 CEREMI is an open, randomised prospective study in parallel groups in healthy volunteers. This study compare the effect of a monotherapy by ceftriaxone or cefotaxime on the emergence of resistance of enterobacteria to 3rd-generation cephalosporins within the intestinal microbiota. Each volunteer will be treated for 3 days by either ceftriaxone 1 gram per day or cefotaxime 1 gram every 8 hours. 