32: Microbiology of dental caries

Chapter 32 Microbiology of dental caries

Dental caries is a chronic endogenous infection caused by the normal oral commensal flora. The carious lesion is the result of demineralization of enamel – and later of dentine – by acids produced by plaque microorganisms as they metabolize dietary carbohydrates. However, the initial process of enamel demineralization is usually followed by remineralization, and cavitation occurs when the former process overtakes the latter. Once the surface layer of enamel has been lost, the infection invariably progresses to dentine, with the pulp becoming firstly inflamed and then necrotic.

Caries is defined as localized destruction of the tissues of the tooth by bacterial fermentation of dietary carbohydrates.

Epidemiology

Dental caries (with periodontal disease) is one of the most common human diseases and affects the vast majority of individuals. Although caries was not uncommon in the developing world, the recent affluence in these regions has resulted in a remarkable upsurge in caries due to the ready and cheap availability of fermentable carbohydrates. In contrast, caries prevalence is falling overall in the developed world due to the increasing awareness of cariogenic food sources and the general improvement in oral hygiene and the dental care delivery systems. Caries of enamel surfaces is particularly common up to the age of 20 years, after which it tends to stabilize. However, in later life, root surface caries becomes increasingly prevalent, due to gingival recession, exposing the vulnerable cementum to cariogenic bacteria.

Classification

Dental caries can be classified with respect to the site of the lesion (Fig. 32.1):

Fig. 32.1 Nomenclature of dental caries. D, dentine; E, enamel; P, pulp. *Also termed occlusal caries.

• pit or fissure caries (seen in molars, premolars and the lingual surface of maxillary incisors)

• smooth-surface caries (seen mainly on approximal tooth surfaces just below the contact point)

• root surface caries (seen on cementum or dentine when the root is exposed to the oral environment)

• recurrent caries (associated with an existing restoration).

Clinical presentation

The primary lesion of caries is a well-demarcated, chalky-white lesion (Fig. 32.2) in which the surface continuity of enamel has not been breached. This ‘white-spot’ lesion can heal or remineralize, and this stage of the disease is therefore reversible. However, as the lesion develops, the surface becomes roughened and cavitation occurs. If the lesion is not treated, the cavitation spreads into dentine and eventually may destroy the dental pulp, finally leading to the development of a periapical abscess and purulent infection (see Chapter 34).

Fig. 32.2 Polarized light microscopic appearance of early enamel caries (ground section). The cone-shaped body of demineralization is evident.

Diagnosis

Diagnosis is usually by a combination of:

1. Direct observation.

2. Probing. Some do not advocate probing as this may create an incipient breach of the enamel and spread the infection from one tooth surface to another.

3. Radiographs. Early white-spot lesions may easily be missed because they cannot be detected by the eye or by radiography. Similarly, it is possible for large carious lesions to develop in pits and fissures with very little clinical evidence of disease.

4. Experimental methods. Methods of potential practical value include laser fluorescence for diagnosis of buccal and lingual caries, and electrical impedance (resistance) to detect occlusal caries.

5. Microbiological tests may be helpful in the assessment of caries (see below).

Aetiology

The major factors involved in the aetiology of caries (Fig. 32.3) are:

Fig. 32.3 Interplay of major aetiological factors in dental caries (all four factors must act simultaneously for caries to occur).

• host factors (tooth, saliva)

• diet (mainly the intake of fermentable carbohydrates)

• plaque microorganisms (i.e. supragingival plaque).

Host factors

Tooth structure

The structure of enamel, and of dentine in root caries, is important: some areas of the same tooth are much more susceptible to carious attack than others, possibly because of differences in mineral content (especially fluoride).

Flow rate and composition of saliva

The mechanical washing action of saliva is a very effective mechanism in the removal of food debris and unattached oral microorganisms. It has a high buffering capacity, which tends to neutralize acids produced by plaque bacteria on tooth surfaces, and it is supersaturated with calcium and phosphorus ions, which are important in the remineralization of white-spot lesions. Saliva also acts as a delivery vehicle for fluoride.

Diet

There is a direct relationship between dental caries and the intake of carbohydrates. The most cariogenic sugar is sucrose, and the evidence for its central role in the initiation of dental caries includes:

• increases in the caries prevalence of isolated populations with the introduction of sucrose-rich diets

• clinical association studies

• short-term experiments in human volunteers using sucrose rinses

• experimental animal studies.

