Hemolytic property is a specific feature of bacteria to obtain iron which is essential for its survival in host tissues. Therefore, it is thought to be one of several factors of virulence. The purpose of this study was to investigate the hemolytic properties of Prevotella nigrescens isolated from the teeth diagnosed as pulp necrosis and apical periodontitis under the presence of hemolysin inhibitors such as NaN₃ and dithiothreitol, heat, various pH and cultural conditions.

The results were as follows;

1. Clinically isolated P. nigrescens strains and standard P. nigrscens ATCC 33563 showed hemolytic activity.

2. P. nigrescens showed higher hemolytic activity against human erythrocytes than sheep or horse erythrocytes.

3. NaN₃ and dithiothreitol (DTT) reduced the hemolytic activity of P. nigrescens in a dose dependent manner (p < 0.05).

4. Optimal pH for the maximum hemolytic activity of P. nigrescens was 4.0 and the hemolysin was stable under the 50℃, but the hemolytic activity was significantly decreased at 95℃.

5. P. nigrescens cultured in 10% CO₂ condition showed higher hemolytic activity than the bacteria cultured in the anaerobic condition.

In order to examine the immunoresponse of host cells to Enterococcus faecalis, this in vitro study monitored the production of Interleukin-2 (IL-2), Interleukin-4 (IL-4) and Transforming growth factor-β1 (TGF-β1) in human lymphocytes. Lymphocytes were activated with PHA in the presence or abscence of sonicated extracts of E. Faecalis (SEF) and further incubated for 72 hours. The level of each cytokine was measured by ELISA. Data were analyzed with Kruskal-Wallis test and Mann-Whitney U test (P < 0.05). PHA-activated group did exhibit higher level of IL-2 and IL-4 than untreated control group. The levels of expression of both cytokines were significantly decreased following the treatment of high (25 µg/ml) and medium concentration (12.5 µg/ml) of SEF (P <0 .05) than those of PHA activated group. But low concentration (5 µg/ml) of SEF showed the similar level of IL-2 and IL-4 production as those of PHA activated group. TGF-β1 was unaffected by SEF treatment. These results suggested that E. faecalis may suppress IL-2 and IL-4 production by lymphocytes and this could be one of possible factors why E. faecalis are found frequently in the teeth with failed endodontic treatment.