Background: Biliary sludge which forms as a result of bacterial adherence and biofilm formation in the biliary system is a recognized cause of blockage of plastic stents. Bacteriological cultures of sludge have revealed a mixed infection with gram-positive and gram-negative bacteria. Animal studies have shown that prophylactic ciprofloxacin, which selectively suppress gram-negative bacteria, results in prolonged stent patency despite colonization of the stents by gram-positive bacteria. Methods: We tested a possible synergistic effect between gram-negative and gram-positive bacteria in adherence and biofilm formation on plastic stents. Clinical isolates of Escherichia coli and Enterococcus were cultured in separate chemostats to achieve a steady growth. Adherence of the two bacteria on plastic stent surface were tested separately by perfusing infected bile with the respective bacteria through different modified Robbins devices containing 10F polyethylene stent pieces up to 4 days. In a second experiment, Enterococcus was perfused through stent pieces precolonized with E. coli for 24 hours. The stent pieces were then removed daily and analyzed by bacteriologic culture and scanning electron microscopy for bacterial adherence and biofilm formation. Results: Gram-negative E. coli were more adherent than gram-positive Enterococcus. Precolonization with E. coli facilitates subsequent attachment of Enterococcus. Conclusions: We concluded that there is a synergistic effect between gram-positive and gram-negative bacteria in adherence and biofilm formation. (Gastrointest Endosc 1998;48:250-7.)