Characterization of the Arginolytic Microflora of Human Oral Biofilms

Objectives: An increased risk of dental caries has been associated with a reduced ability of oral biofilms to produce alkali from arginine via the arginine deiminase system (ADS). The purpose of this study was to begin to isolate and characterize arginolytic bacterial species from caries-free and caries-active subjects to understand the microbiological basis for inter-subject variation in ADS activity and to determine whether antagonistic interactions between Streptococcus mutans and arginolytic isolates could be correlated with caries status. 

Methods: Fifty ADS-positive bacterial strains were isolated and characterized by 16S rRNA gene sequencing. ADS activity of the isolates was measured under growth conditions known to induce or repress ADS gene expression. Antagonism of, and by, S. mutans UA159 of highly ADS-active isolates was also examined using plate inhibition assays.

Results:  Considerable diversity was noted in the arginolytic species from human plaque. Streptococcus gordonii, Streptococcus sanguinis, Streptococcus parasanguinis and Streptococcus mitis were the most prevalent in our samples, but Actinomyces and other species were also identified.  The ability to express high ADS activity was frequently observed in S. sanguinus, S. gordonii and S. mitis, and substantial variation in ADS expression patterns in these strains was noted in response to pH, oxygen, carbohydrate and substrate availability. In general, clinical isolates of S. gordonii showed more pronounced inhibition of the growth of S. mutans.

Conclusion:   This study reveals that there is a diverse arginolytic community in dental biofilms and that the basis for variation in ADS expression between subjects could, in large part, be due to intra-strain variability in the regulation of the ADS genes. Collectively, the results also support that high ADS-expressing strains could have positive and synergistic effects on plaque ecology by moderating plaque pH and directly antagonizing the growth of known caries pathogens.