Characterization of Recombinant, Ureolytic Streptococcus mutans Demonstrates an Inverse Relationship between Dental Plaque Ureolytic Capacity and Cariogenicity

Abstract

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 neutralize plaque acids. To begin to examine the relationship between plaque ureolytic activity and the incidence of dental caries, recombinant, ureolytic strains of Streptococcus mutans were constructed. Specifically, the ureABCEFGD operon from Streptococcus salivarius 57.I was integrated into the S. mutans chromosome in such a way that the operon was transcribed from a weak, cognate promoter in S. mutans ACUS4 or a stronger promoter in S. mutans ACUS6. Both strains expressed NiCl 2 -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. mutans strains to moderate a decrease in pH during the simultaneous metabolism of glucose and urea increased proportionately with the level of urease activity expressed. Specific-pathogen-free rats that were infected with ACUS6 and fed a cariogenic diet with drinking water containing 25 mM urea and 50 M NiCl 2 had relatively high levels of oral urease activity, as well as dramatic decreases in the prevalence of smooth-surface caries and the severity of sulcal caries, relative to controls. Urease activity appears to influence plaque biochemistry and metabolism in a manner that reduces cariogenicity, suggesting that recombinant, ureolytic bacteria may be useful to promote dental health.Despite significant gains in the control and treatment of dental caries, this disease remains widespread and costly to treat. Caries formation results from the acidification of dental plaque driven by microbial metabolism of dietary carbohydrate. The pH of dental plaque can fall to values as low as 4.0 (35), causing dissolution of the tooth enamel (37). In addition, frequent consumption of fermentable carbohydrate leads to extended periods of plaque acidification, which encourages the emergence of an acid-tolerant, cariogenic plaque microflora enriched in organisms such as mutans streptococci and lactobacilli (6). The enrichment of dental plaque with cariogenic microorganisms is generally accompanied by the loss of less acid-tolerant, less cariogenic organisms, which are abundant in so-called healthy dental plaque. These organisms include the sanguis group streptococci and Actinomyces naeslundii (4), which, interestingly, are also capable of neutralizing the acids produced by glycolysis through the generation of ammonia from salivary substrates.Caries development is usually a prolonged process involving cycles of demineralization and remineralization. Periods of plaque acidification and tooth demineralization are normally followed by phases of alkalinization with a return to more neutral plaque pH values (35), which promotes remineralization at the tooth surface (22,23). It is when the phases of demineralization dominate that a carious lesion develo...

Oral Arginine Metabolism May Decrease the Risk for Dental Caries in Children

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The Biofilm Concept: Consequences for Future Prophylaxis of Oral Diseases?

“…It lacks the enzyme lactate dehydrogenase and therefore does not form lactate (Hillman et al, 2000). Other recombinant strains which are ureolytic have also been constructed (Clancy et al, 2000). These strains hydrolyze urea to ammonia, thereby counteracting acidification of the environment.…”

Effects of Arginine on Streptococcus mutans Growth, Virulence Gene Expression, and Stress Tolerance

“…Oral biofilms are highly organized communities of a physiologically and genetically diverse group of microorganisms, many of which are commensal or overtly study showed that subjects with chronic renal failure, who can secrete an amount of urea in saliva that is 10-fold or more higher than that of healthy subjects, seldom developed dental caries despite consuming a diet composed predominantly of carbohydrates (18,19). Furthermore, when rats were infected with S. mutans or a recombinant derivative of S. mutans that expressed the urease genes of Streptococcus salivarius, the rats infected with the strain expressing high levels of urease developed fewer and less-severe carious lesions than those infected with the parental strain when fed a cariogenic diet supplemented with urea (20). More recently, correlations were found between oral alkali generation and caries formation in human subjects.…”