Surface properties and ultrastructure of Porphyromonas gingivalis W50 and pleiotropic mutants

Cell surface ultrastructure and other surface properties of Porphyromonas gingivalis strain W50 and pleiotropic mutants W50/BP1 (brown), and W50/BE1 (beige) were studied. The percentage hydrophobicity of strains W50, W50/BP1, W50/BR1, and W50/BE1 gradually decreased from 24 to 9. Ruthenium red stained cells studied by transmission electron microscopy revealed a layer of extracellular polymeric material of varying thickness depending on the strain. The layer was thickest in W50/BP1 (15-20 nm), strains W50 and W50/BR1 both had a layer of 12-15 nm, while strain W50/BE1 completely lacked this layer. The results clearly showed that the hydrophobicity of P. gingivalis was related not only to the thickness of the layer but also to other factors like the composition of the capsular material, such that only strain W50/BE1, for example, showed no haemagglutinating activity. The surface properties of the pleiotropic mutants appeared to be stable characteristics as cells grown on either solid or in liquid media gave comparable results. The loss of virulence of the beige strain (W50/BE1) is probably partly due to the alteration of these surface properties. Both virulent and avirulent strains, however, possessed extracellular vesicles.