<qa>  What is the drug plasma concentration determined at steady-state?Bacterial analysis of the fecal samples will be performed blindly from the treatment group. Drug plasma concentration will be determined at steady-state T0, T0.5h, T1h, T2h, T4, T6h for cefotaxime and T0, T0.5h, T1h, T2h, T4, T8h for ceftriaxone. Drug concentration in the feces will be measured on the samples obtained between D0 and D7. 