The development of surface-attached biofilm bacterial communities is considered an important source of nosocomial infections. Recently, bacterial interference via signaling molecules and surface active compounds was shown to antagonize biofilm formation, suggesting that nonantibiotic molecules produced during competitive interactions between bacteria could be used for biofilm reduction. Hence, a better understanding of commensal/pathogen interactions within bacterial community could lead to an improved control of exogenous pathogens. To reveal adhesion or growth-related bacterial interference, we investigated interactions between uropathogenic and commensal Escherichia coli in mixed in vitro biofilms. We demonstrate here that the uropathogenic strain CFT073 and all E. coli expressing group II capsules release into their environment a soluble polysaccharide that induces physicochemical surface alterations, which prevent biofilm formation by a wide range of Gram-positive and Gram-negative bacteria. We show that the treatment of abiotic surfaces with group II capsular polysaccharides drastically reduces both initial adhesion and biofilm development by important nosocomial pathogens. These findings identify capsular polymers as antiadhesion bacterial interference molecules, which may prove to be of significance in the design of new strategies to limit biofilm formation on medical in dwelling devices.