Construction and characterization of a recombinant ureolytic Streptococcus mutans and its use to demonstrate the relationship of urease activity to pH modulating capacity.

To begin to understand the contribution of oral microbial ureolysis to the inhibition of dental caries, we sought to construct a recombinant, ureolytic mutans streptococcus and correlate the ureolytic capacity of plaque bacteria with pH moderating ability. Streptococcus mutans GS-5 was transformed with a plasmid containing the urease genes from Streptococcus salivarius 57.I. The recombinant strain, S. mutans AC04, stably maintained the urease genes. High levels of urease activity were detected, with a maximum specific activity of 0.9 mumol of urea hydrolyzed/min/mg cell dry weight when the growth medium was supplemented with 50 microM exogenous NiCl2. Harboring the recombinant plasmid, or growth in NiCl2, did not markedly affect the glycolytic capacity of S. mutans. In vitro pH drop analysis of S. mutans AC04, metabolizing glucose and physiologically relevant concentrations of urea simultaneously, demonstrated that increasing the urease activity of plaque bacteria resulted in a corresponding reduction in the depth and the duration of the glycolytic pH fall. The results demonstrate the feasibility of engineering urease producing S. mutans and suggest that enhancing the ureolytic capacity of dental plaque, particularly cariogenic plaque, may help to offset the progression of the caries process.