Antimicrobial photodynamic therapy effectively reduces Porphyromonas gingivalis infection in gingival fibroblasts and keratinocytes: An in vitro study

Background: Porphyromonas gingivalis possess the ability to invade host cells which prevents this pathogen from eradication by conventional periodontal therapy. Recently, antimicrobial photodynamic therapy (aPDT) was introduced to periodontal treatment as a complementary antibacterial method. The aim of this study was to evaluate the effect of toluidine blue-O (TBO) mediated aPDT on the viability of P. gingivalis invading gingival fibroblasts and keratinocytes in an in vitro model of infection. Methods: Primary human gingival fibroblasts (PHGF) and telomerase immortalized gingival keratinocytes (TIGK) were infected with Pg ATCC 33277. Two concentrations of TBO (0.01 mg/mL, TBO-c1 and 0.001 mg/mL, TBO-c2) and a non-laser red light source (λ = 630 nm) were applied to treat both cell-adherent/intracellular Pg (the adhesion/invasion model) or exclusively the intracellular bacteria (the intracellular infection model). Results: The median viability of cell-adherent/intracellular Pg in infected keratinocytes declined from 1.88 × 105 cfu/mL in infected cells treated with TBO without irradiation to 40 cfu/mL upon irradiation for 10 s with TBO-c1. At higher light doses a complete photokilling of P. gingivalis was observed. Pg from exclusively intracellular infection model was also efficiently eradicated as the residual viability dropped from 1.44 × 105 cfu/mL in control samples to 160, 20 and 10 cfu/mL upon irradiation for 10, 20 and 30 s, respectively. In the infected fibroblasts irradiation significantly reduced bacterial viability but did not completely eradicate the intracellular pathogen. Conclusions: Antimicrobial PDT is effective in reducing the viability of intracellular periopathogens, however those residing within gingival fibroblasts seems to attenuate the photokilling effectiveness of this method.