Porphyromonas gingivalis can invade and survive within epithelial cells which may contribute to the persistence of this organism at sub-gingival sites. A number of antimicrobial strategies including photodynamic therapy (PDT) have been proposed in periodontal disease but the effect this has on organisms within epithelial cells is unknown.

Objectives: To compare the effectiveness of PDT on planktonic cultures of P. gingivalis with that of the same treatment on intracellular organisms.

Method: For studies of planktonic bacteria, Suspensions of P. gingivalis NCTC11834 were subjected to PDT by incubating with erythrosine (22µg/ml for 15 min) followed by irradiation with white light for 15 min. Appropriate controls were included. Suspensions were then serially diluted and viable counting performed on blood agar.

For intracellular bacteria, P. gingivalis (10⁷ cells/ml) were allowed to invade H357 epithelial cells for 90 mins followed by treatment with metronidazole to kill extracellular bacteria. Cells were then exposed to light with and without sensitiser before being lysed in water and viable counts performed as above.

Results: PDT of planktonic cultures of P. gingivalis resulted in complete eradication of bacteria (no viable organisms recovered). Controls showed that erythrosine alone had no effect on bacterial viability (3.66 +/-0.26 x10⁶ cfu/ml) when compared to controls (3.47 +/- 0.26). Light alone produced a slight reduction (2.80 +/- 0.67)

PDT of intracellular organisms resulted in a 10 fold reduction in counts (2.15 +/- 0.06 x10⁵ cfu/ml vs. 2.66 +/- 0.03 x10⁴) relative to controls.

Conclusion: Erythrosine and visible light based PDT has a dramatic effect on the viability of P. Gingivalis grown in suspension. To a lesser extent, the treatment is also effective at reducing the numbers of intracellular bacteria. Such information, may be crucial to the design of therapeutic interventions to treat periodontal disease.

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