Introduction
Receptivity to caries means the predisposition of an individual to be affected by a cariogenous pathology, regardless of the fact that a caries may be detected or not during dental examination. A diagnosis of "presence of caries" assumes the existence of a clinical ability and depends on the operator's skills; similarly, an "early diagnosis of caries" depends more directly on the technical means at our disposal (e.g. radiographic techniques), On the contrary, the diagnosis of "receptivity to caries" precedes all these moments, and allows to identify the patient and to implement the right preventive means, prior to the actual illness. In this situation, saliva may constitute an instrument for the diagnosis of receptivity to caries.

In general, the salivary secretion has become important in the medical field because it is a potential source of biochemical information in certain pathological settings, and as a biological sample it is easy to detect, without the need for any invasive manoeuvre.

Biochemical parameters that have been considered so far include: proteins, electrolytes, lysozyme, alfa-amylase isoenzymes, uric acid, immunoglobulins, ABH antibodies, toxic heavy metals such as lead and selenium, steroids and various medications. The medical fields in which the study of salivary secretion is both important and useful are the study of isoenzimes and of alfa-amylase, the monitoring of medications and the analysis of steroids.

In the odontoiatric field, the saliva is important under many aspects, as it is one of the main components of the oral environment; nonetheless, its role is not often adequately considered, even by the professional people who have a daily and direct contact with it. Also of interest are the relationships between the saliva and some of the odontoiatric illnesses.

A number of studies on the potential relationship between saliva and the carious pathology has been conducted for many years, but research in this field has fully developed only in the last decade. From this point of view, the most important physicochemical and biological properties of the saliva are: the salivary flow, the buffering capacity, the bacterial concentration of specific strains and the comparison between ratings of caries-sensitive versus caries-resistant individuals.

The salivary flow and its significance
Ninety per cent of the salivary flow is due to the parotid and submandibular glands in an almost identical proportion, another 5 % derives from sublingual glands and the remaining 5 % is due to the minor salivary glands. The control of the salivary glands pertains to a nervous center, that is located in the medulla oblungata and is formed by the superior and inferior salivary nucleus. The flow varies around the 24 hours, from 200 ml to 2500 ml, with a mean of 700-800 ml.

Secretory stimuli are mainly the product of two types of reflex and involuntary excitation, a gustative one through the lingual taste-buds and a masticatory one through specific receptors that are located in the parodontal ligament and in the masticatory muscles (Johnson et al. 1987). Under resting conditions, i.e. in the absence of gustative stimuli, there is always a salivary secretion. This "basic" flow is influenced by various factors, with marked individual variations. This type of saliva is more fluid and is secreted in order to maintain a humid environment and to counteract the action of irritants agents, and is therefore very important from a functional point of view.

The saliva originating from stimulation is secreted under involuntary stimuli that are induced by sense of hunger and during the masticatory act; this type of saliva is denser than the former one. The saliva secreted under resting conditions shows noticeable modifications in the flow entity and follows a circadian cycle. Secretion typically increases in the post-meridiem hours and is reduced during night hours. The low secretion at nighttime prevents from frequent swallowing motions during sleep. However, salivary defenses -i.e. the detergent power and the buffering capacity towards the acid environment that is inevitably created- are also reduced, resulting in a progressive lowering of the oral pH. This fact can facilitate the onset of carious processes (Salzmann, 1980).

The salivary flow has an important role in the removal of food debris, bacterial residues and epithelial cells that constitute a pabulum for the bacterial proliferation, by implementing their elimination through the digestive canal (Dawes et al., 1989). It has been demonstrated that a reduction in the salivary secretion has a clear-cut effect in favoring the number and gravity of caries (Steinman et al., 1980): this is also evident in the case of salivary hypo-secretion due to xerostomia, or following radiotherapy for glandular or facial neoplasias (Epstein and Schubert, 1987).

It is commonly accepted that the salivary flow decreases with age (Baum, 1908). Symptoms of dry mouth are more frequent among elderly people; in this respect, some of the illnesses that can cause a reduction of the salivary flow, as Alzheimer disease, are typical of elderly (Ship et al., 1990).

A higher frequency of caries seems to occur in anxious individuals and in patients who are taking antidepressant medications. These conditions are known to cause an involvement of the sympathetic system with subsequent reduction of the salivary flow (Costa et al., 1980; Morse et al., 1981). Due to the severe xerostomia, patients with Sjogren syndrome are at particularly high risk of developing caries.

The buffering capacity
The buffering capacity of saliva is due to its ability to equilibrate the acid environment that inevitably tends to form in the oral cavity, because of the intrinsic acidity of some foods and of the acids produced by acidogenic microorganisms. This property is related to its content of bicarbonate and to the presence of phosphates and amphoteric proteins (Dibdin and Shellis, 1989).

