Bacterial microleakage along the tooth/composite resin dental restoration interface contributes to postoperative sensitivity, recurrent caries, and necrosis. Studies have confirmed that enzymes in human saliva degrade composite resin monomers 2,2-bis [4-(2-hydroxy-3-methacryloxypropoxy) phenyl] propane (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) to release methacrylic acid (MA), bishydroxypropoxyphenyl propane (Bis-HPPP), and triethylene glycol (TEG) at levels of 50 microM in vivo. Studies have found that TEGDMA degradation products alter the growth and gene expression of cariogenic Streptococcus mutans. Specifically, TEG was shown to alter S. mutans gene expression levels of gtfB, a known virulence factor, and yfiV, a putative transcriptional regulator of cell-surface fatty acid genes. The objective of this study was to examine the effect of BisGMA degradation products on the growth and gene expression of S. mutans NG8 cells. Results demonstrated slight inhibition of bacterial growth at Bis-HPPP concentrations of 1.0 x 10(2) and 2.5 x 10(2) microM at pH 5.5. Furthermore, both MA and Bis-HPPP affected gtfB and yfiV expression in a concentration-dependent manner. Because BisGMA is universally used across most dental restorative materials, with millions of placement procedures performed annually, these findings are relevant due to the potential influence of resin monomer-derived biodegradation products on biofilm formation, acid tolerance, and proliferation of S. mutans cells.