Jason M. Tanzer, D.M.D., Ph.D., and Jill Livingston, M.S.:
This review was conducted to evaluate the implication of certain microorganisms in the causation of human tooth decay. It examines the evidence concerning bacterial species identified in both early and current literature to be involved in tooth decay, whether originally implicated by wild animal, experimental animal, or human data. It also discusses the source of this putative infection of humans. Attention is focused on the mutans streptococci, the sanguinis streptococci, other streptococci, the enterococci, the lactobacilli, and certain actinomycetes, all of which are resident in the human mouth.
There is an immense literature on this topic. The present review deals with studies of the microbial causes and associations with dental caries in humans, relying on cross-sectional, case-control, longitudinal, and interventional studies. It addresses tooth decay in young children having only deciduous (primary) dentition, older children and adolescents having mixed and permanent (secondary) dentitions, and adults and seniors, whose secondary dentition often presents varying degrees of root exposure. As such, patients and experimental subjects with incipient enamel lesions (white spots) and established cavitations (cavities) of the tooth crowns and root surface lesions are considered. Studies of so-called secondary or recurrent caries have been excluded from this review, as have studies done in vitro, in experimental animals, or with so-called in situ caries models.
Earlier studies have characterized the biological behavior of the implicated microorganisms. The essentials are summarized below.
Mutans streptococci colonize the host only after the first teeth erupt, and their preferential colonization site is the teeth (Carlsson, Grahnen, Jonsson, 1975; Catalanotto, Shklair, Keene, 1975); they are highly localized on the surfaces of the teeth, and their abundance in the plaque is highest over initial lesions (Duchin, van Houte 1978; Babaahmady, Challacombe, Marsh, et al., 1998); their level of colonization within the plaque is increased by sucrose consumption (Folke, Gawronski, Staat, et al., 1972; Staat, Gawronski, Cressey, et al., 1975); they synthesize molecules from sucrose that foster their attachment to the teeth (Freedman, Tanzer, 1974; Tanzer, Freedman, Fitzgerald, et al., 1974); they are rapid producers of acid from simple carbohydrates and are tolerant to low pH (Edwardsson, 1968; Tanzer, 1989); and they are recovered on cultivation of initial and established carious lesion sites (Clarke, 1924; Littleton, Kakehashi, Fitzgerald, 1970; Keene, Shklair, 1974). Interest in them grew after demonstration of their potency in induction and progression of carious lesions in a variety of experimental animals, including mono-infected gnotobiotes (Fitzgerald, Fitzgerald, 1981). Their virulence expression is strongly associated with consumption of carbohydrates, especially sucrose (Tanzer, Freedman, Fitzgerald, 1985; Kuramitsu, 1993).
Lactobacilli do not avidly colonize the teeth and may be transiently found in the mouth before the teeth erupt; they preferentially colonize the dorsum of the tongue and are carried into saliva by sloughing of the tongue�s epithelium (van Houte, Gibbons, Pulkkinen, 1972); their numbers in saliva appear to be a reflection of the consumption of simple carbohydrates by the host (Staat, Gawronski, Cressey, et al., 1975; Holbrook, de Soet, de Graaff, 1993); they too are highly acidogenic from carbohydrates and are acid-tolerant (Wood, 1961). They are often cultured from established carious lesions (Loesche, Syed, 1973). Some lactobacilli are cariogenic in experimental animals, and their cariogenicity is dependent upon consumption of carbohydrate-rich-diets.
Nonmutans streptococci of several types, including the sanguinis group of organisms, and S. salivarius, are extremely abundant in the mouth; some are tooth surface colonizers, some mucosal colonizers. Some are quite acidogenic from carbohydrates and are acid-tolerant (Guggenheim, 1968; Edwardsson, 1968; Nyvad, Kilian, 1990). Less evidence exists of their virulence in experimental animals.
Enterococci were the first bacteria shown experimentally to induce caries in gnotobiotic animals (Orland, Blayney, Harrison, et al. 1955). Carbohydrate users, acidogenic, and acid-tolerant, they are seldom abundant in the human oral cavity (Guggenheim, 1968; Edwardsson, 1968; Nyvad, Kilian, 1990).
Actinomycetes are abundant in the human mouth and induce root surface caries in hamsters and gnotobiotic animals (Jordan, Keys, Bellack, 1972). They are also carbohydrate users, but are not powerfully acidogenic or acid-tolerant.
