Oral Microbiol Immunol 1992: 7: XAl-XAl

Assessment of the cariogenic potential of Streptococcus mutans strains and its relationship to in vivo caries experience

L. M. D. Macpherson\ T. W. MacFarlane\ D. A. M. Geddes^ K. W. Stephen' Departments of 'Oral Medicine and Pathology and ^Oral Biology, University of Glasgow Dental School, United Kingdom

Macpherson LMD, MacFarlane TW, Geddes DAM, Stephen KW Assessment of the eariogenic potential o/Streptococcus mutans strains and its relationship to in vivo caries experience. Oral Microbiol Immunol 1992: 7: 142-147. Strains of Streptococcus mutans isolated from the plaque of 6 subjects were studied using an in vitro model to determine whether differences in their cariogenic potential could be detected, and if so, whether the results correlated with the caries experience of the individuals. Each strain was incubated with a bovine enamel slab and 5% (w/v) sucrose for 24-h periods. The acidogenic potential was assessed by pH measurement and analysis of acid anion production. Microradiographic and microdensitometric assessment of the enamel, together with measurement of the change in calcium concentration of the reaction mixture were used to determine the demineralizing potential of each strain. Significant differences in cariogenic potential were found between some of the strains tested, and correlations were found between 3 of the test parameters and the decayed-missingfilled-surface score of the individuals. The results suggest that the caries experience of individuals may be related, to some extent, to the cariogenic potential of their S. mutans strains.

A relationship between Streptococcus mutans and the development of dental caries is extremely well documented and a number of biochemical properties are thought to contribute to the cariogenicity of this group of bacteria. These include the ability: to metabolize rapidly many dietary carbohydrates to acid, to continue production of acid in a low pH environment, and to synthesize from sucrose extracellular polysaccharides that can promote bacterial adherence to enamel or dental plaque (12). However, although many cross-sectional and longitudinal studies have shown a correlation between salivary and plaque counts of S. mutans and both caries prevalence and incidence (1, 16, 30), this is often not seen on an individual basis (10, 17, 22). Thus, the usefulness of tests involving 5. mutans counts as predictors of future caries activity in individual patients remains in doubt. In vitro studies have shown that different strains of mutans streptococci vary in their rate of acid production during glycolysis (11, 25), and animal

studies have found differences in cariogenic potential among S. mutans strains (31). This variation among strains may occur also intraorally in humans, and may be one of the reasons why correlations between counts of S. mutans and levels of enamel demineralization are often not seen at individual sites. The aim of this study was to develop an in vitro model to investigate the variation in cariogenic potential of S. mutans strains isolated from different individuals. Further objectives were to determine whether a relationship existed between the cariogenic potential of the strains and the caries activity of the subjects from whom they had been isolated, and to assess the model as a screening test in the detection of individuals with high caries risk. i\/laterial and methods

The methods used in this investigation were based on the model described by Primrose et al. (21) for assessing the cariogenic potential of foods. The

Key words: Streptococcus mutans; cariogenic potential; in vitro rr,odel: intraoral caries L. M. D. Macpherson, Department of Oral Biology, University of Manitoba, Winnipeg, Manitoba, Canada, R3E 0W2 Accepted for publication August 21, 1991

strains of 5. mutans were incubated with bovine enamel slabs and a 5% (w/v) sucrose solution. The acidogenic potential of each strain was assessed by measuring the acid production and pH of the system during the experimental period, and the demineralizing potential was determined by calculating the increase in calcium concentration of the reaction mixture and by performing radiographic and densitometric analysis of the enamel. The cariogenic potential of the different strains, together with salivary microbial counts, flow rate and buffering capacity were then compared with the decayed-missing-filled-surface (DMFS) score and in situ demineralization scores of the individuals, to determine whether any correlations existed between any of the test parameters and caries experience. Preparation of bovine enamei

Incisor teeth were removed from the mandible using a dental drill and dia-

Cariogenic potential of S. mutans strains mond disc, and were then cleaned with pumice and alcohol. Following thorough washing in warm soapy water, longitudinal slabs (approximately 8.0 X 3.5 mm) were cut from the crowns of the teeth, and the surface layer of the enamel was removed to a depth of approximately 100 //m by hand-grinding in a slurry of alumina paste. Each slab was then coated with 2 layers of acid-resistant nail varnish, leaving an exposed enamel window with dimensions of 2 X 2 mm. For each experiment, the slabs were obtained from the same animal.

of the supernatant, the wet weight of bacteria in each vial was adjusted to 90 mg.

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Anaiyticai fechniques

For acid anion analysis, the samples were removed from the freezer and centrifuged at 6500 ^ for 15 min. The supernatant was then removed and the acids Experimenfai procedure present in the fluid were analyzed by To initiate an experiment, a bovine en- isotachophoresis (6) using an LKB 2127 amel slab and 270 lA of a 5% (w/v) Tachophor (LKB Instruments, Broma, sucrose solution were added to the tube Sweden). containing the bacterial cells. The pH Analysis of calcium concentration of the system was then measured using was performed on the supernatant fluid a Corning pH meter, and a 30-//I sample obtained from the baseline and 24-h was removed and stored at — 20°C for samples. A 1-ml ahquot of a calcium subsequent acid anion analysis. A 10-/zl assay solution containing 0-cresolphsample was also taken and placed in thalein complexone (Sigma Chemical, 1 ml of anaerobic blood broth (ABB) Poole, UK) was mixed with 10 //I of Preparation of S. mutans ceiis (Gibco Laboratories, Grand Island, the test sample and the absorbance was The S. mutans strains were obtained as MY). Serial ten-fold dilutions of the read at 575 nm. This value was then fresh isolates from dental plaque sample were carried out in ABB, and 50 referred to a standard curve for determisamples of 6 caries-inactive individuals /

Assessment of the cariogenic potential of Streptococcus mutans strains and its relationship to in vivo caries experience.

Strains of Streptococcus mutans isolated from the plaque of 6 subjects were studied using an in vitro model to determine whether differences in their ...
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