Summer Science Academy Experiment:

Normal Throat Flora


GRADES: 6-12


TIME NEEDED: 15 minutes (day 1); 30 minutes (day 2); 1 hour (day 3)

John Donne's famous phrase, "No man is an island unto himself", has biologic as well as philosophic meaning, for no living organism can survive without support from other life. In the upper respiratory tract, the mouth, throat, and nose offer fertile conditions for the survival and growth of many kinds of microorganisms. Organisms living on skin and mucosal surfaces are referred to as normal flora. Normal flora, though usually remaining relatively constant, may fluctuate in response to host activities, habits, and external influences. Microorganisms that are usually isolates from throats of healthy persons include species of Staphlococci, Streptococci, Corynebacteria (diptheroids), Neisseria, and Branhamella. Most of them are harmless; some are potential pathogens; some virulent organisms may be present but not producing diseases.


In this experiment, you will isolate and characterize microorganisms growing in your throat.


Materials Needed


Sterile tongue depressor

sterile cotton swab

blood agar plates

sterile inoculating loops


Experimental Procedure


Day 1:


Each student will swab his/her partners throat. Use a tongue depressor to hold down the tongue to avoid contaminating the swab with flora of the mouth. Use a sterile cotton swab to sweep the tonsils (ask an instructor if you are unsure of where to wipe the swab). After obtaining the throat specimen, roll the swab back and forth several times on one section of the blood agar plate. Use a sterile inoculating loop to streak out an area of the culture (see diagram). This technique will hopefully assure that the culture will show isolated colonies, instead of just a big smear of many colonies. Incubate the plate overnight at 37o C.


Day 2:


1. Examine your throat culture. Be sure to look through the plate to note any change in the sheep red blood cells of the medium. Some organisms will cause a total clearing of the red blood cells; this is referred to as beta hemolysis. Some organisms will cause an incomplete change or greening; this is referred to as alpha hemolysis. The normal flora streptococci usually produce alpha hemolysis.

2. Note and draw the different types of colonies.

3. Use a sterile inoculating loop to touch a colony and streak it onto a sector of a blood agar plate. Repeat this with each different type of colony present. USE A NEW LOOP FOR EACH COLONY. This will allow you to isolate each colony as a pure culture. Incubate this plate at 37° C overnight.


Day 3:


Perform a Gram stain on each of your pure cultures, using your plate from yesterday. Record the results of the Gram stain for each of the different colonies tested.