Summary of Current Review
Table 1. Studies on the association of microorganisms and dental caries
|
Bacterial Group |
Total |
Interventional |
Longitudinal/ |
Case-Control |
Cross-Sectional |
|
Mutans streptococci |
189 |
25 |
59 |
20 |
85 |
|
Sanguinis/other streptococci |
16 |
1 |
2 |
2 |
11 |
|
Enterococci |
3 |
0 |
0 |
0 |
3 |
|
Lactobacilli |
144 |
9 |
40 |
20 |
75 |
|
Actinomycetes |
27 |
1 |
3 |
3 |
20 |
Randomized Clinical Trials on Mutans Streptococci
Twenty-five interventional studies which monitored the putative cariogenic flora and recorded their effects on caries scores were found in the literature. Several of these applied extremely complex strategies (e.g., Gunay, Dmoch-Bockhorn, Gunay, et al., 1998). Some focused on mitigation of the solubility of the teeth with fluorides, some on repair or sealing of the teeth, some on diet management and/or use of sugar substitutes and thus indirectly on changing the implicated tooth surface flora, and some focused directly on the flora with mechanical plaque control or use of antiseptic agents.
Since the questions for the present review are more straightforward, those multistrategic studies confound interpretations of antibacterial effects with demineralization effects. It is understandable that investigators wish to accept this problem because of the ethical need to offer patients at high risk the best available anticaries strategies. Nonetheless, multistrategy approaches to experimental interventions set a very high threshold for detection of the effects of intervention on the flora and the attribution of anticaries responses to them. Some notable studies have been less confounded, however.
Partial suppression of mutans streptococci by topical chlorhexidine use and dietary counseling in randomized Swedish children (Zickert, Emilson, Krasse, 1983) inhibited mutans streptococcal recoveries and carious lesion development during 3 years, while lactobacillus titers in saliva were not detectably affected.
Treatment of primiparous mothers with 3- to 8-month-old infants in a Swedish community, alternately assigned to treatment or control groups, was aimed at reduction of mutans streptococcal salivary levels by sucrose avoidance counseling, professional toothcleaning (and topical fluoride application), oral hygiene instruction, and excavation of large carious lesions if present, and�if test mothers had salivary mutans streptococcal levels that exceeded a pre-set threshold�by treatment with topical chlorhexidine. This strategy increased the time to colonization by mutans streptococci of young children, time to caries experience of those children, and severity of caries experience of those children (K�hler, Andreen, Jonsson, 1984). There was no significant difference in salivary lactobacilli. Preventive strategies were discontinued when children were detected as colonized. The study ran until children were 36 months old. Four years later, when the children were 7 years old, treated mothers had lower mutans streptococci and lactobacilli than control mothers (K�hler, Andreen, 1994). Far lower percentages of children of treated mothers carried mutans streptococci compared with children of control mothers. The children of test mothers who were carriers also had lower levels of mutans streptococci than control children. Twenty-three percent of the children of test mothers were caries free, compared to 9 percent of the children of control mothers, and total group caries experience for test and control children was 5.2 vs. 8.6 def.
A similar strategy was used to treat 50 to 60-year-old Swedish patients of private dentists (Rask, Emilson, Krasse, et al., 1988). Two randomized groups of high and low risk patients (defined by salivary mutans, salivary flow rate, and buffer capacity) were assigned the test protocol or served as controls who were given standard care as deemed appropriate by their dentists. At year�s end, the treated high risk group had lower caries increments and lower mutans and lactobacillus titers than high risk controls, but there was no difference between the two low risk groups. The intervention was discontinued. Four years later there was no difference in microbiological parameters or caries increment between the treated and untreated high risk and low risk groups, and the one-year differential benefits of the test intercession had been lost.
A 3-year study (Gisselsson, Birkhed, Bj�rn, 1988) of 12-year old Swedish children, using an intervention of chlorhexidine-impregnated dental floss treatment of approximal surfaces compared with placebo-impregnated floss or no floss resulted in about a 50 percent reduction of new DFS in the chlorhexidine-floss compared with the placebo-floss group, and about a 60 percent reduction compared with the no floss group. Chlorhexidine-impregnated floss effects were about 42 percent better than placebo-floss. Salivary monitoring (rather than approximal plaque monitoring) found no differences among the groups, as could be expected.
