Bacterial microleakage along the tooth/composite resin dental restoration interface contributes to post-operative sensitivity, recurrent caries, pulp inflammation and necrosis. Studies have confirmed that saliva can catalyze the degradation of constitutive monomers in dental restorative composites, forming biodegradation by-products (BBPs) such as methacrylic acid (MA), and triethylene glycol (TEG). TEG accelerates the growth of Streptococcus mutans, a major etiological agent of dental caries. Restriction fragment differential display polymerase chain reaction (RFDD-PCR) in conjunction with single strand conformation polymorphism (SSCP) was used to identify S. mutans genes with differential expression when grown in the presence of TEG at pH levels 5.5 and 7.0. Quantitative real-time PCR (q-RT PCR) was utilized to study specific gene expression patterns. TEG modulated the expression levels of glucosyltransferase B (gtfB) (involved in biofilm formation) and yfiV (a putative transcription regulator) in S. mutans. The expression patterns were dependent on the bacterial growth mode (planktonic vs. biofilm) as well as pH (5.5 vs. 7.0). The findings describe the effect of composite resin-derived BBPs on important physiological functions of S. mutans (at BBP concentration levels found in vivo), and indicate the potential influence of BBPs in biofilm formation and microbial survival on surfaces in the oral cavity.