Characterization of the arginolytic microflora provides insights into pH homeostasis in human oral biofilms

Abstract

A selected group of oral bacteria commonly associated with dental health is capable of producing alkali via the arginine deiminase system (ADS), which has a profound impact on the pH of human oral biofilms. An increased risk for dental caries has been associated with reduced ADS activity of the bacteria in oral biofilms. Arginolytic bacterial strains from dental plaque samples of caries-free and caries-active adults were isolated and characterized to investigate the basis for differences in plaque ADS activity between individuals. Fifty-six ADS-positive bacterial strains were identified by 16S rRNA gene sequencing, and their ADS activity levels were compared under standard growth conditions. The spectrum of bacterial ADS activity ranged from 45.2 to 688.0 units (mg protein)(-1). Although Streptococcus sanguinis was the most prevalent species, other Streptococcus sp. were also represented. Biochemical assays carried out using 27 ADS-positive strains under conditions known to induce or repress ADS gene expression showed substantial variation in arginolytic activity in response to pH, oxygen and the availability of carbohydrate or arginine. This study reveals that the basis for the wide spectrum of arginolytic expression observed among clinical strains is, at least in part, attributable to differences in the regulation of the ADS within and between species. The results provide insights into the microbiological basis for intersubject differences in ADS activity in oral biofilms and enhance our understanding of dental caries as an ecologically driven disease in which arginine metabolism moderates plaque pH and promotes dental health.

Figure 1. ADS activity levels of S. gordonii DL1 and arginolytic clinical strains under different environmental conditions

Cultures of TY medium containing: (A) 25 mM galactose with (w/) or without (w/o) 10 mM arginine, (B) 25 mM galactose and 10 mM arginine that had been acidified to pH 5.7 with HCl or buffered at pH 7.0, (C) 10 mM arginine and 25 mM galactose or 25 mM glucose, and (D) 25 mM galactose and 10 mM arginine and the cells were cultured under aerobic (w/O₂) or anaerobic (w/o O₂) conditions. Results represent the mean and standard deviations (error bars) of three independent experiments.

Figure 2. Comparison of the ADS activity levels of arginolytic strains from caries-active and caries-free subjects grown under different environmental conditions

Cultures of TY medium containing: (A) 25 mM galactose with (w/) or without (w/o) 10 mM arginine, ((B) 25 mM galactose and 10 mM arginine that had been acidified to pH 5.7 with HCl or buffered at pH 7.0, (C) 10 mM arginine and 25 mM galactose or 25 mM glucose, and (D) 25 mM galactose and 10 mM arginine and the cells were cultured under aerobic (w/ O₂) or anaerobic (w/o O₂) conditions. CA: caries-active and CF: caries-free subjects. Results represent the mean and standard deviations (error bars) of three independent experiments.