A 3-year intensive program (Carlsson, Struzycka, Wierzbicka, et al., 1988) focused on personalized education, excavation of cavities, fluoride varnish, professional toothcleaning, and oral hygiene instruction. Study participants were randomized by school class and had group instruction on sugar avoidance, toothbrushing, fluoride toothpaste use, and were provided brushes. The personalized program resulted in about a six-fold decline of new DFS in 10 to 12-year-old Polish children and, after 3 years, significant reductions of mutans and lactobacillus salivary counts.
A 2-year randomized group study of 13-year-old Swedish children (Lindquist, Edward, Torrell, et al., 1989) compared supervised chlorhexidine gel treatment to fluoride varnish, topical FeAlF professional application, or an untreated control group. The antibacterial treatment resulted in about a 50 percent reduction of new DFS when compared with the untreated controls and lesser but still substantial and significant DFS reductions compared with the fluoride groups. There was correlated reduction of salivary mutans streptococci in the chlorhexidine group.
Finnish children 10 to12 years old were randomized to either high content xylitol gum use or not, during the first experimental phase (Isokangas, Tenovuo, S�derling, et al., 1991). Two years later, when the controls were randomly recruited for evaluation, it was found that some had begun to use xylitol gum. Approximal plaque mutans levels were lower in the xylitol users, and continuous users of xylitol gum had lower decay scores 6 years after the beginning of their use than nonusers. Mutans streptococci were lower at approximal sites that were clinically and radiographically sound than at decayed sites.
The use of a xylitol chewing gum by Finnish mothers (S�derling, Isokangas, Pienih�kkinen, et al. 2000; Isokangas, S�derling, Pienih�kkinen, et al., 2000) until their children were 3 years old was recently reported to inhibit the colonization of their children and reduce the caries experience of those children during a 5-year period of observation. Mothers were randomized to either xylitol gum use, chlorhexidine varnish, or fluoride varnish applications. The children did not use the gum or receive varnish treatments. The probability of being caries free was 70 percent for nonmutans colonized children compared to about 25 percent for mutans colonized ones at 5 years of age, and the group mean dmf score for the xylitol intercession cohort was 0.83, while scores for the chlorhexidine and fluoride varnish groups were 3.22 and 2.87, respectively.
Sixty-four longitudinal (prospective and retrospective) and case control studies indicate an important role of mutans streptococci in caries. They examined the relationship between salivary titers or plaque relative abundance of mutans streptococci (and often simultaneously quantified other bacteria, especially lactobacilli, actinomycetes, and sanguis streptococci) as well as inception, prevalence, or incidence of carious lesions. Many studies used randomized subjects, some being dental or medical patients; some subjects were almost totally naive dentally. Some studies have used population samples, and some compared cohorts with high or low caries experience, fluoridated or nonfluoridated communities, diverse racial/ethnic groups, diverse socioeconomic groups, diverse methods of paying for dental health care, ambulatory and nonambulatory health status, and diverse ages. The longitudinal, case-control, and cross- sectional (not discussed here) studies involved all of the continents except Antarctica. Several of these diverse studies are cited here (deStoppelaar, van Houte, Backer-Dirks, et al., 1969; Edwardsson, Koch, Obriuk, 1972; Loesche, Straffon, 1979; Alaluusua, Renkonen, 1983; Loesche, Eklund, Earnest, et al., 1984; Kristoffersson, Grondahl, Bratthall, 1985; Lang, Holtz, Gusberti, et al., 1987; Kingman, Little, Gomez, et al., 1988; Wilson, Ashley, 1989; Russell, MacFarlane, Aitchison, et al., 1991; Disney, Graves, Stamm, et al., 1992; Bjarnason, K�hler, Wagner, 1993; Schroder, Widenheim, Peyron, et al., 1994; Drake, Hunt, Beck, et al., 1994; Alaluusua, Malmivirta, 1994; Sigurjons, Magnusdottir, Holbrook, 1995; Hallonsten, Wendt, Mejare, et al., 1995; Grindefjord, Dahloff, Nilsson, et al., 1995, 1996; Twetman, Petersson, 1996).
These and other reports, with few exceptions, support a strong positive statistical association of mutans streptococci with inception or incidence of carious lesions. They often report concomitant positive associations with lactobacilli, especially if saliva, rather than discrete plaque samples, were monitored. They sometimes reported negative associations of sanguinis streptococci with mutans streptococci and with lesions. Some suggest that S. sobrinus are favored in their ability to colonize by preexisting S. mutans colonization. There is also suggestion of an association between S. sobrinus and lactobacilli.
