INRA, Unité des BactÉries Lactiques et Pathogénes Opportunistes, Domaine de Vilvert, 78352 Jouy-en-Josas, France.