Dental caries is a biofilm-related oral disease that continues to afflict the majority of the world's population.1 Although several studies have revealed that the level of mutans streptococci is not necessarily high during the development of dental caries,2 and 3 many studies have identified Streptococcus mutans as one of the main causative pathogens for dental caries. 4 and 5 The ability to produce acids in biofilms is an important virulence trait of this bacterium. 6 and 7 In addition, the bacterium can synthesise extracellular polysaccharides (EP) from sucrose, which allow the bacterial cells to adhere to the tooth surfaces and contribute to the formation of cariogenic biofilms. 8 The elevated levels of EP increase the bulk and structural stability of the biofilms. Thus, if the adhesion of S. mutans to tooth surfaces or the physiological ability (acid production and EP formation) of S. mutans in biofilms can be reduced, the potential for dental caries initiation will be decreased.