These studies often gathered data on other variables of interest � socioeconomic status, sucrose consumption (usually as food types or patterns of consumption), fluoride exposure, oral hygiene status, breast feeding or close personal contact between mothers and their children, and, especially, initial caries status. Some studies asked the clinical examiners to predict the decay experience of study participants.
Some of these studies focused on a related question�the prediction of caries as a function of the sum total of all or many of the variables of interest to cariologists�rather than the microbiological variables targeted in this review. When predictive values were estimated and when multiple regression models included other caries-associated variables (such as candy or soft drink consumption, oral hygiene, SES, and, especially, prior numbers of lesions) and included them in the prediction model, the amount of variance explained by the bacteria of interest became predictably smaller. Prediction of the dependent variable (caries score) by inclusion of the baseline caries score as an independent variable appears inherently tautological in the context of explaining causation of the disease (and is arguably a post hoc, ergo propter hoc problem).
Discernment of microbial etiology from several longitudinal (and cross-sectional) studies was undoubtedly blunted by using salivary (or pooled plaque) monitoring of mutans streptococci as a surrogate for small samples of plaque in areas of high caries risk, as knowledge of the biology of mutans streptococci and expected locations of carious lesions would have seemed to dictate.
Lactobacilli. All of the concerns about confounding and the ambiguity of interpretation in interventional clinical trials stated above for mutans streptococci are applicable to lactobacilli as well. Several of the random clinical trials that yielded data on mutans streptococci also evaluated changes in lactobacilli. Generally, they resulted in inconsistent evidence that inception of carious lesions in children or decreases of incidence were associated with lactobacillus titer changes in saliva (K�hler, Andreen 1994; Rask, Emilson, Krasse, et al., 1988; Carlsson, Struzycka, Wierzbicka, et al., 1988; Lindquist, Edward, Torell, et al., 1989).
Longitudinal and case-control studies are perhaps more informative. Lactobacilli are late colonizers of the mouth (Hemmens, Blayney, Bradel, 1946; van Houte, Gibbons, Pulkkinen, 1972; Carlsson, Grahnen, Jonsson, 1975; Schroder, Widenheim, Peyron, et al., 1994; Babaahmady, Challacombe, Marsh, et al., 1998). Lactobacilli are recovered from carious lesions, but they are later colonizers of those lesions than mutans streptococci (Loesche, Eklund, Earnest, et al., 1984; Crossner, Claesson, Johansson, et al., 1989; Holbrook, de Soet, de Graaff, et al., 1993). Some data suggest that they are favored in their ability to colonize by preexisting colonization by mutans streptococci, especially S. sobrinus. These data thus indicate that lactobacilli are not requisite for the development of lesions. Nonetheless, they may potently contribute to demineralization of the teeth once lesions are established on either crowns or roots (Boyar, Bowden, 1985; Ravald, Hamp, Birkhed, et al., 1986; Fure, Romaniec, Emilson, et al., 1987; Scheinin, Pienih�kkinen, Tiekso, et al., 1994; Grindefjord, Dahllof, Nilsson, et al., 1995; Mazengo, Tenovuo, Hausen, et al., 1996; Fure, 1998). Little information is available concerning the species of lactobacilli that colonizes the human tongue and teeth.
Nonmutans Streptococci. Essentially no data support a causative role for sanguinis streptococci or S. salivarius in human caries. In fact, some data suggest an inverse relationship in the abundance of sanguinis streptococci and mutans streptococci, and also that sanguinis streptococci are inversely related to lesion development (deStoppelaar, van Houte, Backer-Dirks, et al., 1969; Loesche, Straffon, 1979; Bowden, Ekstrand, McNaughton, et al., 1990; Emilson, Ravald, Birkhed, et al., 1993).
Enterococci. Essentially no human data support a significant role of enterococci in the development of human carious lesions or in their prevalence in the human mouth.
Actinomycetes. Actinomycetes are prevalent in the human mouth and are frequently found in association with both carious and sound root surfaces, as well as sound crown surfaces. Evidence of their role in root surface carious lesion induction from interventional, longitudinal, case-control, or cross-sectional data is variable and inconclusive. In fact, these data sometimes suggest that actinomycetes are more reflective of noncariogenic than cariogenic status, in contrast with mutans streptococci and lactobacilli.
