Aim of the project The aim of this prospective randomized placebo controlled study is to prove the positive effect of antibiotic prophylaxis on the incidence of SSI in patients with comparable risk of developing other nosocomial infections in case of acute abdominal procedures for diagnosis such as appendicitis, ileus of small bowel and stomach or small bowel perforation and thus with exclusion of tertiary peritonitis. Secondary aim is to identify the group of patients in which risk of developing nosocomial infection exceeds benefit of ATB prophylaxis. Tertiary aim is to assess whether preoperative microbial screening has its significance for reduction of HAI. After comparing the financial cost of therapy (including SSI and HAI generally) it will be possible to submit background for proposal of recommended procedure for administration of antibiotic prophylaxis in the most common acute surgical procedures with regard to risk factors for genesis of nosocomial infections. Such prospective randomized placebo controlled study has not been conducted in the Czech Republic; further study with similar target group of patients (very common in surgical praxis) has not been published in world literature.