THE VIRTUAL EDGE: Lab 25 Environmental Microbiology III and Food Microbiology I

Caries Susceptibility Test:

(Adapted from I. Edward Alcamao, Laboratory Fundamentals of Microbiology, 4th ed.)

Members of the bacterial genus Lactobacillus are characterized by having a variable morphology ranging from long and slender rods to short coccobacillary forms.  Interestingly, the bacteria are Gram-positive in young, active cultures but become Gram-negative with age.   As a group, Lactobacillus are most noted for their role in the production of many foods such as dairy products (cheeses, yogurt, acidophilus milk), fermented vegetable foods (sauerkraut, pickles), beverages (beer, wine, juices), and sour dough.  Lactobacilli also are part of the normal flora of the human body in the mouth, intestinal tract and genitourinary tract of females.

A significant correlation has been noted between the amount of Lactobacillus in the saliva and an individual's susceptibility to dental caries (cavities).  An estimate of the Lactobacillus population may be obtained by incubating saliva in a selective medium and noting the rate of acid production as a function of incubation time.  The procedure is named the Synder test after its developer, Marshall L. Snyder. 

The test involves the growth of lactobacilli in Snyder test agar.  This is a selective medium that has a low pH (pH 4.8) to inhibit the growth of other bacteria and the pH indicator bromcresol green.  The indicator is green at pH 4.8 and yellow at pH 4.4 and below.  As lactobacilli proliferate, the acid they produce causes a decrease in the pH and an accompanying color change.  By reference to a standard table, an estimate of the Lactobacillus population may be made.


In today's lab we will perform a Synder test on our own saliva using the following procedure.

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