First Author:   Giscard Assoumou-Ella LABAT, Omar Bongo University g.assoumouella@gmail.com   Libreville Libreville, Gabon   077991460(Phone) 077991460(Fax) Second Author: Streley Paul N'nang-Ndong LABAT, Omar Bongo University versenepaul47@gmail.com   Libreville, Gabon