To study the effect of resin infiltration to the surface microhardness (SMH) and Streptococcus mutans (S.mutans) adhesion of artificial caries models.

After extraction for orthodontic treatment, caries-free human premolars were cut into 17 enamel blocks. Five enamel blocks were used to study the effect of resin infiltration on SMH. Microhardness of each enamel block was detected before and after demineralization and after resin infiltration. The remaining 12 enamel blocks were demineralized and used to study the effect of resin infiltration on S.mutans adhesion. Six samples treated with resin infiltration were detected S.mutans biofilm at 4 h and 24 h, and other six samples wer referred to the blank control group. Microhardness of enamel surfaces were detected by Vicker′s microhardness tester. Biomass, thickness and percentage of live bacteria of S.mutans biofilm were detected by confocal laser scanning microscope (CLSM) . The values were analyzed by paired t test and independent t test.

The mean microhardness value of enamel surfaces decreases from (316.07 ± 13.54) HV to (22.44 ± 1.73) HV after 24 hours′ demineralization, and increases to (139.45 ± 21.46) HV after resin infiltration significantly (Before demineralization-After demineralization t= 55.879, P<0.001; After demineralization-After treatment t=-14.400, P<0.001; Before demineralization-After treatment t= 14.090, P<0.001) . CLSM images showed reduced biofilm formation on resin-infiltrated specimens[ (0.59 ± 0.24) , (9.53 ± 1.49) μm³/μm²] compared to blank controls [ (1.01 ± 0.30) , (15.47 ± 7.32) μm³/μm²] both at 4 h and 24 h (t_{4 h}= 3.232, P_{4 h}= 0.005; t_{24 h}= 2.384, P_{24 h}= 0.042) , while the percentage of live bacteria in the biofilm of resin-infiltrated specimens [ (48.73 ± 8.54) %, (60.49 ± 5.33) %] was higher than that of the control [ (31.84 ± 7.30) %, (34.87 ± 10.72) %] (t_{4 h} = -4.508, P_{4 h}<0.001; t_{24 h} = -6.419, P_{24 h}<0.001) ; the thickness of biofilm of resin-infiltrated specimens at 24 h [ (6.44 ± 1.51) μm] was higher than that of the control [ (12.78 ± 7.17) μm] (t = 2.592, P = 0.030) .

Resin infiltration increased microhardness value of demineralized enamel lesions, which showed excellent mechanical characters, and resin infiltration can inhibit S.mutans biofilm formation.