Just as modern molecular and genetic methods are now used in forensic science, they are also used to trace the spread of infection. They provide perhaps the strongest evidence of the source of transmission of infection. That evidence will be briefly abstracted here. Nonetheless, other evidence of the source of transmission of the bacteria etiologically involved in caries from experimental and longitudinal studies is consistent with even more compelling genetic investigations. Convincing data on the source of infection by cariogenic bacteria almost entirely pertain to mutans streptococci (see table 2).
Table 2. Studies on the transmission of bacterial species implicated in dental caries
|
Bacterial Group |
Total |
Molecular and genetic tracing: bacteriocin/ |
Interventional |
Longitudinal/ Case-Control |
Cross-Sectional |
|
Mutans streptococci |
40 |
17
|
8 |
13 |
1 |
|
Sanguinis/other streptococci |
1 |
0 |
0 |
1 |
0 |
|
Enterococci |
0 |
- |
- |
- |
- |
|
Lactobacilli |
7 |
- |
4 |
3 |
0 |
|
Actinomycetes |
0 |
- |
- |
- |
- |
Study of mutans streptococci isolated from children and their parents/siblings/caretakers as to bacteriocin typing, phage typing, mutacin typing, endonuclease DNA mapping, and ribotyping establish that these bacteria are transmitted to humans early in their lives, mainly from their mothers (Berkowitz, Jordan, 1975; Berkowitz, Jones, 1985; Caufield, Ratanapridakul, Allen, et al., 1988; Kulkarni, Chan, Sandham, 1989; Caufield, Walker, 1989; Li, Caufield, 1995; Emanuelsson, Li, Bratthall, 1998; Redmo Emanuelsson, Wang, 1998; Gronroos, Saarela, Matto, et al., 1998). Only two reports suggest significant patrilineal transmission. While it is common for children to share more than one genotype or bacteriocin type of mutans with their mothers, failure to detect all of the types among mother/child pairs suggests that some may be lost with time. New genotypes have been reported to colonize children during longitudinal studies, suggesting that extrafamilial transmission also occurs.
Longitudinal study of children led investigators to propose the existence of a "window of infectivity" by mutans streptococci (Caufield, Cutter, Dasanayake, et al., 1993), but that concept does not appear well-supported. Children become colonized both before and after the "window" period (Aaltonen, Tenovuo, 1994; van Loveren, Buijs, Bokhout, et al., 1998; Straetemans, van Loveren, de Soet, et al., 1998; Mohan, Morse, O�Sullivan, et al., 1998). Also, as reported in essentially all of the studies of adults (cited above), virtually all dentate adults appear colonized to some degree by mutans streptococci. There are likely to be other events of transmission or, alternatively, the methods historically used to cultivate mutans streptococci may fail to detect transmission which has in fact occurred.
Interventional studies of transmission are clearly inhibited by the ethical impossibility of exchanging children with mothers shortly after birth. Nonetheless, controlled experiments aimed at altering the probability of transmission of mutans streptococci from mothers to their children support the concept that the mother is the usual source of transmission to her child (K�hler, Andreen, 1994; Brambilla, Felloni, Gagliani, et al., 1998; S�derling, Isokangas, Pienih�kkinen, et al., 2000).
There are few data on the source of transmission of lactobacilli to children. Despite the use of very specific selective media for the cultivation of lactobacilli, speciation of them is laborious and is usually not done in an epidemiological context. Also, the literature does not yield studies of the genetics of the lactobacilli in the mouth, vagina, and gastro-intestinal tract of mothers and their children. It is clear that while lactobacilli can be found in the mouths of infants, they appear to be transient and are not a common feature of the oral cavity until after teeth erupt or obturators are placed for cleft palate management. There is even less information on the source of colonization of the mouth by sanguinis group streptococci, enterococci, and actinomycetes. S. salivarius is long known to colonize the mouth, usually within a day of birth.
Conclusion
Evidence from the current review strongly supports a central role of the mutans group of streptococci in the initiation of caries on the smooth surfaces and fissures of the crowns of the teeth of adults and children, and suggests that they have a potent etiologic role in the induction of root surface caries. Lactobacilli are also implicated as important contributory bacteria in tooth decay, but their role in induction of lesions is not well supported. Evidence that other streptococci, enterococci, or actinomycetes are prominent etiological agents of dental caries in humans is equivocal at best. The mutans streptococci are spread vertically in the population, mostly but not exclusively from mothers to their children.
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