<qa>  What is the effect of monotherapy on the emergence of resistance to 3rd generation ceCEREMI 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. 