Antibacterial activities of the semiconductor TiO₂ particles have been demonstrated in recent years [1]. The purpose of this study was to examine the antibacterial effect of TiO₂ microparticles on Streptococcus mutans (S.mutans).

We used TiO₂ 100-nm- ST41TiO₂ microparticles in the present study (specific gravity of 3.8, Ishihara Sangyo, Osaka, Japan). Three strains of S.mutans (NCTC 10449, GS-5 and Ingbritt) were pre-cultivated in trypticase soy broth (TSB:BBL, Becton Dickinson and CO., Cockeysville, MD,U.S.A) at 37C, for 24h. Following pre-cultivation, the culture was diluted with 0.9% NaCl, and 1x10⁵ cells/ml were mixed with different amounts of TiO₂ microparticles (0.1, 0.5, 1.0, 3.0 and 5%W/V). The samples were subsequently cultivated under 485lm light-emitting diode (LED) in an aerobic growth chamber (Biotron®: Japan Medical and Chemical Co. Ltd., Japan), at 37°, 55% relative humidity, while being stirred (Type S-200: Taiyo Scientific Ind. Co., Japan). The light exposure time was 2 h. Samples of the treated suspension were incubated on TS agar plates for 48 h at 37°, 55% relative humidity. After 48 h of cultivation at 37C , Hitach-High- Tech Co.Ltd., Japan, the viable cells were counted voluntarily (at random). Following the reported method [2], we made same samples (5 x 5 mm²) and set in a mouthpiece. To evaluate the inhibitory effect, the samples were washed in saline. After these methods and drying, these samples fixed using 2.5% glutaraldehyde-1.0% osmium tetroxide, dehydrated, freeze dried, and coated with Pt-Pd ; it was then observed by S-3500 scanning electron microscope (SEM : Hitachi-High-Tech Corporation, Tokyo Japan). Statistical analysis of the packing density (the percentage occupying the plate) was conducted using the signed-rank test and the rank-sum test.

The ratio of viable cells to the total cell population (reduction ratio) of S.mutans were decreased in three strains in solution of the containing the potocatalyticTiO₂ microparticles. Reduction ratio of NCTC 10449 was 10%, that of GS-5 was 4% and that of Ingbritt was 0%. These results show that the effect of photocatalytic TiO₂ microparticles varied depending on the bacterial strains (p < 0.01; Table 1).

Table 1: Reduction ratio of viable cells of three Streptococcus mutans strains according to different TiO₂ microparticle concentrations.

The present study objectively confirmed the antibacterial effect of photocatalytic TiO₂ microparticles on three different strain of S.mutans based on the caluculation of viable cell count and packing densityes on TSA plates, and as indicated by SEM.

1. Nagame S, Oku T, Kambara M, Konishi K. Antibacterial effects of the powdere semiconductor TiO₂ on the viability of oral microorganisms. J Dent Res. 1989: 68: 1696-1697.
2. Nagame S, Arita S. Antibacterial effect of synthetic dental resin containing TiO₂ microparticles. J Osaka Dent Univ. 2023; 57: 83-88.