Cranberry juice has long been believed to benefit the prevention and treatment of urinary tract infections (UTIs). As the first step in the development of infection, bacterial adhesion is of great research interest, yet few studies have addressed molecular level adhesion in this context. P‐fimbriated Escherichia coli play a major role in the development of a serious type of UTI, acute pyelonephritis. Experiments were conducted to investigate the molecular‐scale effects of cranberry juice on two E. coli strains: HB101, which has no fimbriae, and the mutant HB101pDC1 which expresses P‐fimbriae. Atomic force microscopy (AFM) was used to investigate both bacterial surface characteristics and adhesion forces between a probe surface (silicon nitride) and the bacteria, providing a direct evaluation of bacterial adhesion and interaction forces. Cranberry juice affected bacterial surface polymer and adhesion behavior after a short exposure period (<3 h). Cranberry juice affected the P‐fimbriated bacteria by decreasing the adhesion forces between the bacterium and tip and by altering the conformation of the surface macromolecules on E. coli HB101pDC1. The equilibrium length of polymer (P‐fimbriae) on this bacterium decreased from ∼148 to ∼48 nm upon being exposed to cranberry juice. Highly acidic conditions were not necessary for the prevention of bacterial adhesion, since neutralization of cranberry juice solutions to pH = 7.0 allowed us to observe differences in adhesion between the E. coli strains. Our results demonstrate molecular‐level changes in the surfaces of P‐fimbriated E. coli upon exposure to neutralized cranberry juice. © 2005 Wiley Periodicals, Inc.