The use of bacteria from the genus Bacteroides for inducing weight loss in a human. The bacteria can be selected from B. thetaiotaomicron, B. adolescentis, B. fragilis, B. vulgatus, B. distasonis, B. ovatus, B. stercoris, B. merdae, B. uniformis, B. eggerithii and B. caccae. The bacteria can be used as a live, lyophilized culture.