The bicarbonate/CO2 system is the most important salivary buffering system acting at normal pH levels, while the amphoteric proteins seem to work at lower pH levels. The phosphate/HPO4 system has a minor role. The salivary amino acids and peptides are also important in the pH regulation, by producing an increase of it (Andersson et al., 1984). The same is true for the urea, that can neutralize acids after being converted to ammonia.

Some authors have found a superior buffering capacity in the saliva of caries-resistant subjects, compared to people affected by several carious processes. According to Ericson and Makinen (1986), measuring the buffering capacity of saliva, together with other indicators of potential increase of cariogenous risk, e.g. an excess presence of Streptococcus mutans and Lactobacilli, can be very helpful in the diagnosis of caries-receptivity.

The oral bacterial flora
A great number of bacterial species can be found in the saliva in an equilibrium state. When this equilibrium is altered by multiple causes, a number of diseases may develop. A great number of bacterial species may be related to the carious pathology. Their presence also varies with the anatomical site: for example, Streptococcus salivarius can be found in caries involving the cervical third of a tooth.

The two most important bacterial species are Streptococcus mutans and Lactobacilli, which can produce substantial quantities of acid. If a metabolic substrate enhances their acidogenic activity, the saliva and its control mechanisms may not be able to compensate it any further. In this case, the oral environment reaches critical acidity levels (pH around 4.8), that cause the demineralization of the tooth hard-tissue. Therefore, it is possible to observe the onset of those processes, that - if not early identified or prevented - may lead to caries formation.

In a predisposed subject, microbiological tests are therefore of great importance. These tests estimate the concentration of S. mutans and Lactobacilli in the oral cavity, as their number is directly related to the likelihood of developing a caries.

Methods
Salivary testing is routinely performed in the clinical practice. A typical kit used by laboratory technicians includes a plastic pipette, that draws the patient's saliva from a graduated cylinder and allows the measurement of the salivary flow (expressed as ml of saliva produced in 5 minutes). Normal readings should be in the 1 ml/min range.

The buffering capacity is determined by leaving a drop of saliva on a special 3.3 pH titrated test strip for 5 minutes. The strip colour is then determined using a colorimetric scale on a comparison chart, and the correspondence with one of the three reported examples (yellow = pH 3.3; green = pH 5.5; blue = pH > 7) is assessed.

The salivary secretion is then placed in a cylinder containing an agar-based culture medium. Bacytracine is used to make the medium selective for the micro-organism to be analyzed. The tube is then placed in an incubator at 37° C for 48 hours. After this time, results are evaluated by comparing the density of bacterial colonies that have developed with a comparison chart. For the two species considered, normal readings should be = / < 105 cfu (colony forming unity)/ml).

Evaluation of the results
An excess amount of Lactobacilli in the saliva indicates a risk factor for the patient, owing to a diet too high on sugar, combined with poor oral hygiene. This bacterial species is responsible for the synthesis of acids that contribute to oral pH lowering.

An excess presence of Streptococci, i.e. the bacteria more directly related to the pathogenesis of the carious process, indicates a higher-than-normal amount of bacterial plaques, that in turn is related either to an individual predisposition or to poor oral hygiene.

Test implementation and evaluation of the three indicators considered (flow, buffering capacity, bacterial concentration) allow the dentist to identify subjects who are at low or high risk of caries, i.e. to formulate a diagnosis of receptivity to caries. The former individuals need not necessarily to modify their habits, and should simply undergo periodic check-ups by their dentist. On the contrary, patients at high risk should follow an intensive and effective prevention regimen. In case of a reduction of the salivary flow, it is imperative to investigate on the possible causes of hyposecretion (e.g. medications, Sjogren syndrome); at the same time, patients should limit the sugar intake, by following a fibre-rich diet, and by using sugar-free chewing-gum to stimulate salivation. A program of fluoride prophylaxis should be also implemented, according to the following conditions:

• normal fluoride prophylaxis (fluoride toothpaste)
• mean fluoride prophylaxis (fluoride toothpaste + fluoride gel)
• intense fluoride prophylaxis (fluoride toothpaste + fluoride gel + professional application of fluoride varnish)

The patient should be notified of the testing results by a written report indicating his values and the reference ones; he should be also informed on the therapy plan and on the frequency of the necessary check-ups.

On this basis, the importance of a close relationship between the dentist and the internist or any other specialist (pediatrician, geriatrician, diabetologist, etc.) should be stressed, in order to identify and analyse any at-risk condition (diabetes, radiotherapy, xerostomy-inducing syndromes, etc.).