Virulence of Streptococcus mutans: restoration of pathogenesis of a glucosyltransferase-defective mutant (C4)

Evaluation of cariogenic bacteria

OBJECTIVES

The evaluation of Mutans streptococci (MS) is one of the index for caries risk. Dentocult(TM) and CRT(TM) are commercial kits to detect and evaluate MS, conveniently. However, the evaluation of MS has also been carried out simply using an instruction manual. But the instruction manual is not easy to use for evaluation of MS. The aim of this study was to examine the utility of modified Mitis-Salivalius Bacitracin (MSB) agar medium compared with MSB agar medium and commercial kits, and to establish a convenient kit (mMSB-kit) using modified MSB agar.

METHODS

The MS in stimulated saliva from 27 subjects were detected by MSB, modified MSB agar medium and commercial kits. Laboratory and clinically isolated strains of MS were similarly evaluated. The ratios of MS in detected bacteria were compared by ELISA.

RESULTS

The scores using an mMSB-kit on the basis of modified MSB agar medium were tabulated. Saliva samples showed different levels of MS between culture methods and the commercial kit. Some samples which were full of MS were not detected by the commercial kit. The detection of MS by modified MSB agar medium and mMSB-kit were significantly higher when compared with MSB agar medium,CRT(TM), (P< .01) and Dentocult SM(TM) (P<.05).

CONCLUSION

The sensitivity for detection of MS is higher for modified MSB agar medium when compared with MSB agar medium. The mMSB-kit can be used simply, and can be an important contributor for the evaluation of MS as a caries risk factor.

Genetic approaches to the study of oral microflora: a review

As the study of oral microorganisms intensified almost 2 decades ago, the application of genetic techniques resulted in important contributions to the understanding of this clinically and ecologically important group of bacteria. The isolation and characterization of mutants of cariogenic streptococci helped to focus attention on traits that were important in colonization and virulence. Such classic genetic approaches gave way to molecular genetic techniques, including recombinant DNA methodology in the late 1970s. Gene cloning systems and methods to move DNA into cells have been developed for oral streptococci. Many streptococcal genes thought to be important in colonization and virulence have since been cloned and their nucleotide sequence determined. Mutant strains have been constructed using defective copies of cloned genes in order to create specific genetic lesions on the bacterial chromosome. By testing such mutants in animal models, a picture of the cellular and molecular basis of dental caries is beginning to emerge. These modern genetic methodologies also are being employed to develop novel and efficacious cell-free or whole cell vaccines against this infection. Genetic approaches and analyses are now being used to dissect microorganisms important in periodontal disease as well. Such systems should be able to exploit advances made in genetically manipulating related anaerobes, such as the intestinal Bacteroides. Gene cloning techniques in oral anaerobes, Actinomyces and Actinobacillus, are already beginning to pay dividends in helping understand gene structure and expression. Additional effort is needed to develop facile systems for genetic manipulation of these important groups of microorganisms.

In vitro inhibition of adherence of Streptococcus mutans strains by nonadherent mutants of S. mutans 6715

Four nonadherent mutants from Streptococcus mutans 6715 mutant UAB66 (serotype g) with similar phenotypes were shown to inhibit the adherence of adherence-proficient S. mutans serotypes c and g strains. One mutant, UAB108, was shown to inhibit adherence by wild-type strains representing serotypes a, d, and e as well. This inhibition of adherence was seen with pairs of strains grown in partially defined (PD) medium supplemented with 1% sucrose in both microtiter plates and glass tubes. The inhibiting factor was present in culture supernatant fluids of inhibiting strains grown in PD medium plus 1% sucrose and was heat stable. Ethanol precipitation of culture supernatant fluids of these strains yielded a water-soluble polymer which effectively inhibited the adherence of UAB66. This polymer, isolated from UAB108, was also shown to inhibit the adherence of UAB66 at lower concentrations than that needed to inhibit adherence with dextran T10. Partially purified glucosyltransferase, isolated from the culture supernatant fluids of glucose-grown UAB108, produced a water-soluble glucan which was shown to inhibit the adherence of UAB66 as well. The methods developed permit rapid screening for strains or mutants of strains or both that inhibit adherence or plaque formation or both by wild-type strains of S. mutans.

In vitro and in vivo complementation of Streptococcus mutans mutants defective in adherence

Previous studies have shown that adherence-defective mutants of Streptococcus mutans PS14, serotype c, can be grouped into several different phenotypic groups. In this study a method was developed to test for complementation between pairs of nonadhering mutants which possess different genotypic defects. Mutant strains UAB95 and a spectinomycin-resistant derivative of UAB95 (UAB516) were found to exhibit increased levels of adherence when grown together with UAB230 in media containing sucrose as compared to the adherence of each strain grown separately. An increase in caries was also observed in gnotobiotic rats mixedly infected with the two mutants as compared to either strain alone. Tests revealed that UAB95 produced more water-insoluble glucan than its parent strain but had a defect in glucan binding. UAB230 was found to produce levels of a defective glucan that could not be bound by mutant or wild-type cells. Our results suggest that UAB95 produces a water-insoluble glucan which is bound by UAB230, thus allowing complementation for adherence and caries production.

