Streptococcus mutans, a primary etiologic agent of dental caries, is adept at driving the formation of pathogenic biofilms. Certain oral commensal streptococci can potently antagonize S. mutans, but the effects are dependent on environmental factors. Using a known pathogenic strain of S. mutans (UA159) tagged with a green fluorescent protein and an antibiotic resistance marker, we examined the abilities of 50 low-passage commensal oral Streptococcus strains to compete in a two-species biofilm model with S. mutans when growing on the amino sugar N-acetylglucosamine (GlcNAc), compared to the same set of strains growing on glucose. Growth on GlcNAc substantially improved the competitive fitness of the commensals. A subset of commensal strains was selected and the effects of GlcNAc on the ability of the commensals to protect an oral Streptococcus species from killing by S. mutans were examined using a deferred antagonism assay. Collectively, the results demonstrate that GlcNAc can enhance the probiotic properties of beneficial health-associated commensals. This research establishes a foundation for designing novel therapeutic approaches that incorporate amino sugars to moderate the pathogenic potential of oral biofilms, thus decreasing the tremendous burden of dental caries and other oral infectious diseases.