Abstract: Streptococcus mutans is a Gram-positive facultative anaerobic bacterium, known as the principal dental pathogen associated with caries. It resides in the diverse, multi-species oral biofilm and is constantly challenged by the dynamic environmental perturbations and other competing bacterial specks A number of virulence factors, including acid production, acid tolerance, glucan mediated biofilm formation, and bacteriocin (mutacin) production, enable S. mutans to compete against other oral bacteria, become the dominant species in the dental biofilm and eventually initiate the development of cariogenic lesions. Biofilm formation which involves initial adhesion, co-adhesion, growth and maturation and finally detachment of certain cells from the mature biofilm to reach new adhesion sides plays an essential role in the pathogenesis of S. mutans. This dissertation addresses the functions of surface components associated with biofilm formation and pathogenesis of S. mutans. This bacterium employs two different mechanisms, a sucrose dependent and a sucrose independent one, to promote adherence to the tooth surface. In chapter 1, WapA, a cell wall anchored protein which is involved in the sucrose independent adhesion mechanism, was demonstrated to play an important role in maintaining general cell morphology and initiating the surface attachment. More interestingly, the expression of wapA is repressed by sucrose in a dose dependent manner, which suggests coordination between the different mechanisms in S. mutans biofilm formation. The sucrose dependent adhesion is considered as the predominant strategy in S. mutans attachment to the surface. In the following chapter, atomic force microscopy (AFM) was used to probe the local cell-surface interactions associated with the glucan polymers and related proteins under in situ conditions. The measurement demonstrates that the local adhesion strength increases in a time dependent process. This finding suggests that in the presence of sucrose S. mutans attaches to surfaces predominantly through glucans. In addition, a possible role of the glycosyl-transferases (Gtf proteins) in sucrose independent attachment is supported by the decreased adhesion properties of a gtfBCD mutant compared to the wild-type. In the last chapter, an interesting regulatory network which connects virulence factor expression and stress tolerance development to the biofilm formation was identified and further studied.