1. Laboratoire de Microbiologie Alimentaire (EA 3213), IBFA, Université de Caen-Basse Normandie, Esplanade de la Paix, 14032 Caen Cedex, France.
2. Present address: Emmanuel Denou, Nestlé Research Center, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland.