Inclusion Criteria:  Patients will be eligible providing that they meet all the following criteria:  having a surgery for a suspicion of post-operative peritonitis; free, informed and written consent of the patient, or his relatives or the trusted person previously designated or failing that, inclusion in an emergency situation  Exclusion Criteria:  Patients will not be eligible if they meet at least one of the following criteria  age under 18 years; current pregnancy; poor chance of survival (SAPS II > 65 points within 12 hours before inclusion); patient for whom a limitation of care has been decided patient included in another therapeutic trial involving antibiotics agents patient who has already participated in this study patients unable to give consent (under guardianship or curatorship) patient not affiliated to social security  Secondary exclusion criteria: to be checked prior to randomization:  not requiring admission in ICU for the management of the perioperative period; surgical samples not collected during reoperation for microbiological purpose; the two surgical samples that cannot be processed at the same time by the microbiology laboratory (the time slots will be defined for each center according to the local organization)