BACKGROUND AND OBJECTIVE:Porphyromonas gingivalis, an anaerobic bacterium associated with adult periodontal disease, employs a number of pathogenic mechanisms, including protease/adhesin complexes (gingipains), fimbriae and hemagglutinins, to maintain attachment within colonized hosts. Here we examined the binding of gingipains and whole, live P. gingivalis cells to immobilized extracellular matrix proteins in the presence of soluble forms of the same proteins, to investigate whether this may constitute a colonization mechanism in the oral environment. MATERIAL AND METHODS:Binding of purified gingipain molecules and whole bacterial cells to immobilized matrix proteins was examined in the presence and absence of soluble competitors using enzyme-linked immunosorbent assays. RESULTS:Purified gingipains or whole, live bacteria preferentially bound immobilized forms of matrix proteins, even in the presence of soluble forms of the same proteins. Fimbriae appeared to be redundant for adhesion to immobilized proteins in the presence of the gingipains, indicating that the protease/adhesins and hemagglutinins may be more important for adhesion under these conditions. CONCLUSION:The data presented here provide evidence for a model of adhesion for P. gingivalis within the fluid environment of the oral cavity, where preferential binding of matrix-located proteins over soluble forms facilitates colonization of the host.