Abstract
* Corresponding author. Mailing address: Laboratoire de Microbiologic ORSTOM, Université de Provence, 3 Place Victor Hugo, 13331 Marseille Cedex 3, France. Phone: (33) 91.10.64.80. Fax: (33) 91.10.64.81. Electronic mail address: rogerpa{at}loma.orstom.fr .