Effects of sub-minimum inhibitory concentrations of antimicrobial agents on Streptococcus mutans biofilm formation

Many studies have demonstrated that sub-minimum inhibitory concentrations (sub-MICs) of antimicrobial agents can inhibit bacterial biofilm formation. However, the mechanisms by which antimicrobial agents at sub-MICs inhibit biofilm formation remain unclear. At present, most studies are focused on Gram-negative bacteria; however, the effects of sub-MICs of antimicrobial agents on Gram-positive bacteria may be more complex. Streptococcus mutans is a major cariogenic bacterium. In this study, the S. mutans growth curve as well as the expression of genes related to S. mutans biofilm formation were evaluated following treatment with 0.5× MIC of chlorhexidine (CHX), tea polyphenols and sodium fluoride (NaF), which are common anticaries agents. The BioFlux system was employed to generate a biofilm under a controlled flow. Morphological changes of the S. mutans biofilm were observed and analysed using field emission scanning electron microscopy and confocal laser scanning microscopy. The results indicated that these three common anticaries agents could significantly upregulate expression of the genes related to S. mutans biofilm formation, and S. mutans exhibited a dense biofilm with an extensive extracellular matrix following treatment with sub-MICs of NaF and CHX. These findings suggest that sub-MICs of anticaries agents favour S. mutans biofilm formation, which might encourage dental caries progression.