Bacterial viability was determined following epithelial cell challenges. From each challenge assay reported in Fig. 4, supernatant containing bacteria was removed at 4, 8, 12, and 24 hours, plated in blood agar plates and colony forming units were counted. P. gingivalis strains cultured were 33277, KDP128, W50, E8 and K1A. For all strains > 80% viability persisted until 8 h, where upon viability decreased to approximately 30% at 12 h and 1–2% at 24 h. Values represent the means ± SD of at least two experiments.