Inhibitory Effects of the Heat-Killed Lactic Acid Bacterium on the Growth of .

BACKGROUND

, a gram-negative obligate anaerobic bacterium, is a major pathogen involved in the onset and progression of periodontal disease, a chronic inflammatory disorder observed in approximately two-thirds of the Japanese population older than age 30 years. cells produce and secrete gingipain, a powerful proteolytic enzyme, on their surfaces and in external environments.

OBJECTIVES

The effects of heat-killed (Hk), a lactic acid bacterium, on the growth of were evaluated in vitro by measuring the viable cell count of and gingipain activity.

METHODS

Hk solution (1.63 or 163 mg/mL) was added to 1 mL culture to generate a final Hk concentration of 0.64 or 64 mg/mL. The cultures were incubated anaerobically. The number of viable cells and gingipain activity were measured after incubation for 0, 12, 24, 48, and 72 hours. The number of viable cells was calculated by counting the number of colonies after culture. Gingipain activity was quantified by adding a chromogenic substrate to culture medium and measuring the absorbance of the reaction solution with a plate reader. Mean (SE) was calculated for viable cell counts and gingipain activity, and Wilcoxon rank-sum test was used to test for significant differences.

RESULTS

The counts of viable cells in the control group increased as incubation time progressed for 12, 24, 48, and 72 hours; similar results were observed in the low-concentration Hk group. In the high-concentration Hk group, the increase in the viable cell count was significantly inhibited compared with that of the control group. Furthermore, gingipain activity in the low- and high-concentration Hk groups was significantly inhibited over time compared with that of the control group. Although the pH of the culture solution tended to decrease in the high-concentration Hk group, it was not considered to have affected the growth of .

CONCLUSIONS

Hk exhibits inhibitory effects on the growth of and gingipain activity.