Alkali Generation Capacity of Oral Bacteria

Alkali generation by oral bacteria plays a key role in plaque pH homeostasis and thus may be a major impediment to dental caries. Oral bacteria can produce alkali by hydrolyzing urea or arginine to ammonia, either by urease enzymes or the arginine deiminase system (ADS), respectively. Objectives: To develop a protocol for detection and quantification of arginolytic and ureolytic bacteria from oral samples; and to correlate their number with the alkali-generating capacity of samples and caries-status. Methods: Bacteria were isolated from supragingival plaque and whole un-stimulated saliva of 11 caries-free (CF; DMFT = 0) and 3 caries-active (CA; DT ≥ 2) subjects. The ADS- and urease-positive phenotypes of bacterial isolates were identified by quantitation of the ammonia generated from the incubation of bacteria in a mixture containing 50 mM arginine-HCl or 50 mM urea. Ammonia was detected by the Nesslerxs Reagent (Sigma, USA). Controls for background and interference were included for each reaction. Results: A total of 2,464 bacterial strains were recovered from oral samples, including 1,117 capable of generating alkali from the test substrates. Of the alkali-generating bacteria, 26% and 21% were arginolytic bacteria; and 14% and 39% were ureolytic bacteria recovered from plaque and saliva samples, respectively. There were no significant associations between the number of alkali-producing bacterial strains and the capacity of oral samples to generate alkali or the caries-status. Bacteria from different oral ecosystems differed widely in their capacity to break down arginine or urea. Conclusions: A protocol for detection of oral bacteria capable of producing ammonia from arginine and urea over a large range of bacterial clinical-isolates was developed. The results support that variations in the capacities of individual species to catabolize urea and arginine, and/or host factors, may legislate the ammonia-generating capacity of oral biofilms. Supported by UFCD Student Summer Research Fellowship.