Abstract: The present invention relates to a composition comprising a therapeutically effective amount of Bacteroides thetaiotaomicron or an extract thereof for use in the prevention or treatment of an infection in a subject who is immunodepressed, who is immunosuppressed, who has an immature immune system, and/or who has a weakness, and in which the composition comprises at most six species of bacteria and/or extracts thereof. The infections concerned are mainly intestinal infections, in particular those which affect non-human mammals and/or humans.