Interventional ERCP in patients with cholestasis. Degree of biliary bacterial colonization and antibiotic resistance. Biliary obstruction together with bacterial colonization of the bile duct may lead to development of acute cholangitis. The aim of our prospective study was to investigate the presence and degree of biliary bacterial colonization by means of bile aspiration during ERCP in patients with biliary obstruction. Furthermore, we evaluated antibiotic therapy regimens, which would cover the bacterial species obtained by ERCP and subsequent culture in each patient. In addition, analysis of risk factors was performed that would predispose to the development of cholangitis.80 patients with clinical and laboratory evidence of biliary obstruction underwent ERCP with initial aspiration of bile via the cannulation catheter. This material was used to culture aerobic and anaerobic bacteria and determine the colony count/ml bile, followed by identification of each species and antibiotic resistance testing. The minimal inhibitory concentration for Levofloxacin, Ciprofloxacin, Piperacillin, Ampicillin, Ceftriaxone, Imipenem, Gentamycin und Metronidazole was determined. Immediately after the ERCP or if the body temperature (after ERCP) rose to > 38 degrees C blood cultures were obtained. In 45 (56 %) patients biliary colonization with bacteria could be identified (56 %). In 20 patients a single isolate was cultured, in 25 cases mixed infection was present. A total of 83 species were isolated. The most common bacteria were E. coli, Enterococcus and Klebsiella. 9.6 % of all isolates were obligatory anaerobes. In 9 of 80 patients bloodcultures tested positive for bacterial growth (rate of bacteremia: 11.3 %). 10 patients had acute cholangitis clinically before ERCP, 13 patients developed signs of infection after ERCP. Statistically significant factors contributing to the risk of infection were age of the patient, the clinical condition of the patient before ERCP and the biliary colony count. Patients with development of infection after ERCP showed a significantly higher incidence of bacterial colonization of the biliary tree and a higher colony count. In all bacterial species Imipenem (4.5 %) or Levofloxacin (2.2 %) exhibited the lowest rate of in-vitro resistance. Based on these data, the implementation of Levofloxacin in combination with anaerobic coverage is advantageous as a calculated therapy for patients with acute cholangitis.