Abstract: Compounds isolated from plant materials, particularly plants of the genus Vaccinium, which have biological activity measurable as inhibition with adhesion of bacterial cells to surfaces, and an extract of such plant materials which is significantly enriched for the anti-adhesion activity. The specific compounds include procyanidins (also known as “condensed tannins”), leukocyanin, leucodelphinin, flavonol glucosides including myricetin-3-pyranoside and proanthocyanidin extracts. These proanthocyanidin extracts are capable of inhibiting agglutination reactions of P-type E. coli. The extracts containing proanthocyanidins contain at least one A-type interflavanoid bond. Methods of making an extract.