Appropriate patients to be enrolled in the study are all patients with following diagnosis operated at the site: Acute appendicitis (classical or laparoscopy appendectomy), ileus of the small bowel (adhesiolysis, enterotomy with suction or resection of small bowel), perforated stomach or duodenal ulcer (classical excision of the ulcer and suture of the GIT wall), i.e. patients without stercoral peritonitis. Patient will be definitely included if they signed the preoperative informed consent form which they were able to understand well and whose diagnosis was confirmed during surgery. Patients will be excluded if they are younger than 18 years of age, pregnant females, and patients with known allergy to administered ATB and also if their mental state does not allow appropriate instruction and signing of the informed consent and further patients who used ATB in the course of last 2 weeks.