This study was investigated to observe the effect of Treponema denticola (TDC) and Treponema lecithinolyticum (TLC) on cultured human periodontal ligament cells. Several experiments were performed including MTT test for the inhibition effect of cell proliferation, LDH test for the cytotoxicity , gelatin zymography for the gelatinase activation and observation of cell morphology change using the phase-contrast microscopy. The results were as follows.

1. The effect of concentration on cell proliferation with time showed an inhibitory effect at high concentration(150µg/well) for TLC and at low concentration(9.4µg/well) for TDC.

2. The effect of time on cell proliferation with concentration showed an inhibitory effect at 150µg/well on 2-day incubation for TLC and at 9.4µg/well on 2-day incubation for TDC.

3. The effect of heat-treated TDC and TLC on the inhibition of cell proliferation showed the difference in the heat-treated group compared to the non-heat treated group for TDC, whereas no difference was found for TLC.

4. The morphological changes which were observed from the phase-contrast microscopy showed the difference in the test group compared to the control group. The loss of spindle-like appearance, cell-to-cell detachment and inhibition of cell proliferation were observed.

5. There was no difference of the cytotoxicity effect between the test group and the control group in the LDH test.

6. The active form of progelatinase A with molecular weight 72kDa was activated in both TDC and TLC on the gelatin zymography.

Regarding to the above results, TDC and TLC have an effect on periodontal ligament cells by playing an inhibitory role in cell proliferation and appears to activate progelatinase A which degrades type IV collagen.