Infection of HGF cells by P. gingivalis. Wild type (WT) or triple gingipain mutant (MT) P. gingivalis was added to monolayers of HGF cells at the indicated bacteria-to-cell ratios (m.o.i). Only a few representative multiplicities are shown. Invasion was determined after killing of the external bacteria with antibiotics and staining the cells with DAPI (Blue) and P. gingivalis Fim A antibody (Pg; Red) as described under Experimental Procedures. Visually detectable red cells in the 'Pg' fields were counted in three independent observations and expressed as percentage of all the blue cells in the 'DAPI' fields.