Sucrose is highly soluble and diffuses easily into dental plaque, acting as a substrate for the production of extracellular polysaccharides and acids. Cariogenic streptococci produce water-insoluble glucan from sucrose, which, in addition to facilitating initial adhesion of the organisms to the tooth surface, serve as a nutritional source and a matrix for further plaque development. The relationship between sucrose and dental caries is complex and cannot be simply explained by the total amount of sugar consumed. The frequency of sugar intake rather than the total amount of sugar consumed appears to be of decisive importance. Also relevant are the stickiness and concentration of the sucrose consumed, both factors influencing the period for which sugar is retained in close contact with the enamel surface.

Carbohydrates other than sucrose, e.g. glucose and fructose, are also cariogenic, but less so than sucrose. Polyol carbohydrates, ‘sugar alcohols’ (e.g. xylitol), with low cariogenicity have been produced and are sought after as sugar substitutes in products such as chewing gum and baby foods.

Microbiology

Microorganisms in the form of dental plaque are a prerequisite for the development of dental caries. The different types of plaque and the factors involved in their development are described in Chapter 31.

Specific and non-specific plaque hypothesis

Although mutans streptococci have been recognized as the major group of organisms involved in caries, there is some controversy as to whether one or more specific groups of bacteria are principally involved in caries – the specific plaque hypothesis – or whether the disease is caused by a heterogeneous mixture of non-specific bacteria – the non-specific plaque hypothesis.

There is conflicting opinion for and against the specific plaque hypothesis:

• mutans streptococci are involved in the initiation of almost all carious lesions in enamel

• mutans streptococci are important, but not essential

• the association of mutans streptococci and caries is weak and no greater than for other bacteria.

Given the extreme variation in the composition of supragingival plaque from the same site in the same mouth at different times, it is unlikely that the initiation and progression of all carious lesions are associated with specific organisms such as Streptococcus mutans. Further, other plaque bacteria also possess some of the biochemical characteristics thought to be important in cariogenicity. Therefore, it seems likely that combinations of bacteria other than mutans streptococci and lactobacilli may be able to initiate carious lesions, and the plaque flora may be non-specific in nature. The current evidence implies that some bacteria (mutans streptococci, Lactobacillus spp. and Actinomyces spp.) may be more important than others in the initial as well as subsequent events leading to both enamel and root surface caries.

The role of mutans streptococci

There is a vast literature on the role of the mutans streptococci in caries. ‘Streptococcus mutans’ is a loosely applied group name for a collection of seven different species (S. mutans, S. sobrinus, S. criceti, S. ferus, S. ratti, S. macacae and S. downei) and eight serotypes (a–h). S. mutans serotypes c, e, f and S. sobrinus serotypes d, g are the species most commonly found in humans, with serotype c strains being the most prevalent, followed by d and e. The others are rarely encountered. The evidence for the aetiological role of mutans streptococci in dental caries includes the following:

• correlations of mutans streptococci counts in saliva and plaque with the prevalence and incidence of caries

• mutans streptococci can often be isolated from the tooth surface immediately before the development of caries

• positive correlation between the progression of carious lesions and ‘S. mutans’ counts

• production of extracellular polysaccharides from sucrose (which help to cement the plaque organisms together and to the tooth surface)

• most effective streptococcus in caries studies in animals (rodents and non-human primates)

• ability to initiate and maintain microbial growth and to continue acid production at low pH values

• rapid metabolism of sugars to lactic and other organic acids

• ability to attain the critical pH for enamel demineralization more rapidly than other common plaque bacteria

• ability to produce intracellular polysaccharides (IPSs) as glycogen, which may act as a food store for use when dietary carbohydrates are low

• immunization of animals with specific S. mutans serotypes significantly reduces the incidence of caries.

Note: not all strains of mutans streptococci possess all of the above properties; thus, some strains are more cariogenic than others. Caries may therefore be an infectious disease in a minority, with a highly pathogenic strain being transmitted from one individual to another. Despite this apparently strong relationship between S. mutans and caries, a number of longitudinal studies in children have failed to find such a strong correlation.

The role of lactobacilli

Lactobacilli were previously believed to be the causative agents of dental caries. They were considered to be candidate organisms for caries because of:

• their high numbers in most carious lesions affecting enamel (many studies have now shown its high prevalence in root surface caries too)

• the positive correlation between their numbers in plaque and saliva and caries activity

• their ability to grow in low-pH environments (below pH 5) and to produce lactic acid

• their ability to synthesize both extracellular and IPSs from sucrose

• the ability of some strains to produce caries in gnotobiotic (germ-free) rats

• the fact that their numbers in dental plaque derived from healthy sites are usually low.

On the negative side, however, lactobacilli are rarely isolated from plaque before the development of caries, and they are often absent from incipient lesions.

Although the role of lactobacilli in the carious process is not well defined, it is believed that:

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Tags: Essential Microbiology for Dentistry 4e

Jan 4, 2015 | Posted by mrzezo in General Dentistry | Comments Off

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