Characterization of recombinant, ureolytic Streptococcus mutans demonstrates an inverse relationship between dental plaque ureolytic capacity and cariogenicity

Dental caries results from prolonged plaque acidification that leads to the establishment of a cariogenic microflora and demineralization of the tooth . Urease enzymes of oral bacteria hydrolyze urea to ammonia, which can neut ralize plaque acids. To begin to examine the relationship between plaque ur eolytic activity and the incidence of dental caries, recombinant, ureolytic strains of Streptococcus mutans were constructed. Specifically, the ureABC EFGD operon from Streptococcus salivarius 57.I was integrated into the S. m utans chromosome in such a way that the operon was transcribed from a weak cognate promoter in S. mutans ACU4 or a stronger promoter in S. mutans ACUS 6. Both strains expressed NiCl2-dependent urease activity, but the maximal urease levels in ACUS6 were threefold higher than those in ACUS4. In vitro pH drop experiments demonstrated that the ability of the recombinant S. mut ans strains to moderate a decrease in pH during the simultaneous metabolism of glucose and urea increased proportionately with the level of urease act ivity expressed. Specific-pathogen-free rats that were infected with ACUS6 and fed a cariogenic diet with drinking water containing 25 mM urea and 50 mu M NiCl2 had relatively high levels of oral urease activity, as well as d ramatic decreases in the prevalence of smooth-surface caries and the severi ty of sulcal caries, relative to controls. Urease activity appears to influ ence plaque biochemistry and metabolism in a manner that reduces cariogenic ity, suggesting that recombinant, ureolytic bacteria may be useful to promo te dental health.