Pseudomonas aeruginosa undergoes phase variation in the expression of the phosphorylcholine (ChoP) epitope, a structure crucial for the virulence of several respiratory pathogens. In this study, ChoP expression analysis comparing organisms from acute and chronic infections revealed that expression of ChoP at 37 degrees C was higher among strains from chronic infections. Coimmunoprecipitation experiments and mass spectrometry analysis demonstrated that ChoP was on the protein elongation factor Tu. The presence of ChoP at the surface was confirmed by immunofluorescence and flow cytometry analysis of intact bacteria. Pretreatment of bronchial epithelial cells or mice with a platelet-activating factor receptor (PAFR) antagonist reduced adhesion and invasion of the ChoP-positive P. aeruginosa isolates. Results of this study suggest that ChoP expression may represent a novel phenotype expressed by the chronic infection isolates that could mediate P. aeruginosa colonization of the epithelial airway by means of the interaction with the PAFR.