〔Abstract〕 Objective To study the difference between the high temperature requirement serine proteinase A(HtrA) gene-deficient Streptococcus strains and high-virulent strains of Streptococcus mutans(S.mutans) in the formation of biofilm in vitro on non-stressful environment.Methods Taking the high virulence strain of HtrA screened from high caries-sensitive deciduous teeth obtained in the early stage and the high cariogenic HtrA gene defectivestrain and the international standard strain UA159 as the research objects, and the growth curve of them was evaluated by spectrophotometer turbidimetry. The biofilm model of the three strains was constructed in vitro,the morphology and structure of the biofilm were observed under scanning electron microscope( SEM),and the amount of biofilm formation was evaluated by crystal purple staining. Results Compared with HtrA gene-deficient strains,the bacterial concentration( OD600 value) of HtrA high virulent strains and UA159 standard strains was higher than that of HtrA gene deficient strain in each stage of S. mutans proliferation in the three groups,and the differences were statistically significant( P<0. 05). Under scanning electron microscope,the density of HtrA high virulence strain was the highest,followed by UA159 standard strain,and the density of HtrA deficient strain was the lowest.Semi-quantitative determination of biofilm crystal purple for 4,8,12,24 h,there was significant difference in OD600 value between HtrA high virulence strain and HtrA gene deficient strain( P<0. 001). Conclusion The formation of S. mutans biofilm is affected by HtrA,and the deletion of HtrA gene reduces bacterial adhesion and slows biofilm formation.