<qa>  What is the first antibiotic to be administered?Open, randomised prospective study in parallel groups in healthy volunteers. Each volunteer will be treated for 3 days by either ceftriaxone 1 gram per day or cefotaxime 1 gram every 8 hours. Fecal samples will be collected before 3 samples, D-14, D-7 and D-1 and after the first administration of the antibiotic D1, D2, D3, D4, D7, D10, D15, D30, D90, D180. 