The bacterial strain used in this study was P. gingivalis ATCC 33277. P. gingivalis was grown in brain heart infusion broth (BHI broth; Difco Laboratories, Detroit, MI, USA) supplemented with 5 mg/ml yeast extract, 5 μg/ml hemin and 1 μg/ml vitamin K1 (BHIY-HK broth) for 18 h. P. gingivalis was also grown on BHI blood agar medium containing defibrinated 5% sheep blood at 37 °C for 5 days under anaerobic conditions (85% N2, 10% H2, 5% CO2). P. gingivalis was washed twice with PBS before each experiment to avoid the influence of medium pigments.