L-Arginine Modifies the Exopolysaccharide Matrix and Thwarts Streptococcus mutans Outgrowth within Mixed-Species Oral Biofilms

L-Arginine, a ubiquitous amino acid in human saliva, serves as a substrate for alkali production by arginolytic bacteria. Recently, exogenous L-arginine has been shown to enhance the alkalinogenic potential of oral biofilm and destabilize its microbial community, which might help control dental caries. However, L-arginine exposure may inflict additional changes in the biofilm milieu when bacteria are growing under cariogenic conditions. Here, we investigated how exogenous L-arginine modulates biofilm development using a mixed-species model containing both cariogenic (Streptococcus mutans) and arginolytic (Streptococcus gordonii) bacteria in the presence of sucrose. We observed that 1.5% (wt/vol) L-arginine (also a clinically effective concentration) exposure suppressed the outgrowth of S. mutans, favored S. gordonii dominance, and maintained Actinomyces naeslundii growth within biofilms (versus vehicle control). In parallel, topical L-arginine treatments substantially reduced the amounts of insoluble exopolysaccharides (EPS) by > 3-fold, which significantly altered the three-dimensional (3D) architecture of the biofilm. Intriguingly, L-arginine repressed S. mutans genes associated with insoluble EPS (gtfB) and bacteriocin (SMU. 150) production, while spxB expression (H2O2 production) by S. gordonii increased sharply during biofilm development, which resulted in higher H2O2 levels in arginine-treated biofilms. These modifications resulted in a markedly defective EPS matrix and areas devoid of any bacterial clusters (microcolonies) on the apatitic surface, while the in situ pH values at the biofilm-apatite interface were nearly one unit higher in arginine-treated biofilms (versus the vehicle control). Our data reveal new biological properties of L-arginine that impact biofilm matrix assembly and the dynamic microbial interactions associated with pathogenic biofilm development, indicating the multiaction potency of this promising biofilm disruptor.