An improved method for measuring aggregation of certain streptococcal bacteria found in dental plaque

Various cells were incubated with either dextran or sucrose and the reaction was terminated by the addition of a 20 per cent solution of formaldehyde. A reaction mixture consisting of cells and aggregated cells was applied to a step-gradient glycerol column (0-60 per cent) and 0.5 ml fractions were collected from the bottom of the column. Non-aggregated cells remained in the top layer of the column. Aggregated cells settled in the 30 per cent glycerol layer as determined by either spectrophotometric or radioactive methods. The amount of aggregated cells demonstrated in this layer increased with either incubation time or concentration of sucrose, dextran or cells. Cells of Strep, mutans strains 6715, AHT, 10449, OMZ-176, OMZ-175 and two mutants of Strep. mutans strain 6715 (designated C4 and C307) aggregated strongly. Cells of Strep. mutans strain LM7 aggregated weakly, whereas cells of Strep. mutans 6715 mutant UAB165, a mutant defective in aggregation. Strep. mutans strain BHT, Strep. salivarius and Strep. sanguis did not aggregate in the presence of either sucrose or dextran. Experiments testing co-aggregation between two different types of cells were done by using 3H-labelled and 14C-labelled cells. Strep. mutans strain 6715 co-aggregated with Strep. sanguis in the presence of sucrose or dextran, whereas Strep. mutans strain 6715 did not aggregate with Strep. salivarius. The method separates aggregated from non-aggregated cells and enables quantitation of sucrose- or dextran-induced aggregation and co-aggregation.

Isolation and characterization of Streptococcus mutans mutants defective in adherence and aggregation

A method was developed which enriched for mutants of Streptococcus mutans that exhibit defects in adherence to glass, aggregation, or both. Mutants were isolated from derivatives of strains PS14 (serotype c) and 6715 (serotype g) after mutagenesis with either ethyl methane sulfonate or nitrous acid. Cell survival after mutagenesis was kept above 1 to 2% to enhance the probability that mutants resulted from single mutational events. A total of 117 mutants were isolated; they also displayed non-wild-type colony morphology on mitis salivarius agar. These mutants were examined for (i) adherence and aggregation after overnight growth in sucrose-containing medium, (ii) aggregation of nongrowing cells in the presence of 200 microgram of sucrose per ml or 20 microgram of dextran per ml, and (iii) dextranase production on blue dextran agar plates. Although we isolated mutants which exhibited a variation from the parent strain in only one of the traits tested, the majority of mutants exhibited defects in two or more characteristics. Thirty-eight stable mutants of independent origin were categorized into 13 separate phenotypic groups.

Oral ecology and virulence of Lactobacillus casei and Streptococcus mutans in gnotobiotic rats

Lactobacilli comprise a small percentage of the normal oral microbial flora of humans and are isolated commonly from saliva and frequently from an active caries lesion. We have compared the pathogenesis and colonization pattern of Lactobacillus casei with that of Streptococcus mutans strain 6715 in gnotobiotic rats. Of the two L. casei strains tested, L. casei strain ATCC 4646 caused slightly more caries than L. casei strain ATCC 11578. However, the level of caries induced by either L. casei strain was significantly lower (P less than 0.01) than that observed in similar-aged rats monoassociated with S. mutans strain 6715. When groups of rats were infected with mixtures of L. casei strain ATCC 4646 and S. mutans strain 6715, or with L. casei followed by S. mutans, higher numbers of L. casei than S. mutans were found associated with the tongue and in saliva; S. mutans always predominated in plaque. The level of caries observed in these groups of rats was similar to that seen with rats monoassociated with S. mutans except when L. casei comprised greater than 1% of the plaque microflora. In this latter situation, the level of caries was significantly lower (P less than or equal to 0.05) than that obtained in S. mutans-monoassociated rats. The results of this study suggest that L. casei colonizes sites in the oral cavity (including the tongue and saliva) other than the tooth surface in rats. The effect of L. casei in plaque toward reduction of S. mutans-induced dental caries in rats is discussed.

Virulence of Streptococcus mutans: in vivo reversion of a low-virulence mutant results in partial displacement and pathogenesis

A mutant of Streptococcus mutans 6715 wild type (WT), designated C4, has been shown previously to be defective in glucosyltransferase synthesis of insoluble glucan and to have low virulence in monoassociated gnotobiotic rats. The present investigation was concerned with the detection of WT-like variants of C4 in monoassociated rats, the supplantation of C4 by these WT-like organisms, and finally, the pathogenic potential of these WT-like organisms in gnotobiotic rats. In the first series of longitudinal studies with C4-monoassociated rats, WT-like organisms were detected at a low frequency (0.001%) in oral swab samples from only one of four cages of animals analyzed on day 7 after infection (age 27 days). The frequency of variants isolated from animals in the one cage increased, and by age 45 days these organisms represented approximately 1% of the mandibular plaque flora. After random redistribution of rats in the four cages (age 45 days), microbial analysis of oral swab samples (age 60 days) demonstrated the presence of variants in samples taken from rats in all four cages. The frequency of recoverable variants increased in older animals (age 90 days) and correlated with high caries activity. WT-like organisms were transmissible, since offspring (age 45 days) from these animals had high levels of variants as well as high caries activity. Similar results were obtained in a second longitudinal study; however, variants, although present in all four cages, were not detected until rats were 45 days old. All variant isolates exhibited morphological, biochemical, and in vivo virulence characteristics more similar to S. mutans 6715 WT than to C4. In vitro mixing experiments with C4 and either WT or a selected variant suggested that C4 was rapidly displaced by WT organisms. The results of this investigation demonstrate that the glucosyltransferase-defective, low-virulence C4 reverts to virulent WT-like organisms in vivo which compete more favorably for smooth surfaces than C4. Subsequently, these variants reached significant numbers in plaque which correlated with increased dental caries.