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Dextran glucosidase (EC 3.2.1.70) isolated from cell extracts of Streptococcus mitis ATCC 903 was purified 925-fold by DEAE-Sephadex, Sephadex G-100 and hydroxylapatite chromatography. The enzyme showed normal Michaelis-Menten kinetics when tested with p-nitrophenyl-α-D-glucopyranoside (pNPG), dextran (MW 9,400) or isomaltose as substrates. The apparent K_m values were 1.60, 4.95 and 15.7 mM, respectively. The molecular weight of the enzyme was estimated to 54,000 and the pi was 4.45. Among the metal ions tested (1 mM solutions), Hg²⁺ and Zn²⁺ inhibited the enzyme activity completely. In spite of an inhibiting effect of EDTA, no cationic metallic cofactor was found. Glucose exerted competitive inhibition of enzyme activity (K_I = 2.5 mM). Optimal enzyme activity was found at pH 7.2 in the temperature range of 37–40 °C and at a low salt concentration (I = 0.06).

The potential pathogenic role of mutans streptococci in the etiology of dental caries is well-documented. Mutans streptococci are sensitive to chlorhexidine (CHX), and several methods for the clinical use of CHX have been described. An important target group for caries-preventive measures is patients with impaired salivary secretion due to the use of therapeutic drugs such as psychotropics. The aim of the present study was to compare two methods for antimicrobial treatment in such patients. Twenty-four volunteering patients at a hospital clinic were randomly divided into three groups: Group A, in which each participant was treated with 10% CHX gel in gel trays on two consecutive days, followed by application of a fluoride varnish; Group B, in which the patients were treated with the same CHX gel as above; and Group C, which was used as a control. After treatment, there was a reduction of mutans streptococci in the two groups treated with CHX. In group A, the sames collected one, three, and five weeks after the treatment were significantly lower than baseline values. No clearcut reduction of the number of mutans streptococci was observed in the subjects not treated with CHX. These data indicate that treatment with CHX gel in gel trays is superior to polishing the teeth with CHX gel. From a clinical point of view, our observations suggest that it is important to monitor the effect of antimicrobial treatment individually in order to optimize preventive programs in patients with impaired salivary secretion.

The aim of this study was two-fold: firstly, to study the effect of high fluoride concentrations on carbohydrate metabolism in Streptococcus mutans present in biofilms on hydroxyapatite; and, secondly, to evaluate the effect of fluoride-bound hydroxyapatite on lactic acid formation in growing biofilms of Strep. mutans. Biofilms of a clinical strain of Strep. mutans on saliva-coated hydroxyapatite beads were incubated with sodium fluoride over a wide range of concentrations. At high fluoride concentrations (>10 mM) the incorporation of [14C]-labeled glucose decreased by 80-85%, at both pH 7.0 and 5.6. At lower fluoride concentrations, the effect of fluoride on the incorporation of labeled glucose was pH-dependent in both biofilm cells and in planktonic cells. At pH 7.0, fluoride at concentrations < 10 mM had little or no effect. Pretreatment of hydroxyapatite discs with fluoride varnish (Fluor Protector) or fluoride solutions caused a statistically significant reduction of lactic acid formation in associated, growing biofilms of Strep. mutans. Fluoride varnish and 0.2% (47.6 mM) sodium fluoride solution exhibited a statistically significant inhibitory effect on lactate production.

Extracts of Streptococcus mitis ATCC 903 were analysed for P-fructofuranosidase and a-glucosidase activities by isoelectric focusing in thin-layer polyacrylamide gels combined with zymogram procedures. Three bands of activity were visualized in the gels after incubation with sucrose (PI 4.05, 4.25 and 4.85) and three other bands after incubation with p-nitrophenyl a-D-ghcopyranoside (PI 3-90,4.45 and 4.65). The enzymes responsible for the reaction with sucrose were identified as P-fructofuranosidases (EC 3.2.1 .26) for the following reasons : identical enzyme bands were visualized in the gels after incubation with raffinose; no enzyme bands appeared in the gel after incubation with the a-glucosides maltose, turanose, trehalose and melezitose ; and the soluble fraction hydrolysed sucrose to equimolar amounts of glucose and fructose. I N T R O D U C T I O NStreptococcus mitis comprises a significant proportion of the bacteria of dental plaque and saliva and on the buccal mucosa (Gibbons & van Houte, 1975). It is capable of sucrose metabolism, as are all other oral viridans streptococci (Facklam, 1977). However, in contrast to s. mutans, s. sanguis and s. salivarius, s. mitis is generally devoid of glucosyl-and fructosyltransferase activities (Facklam, 1977). Apparently S. mitis utilizes other enzymic reactions to bring sucrose into the glycolytic pathway.Two types of enzymes hydrolysing the glycosidic bond in sucrose have been demonstrated in micro-organisms (Myrback, 1960) : P-fructofuranosidases (P-D-fructofuranoside fructohydrolase, EC 3.2.1 .26 ; invertase) and a-glucosidases (a-D-glucoside glucohydrolase, EC 3 . 2 . 1 .20). Sucrose phosphorylase (sucrose : orthophosphate a-glucosyltransferase, EC 2 . 4 . 1 .7), which catalyses the phosphorylytic decomposition of sucrose, has been found in some bacteria (Silverstein et al., 1967).In the present investigation the soluble fraction of extracts of S. mitis was analysed by isoelectric focusing in thin-layer polyacrylamide gels combined with zymogram procedures in order to separate and visualize enzymes active on P-fructofuranosidic and a-glucosidic linkages. The zymogram patterns showed three enzyme bands with substrate specificities characteristic of P-fructofuranosidase and three enzyme bands with a-glucosidase activity. Preparation of extracts. Bacteria were harvested 1 h after the exhaustion of the energy source or at the METHODS Growth medium and cultivation technique. Streptococcus mitis