MOUTH FLORA:  SUSCEPTIBILITY TEST

PREPARATION

Provide a tube of Snyder Test Agar for each student that has been boiled for 10 minutes then held in a water bath at 45o C.  Block of paraffin/or sugarfree gum for each student.  Sterile container for saliva.  Sterile 1 ml pipette for each student.

STUDENT INSTRUCTIONS

The human mouth provides a warm, moist environment for many species of organisms.  Within your mouth lives a tremendous amount of microorganisms, including fungi, protozoa, viruses and bacteria.

Dental caries or cavities are caused by bacteria and saliva is known to contain over a million bacteria per milliliter.  All cavities begin with the formation of plaque.  Plaque is a gummy substance on the surface of the enamel that is a mixture of various bacteria and the end products of carbohydrate hydrolysis and fermentation.  The production of acids by these bacteria causes tooth decay.  At a pH of 5.5 or lower, demineralization of the tough, protective enamel begins.

Streptococcus and lactobacilli are two of the biggest acid producers and it now seems that Streptococcus mutans seems to be the most important one and is usually the one that initiates tooth decay.

In this exercise you are estimating your susceptibility to dental caries by measuring the rate at which your own oral lactobacilli generate acid.

The low pH and reduced environment provided in Snyder Test Agar inhibits most oral flora.  Lactobacilli thrive, producing acid from the medium’s dextrose.  If your lactobacilli drop the pH of the medium below about 4.6, the bromcresol green indicator changes from green to yellow.  This color change shows that your flora produce enough acid to decalcify teeth procedure.
 

  1. Obtain a tube of liquified Snyder Test Agar that has been boiled for 10 minutes and then cooled to 45* C.
  2. Obtain a piece of sugarfree gum or paraffin and chew for 3 minutes without swallowing.  Collect your saliva in a sterile container.  Chewing removes the bacteria from your teeth.
  3. Shake your saliva to resuspend the microorganisms.
  4. With a 1ml pipette, transfer 0.2 ml of saliva to tube of test agar.
  5. Mix the contents of the tube by rotating the tube vigorously between the palms of the hands.
  6. Incubate the tube at 37* C.  Examine the tube at 24-hour intervals to see if the bromcresol green indicator has changed to yellow.  If the medium turns yellow in 24-48 hours the individual is said to susceptible to caries.