Dental caries results from microbial acid and plaque formation. Plaque sets the stage for caries because it collects the acid forming bacteria on the tooth surface, supplies an anaerobic environment for fermentation, traps the acids and excludes the protective saliva. Caries lesions are basically the outcome of chemical attack on the enamel and dentin. Demineralization of the tooth alternates with periods of re-mineralization. If demineralization exceeds re-mineralization, a subsurface carious lesion becomes a clinical cavity with extension of the decay into the dentine (1). For determining the rate and amount of acid produced by microorganisms in saliva, Snyder (2, 3) described a colorimetric method. The procedure makes use of an agar medium that is known as Snyder Test Agar. Later on Alban (4) modified the procedure and reported it to be more accurate than the original procedure.
This is a differential medium based on the rate of acid production from dextrose, by oral acidogenic microorganisms from buccal cavity and is evidenced by a change in color of the indicator - bromo cresol green from blue-green to yellow (3). Peptic digest of animal tissue provides carbon, nitrogen, vitamins and minerals. Dextrose is the carbohydrate source and bromo cresol green is the pH indicator.
Snyder Test Procedure:
Collect specimens of saliva before breakfast, before brushing the teeth or just before lunch or dinner. Collect specimen of saliva in a sterile tube or bottle after patient chews paraffin for 3 minutes. Shake the specimen thoroughly and transfer 0.2 ml of this to a sterile Snyder Test Agar tube melted and cooled to 45°C. Mix the inoculum by rotating the inoculated tubes and incubate at 37°C for 72 hours in an upright position. The rate of acid production is graded as, marked for 24 hours, moderate and slight if color changes within 48 and 72 hours respectively (5).
| Incubation hours | Color | Caries activity |
| 24 | yellow | marked |
| 48 | greenish yellow | moderate |
| 72 | yellowish green | slight |
Alban Modified Test Procedure:
Collect the saliva specimen (unstimulated) to just cover the medium in the tube. When specimen collection is difficult, dip a sterile cotton swab into the saliva under the tongue or rub on tooth surfaces and place the swab just below the surface of the medium. Incubate the tubes at 35°C along with uninoculated control. Examine tubes daily for 4 days and compare the color change with the control tube.
Record the results as:
| No color change as negative | = - |
| Color beginning to change to yellow | = + |
| Half medium yellow | = ++ |
| Three fourths of medium yellow | = +++ |
| Total medium yellow | = ++++ |
The daily readings indicate the rapidity and amount of acid production. To establish a reference point at least two specimens collected within 2-4 days must be obtained.
Storage and Shelf-life:
Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on label.
References:
1. Lewis and Ismail, 1995, Can. Med. Assoc. J., 152:836.
2. Snyder, 1941, J. Dent. Res., 20:189.
3. Snyder, 1941, J. Am. Dent. Assoc., 28:44.
4. Alban, 1970, J. Dent. Res., 49:641.
5. MacFaddin J. F., 1985, Media for Isolation-Cultivation-Identification-Maintenance of Medical Bacteria, Vol. I, Williams & Wilkins, Baltimore, Md.