BIOL 260 Microbiology
This lab is designed to demonstrate how an epidemiologist might track the spread of a communicable
disease back to the original source.
1. Each person will receive a hard-candy that is wetted with a broth medium and identified with a
number. Write down your 1.D. number on your data sheet.
2. Put on a pair of clean disposable gloves. Pick up the candy with one hand, preferably the one
you do not write with, and thoroughly smear the palm and fingers of that glove. Drop the
candy back into its original dish.
3. One person in the class will unknowingly receive a candy that has been contaminated with the
bacteria Serratia marcescens. The presence of this bacterium represents a communicable
disease. This bacterium is a mild pathogen. Touch as few things as possible once you have
exposed your glove to the candy.
4. You will shake gloved hands with four different people. Keep track of their I. D. numbers and the
order in which you came in contact with them. Make sure the gloves are well contacted: smear
the palms together as well as possible.
5. After shaking hands with all four contacts, rub the palm and fingers of your glove on the surface
of a sterile petriplate containing nutrient agar. Try not to break the surface of the agar but have
as much of the glove as possible contact the agar. Remove your gloves by turning them inside
out as you pull them off your hands. Dispose of them in the “BIOHAZARD” waste containers
only. Dispose of your candy, dish and everything used in this part of the lab into the
“BIOHAZARD” waste containers only.
6. Incubate your plate overnight at 30°C. Serratia marcescens is a species of Gram-negative
bacteria that produces a prodiglosin, a type of reddish pigment that can range from orange to
brick-red in color. Check your petri plate for the presence of bacteria with red pigmentation. If
these bacteria are present on your plate you were infected with our hypothetical communicable
disease. Report the people you came in contact with to the rest of the class. Record the data
collected on the accompanying data sheet.
7. Identify the initial carrier by eliminating anyone who came in contact with someone who was
1. Is it possible to know precisely who is the initial carrier? Why or why not?
2. Who Is the initial carrier of the “disease’?
3. Does everybody who comes in contact with an infected person get the “disease”? Why or why
4. Some of the plates will have bacteria growing on them that are not pigmented. Where do the un
pigmented bacteria come from?
Epidemiology Data Sheet
I.D. Numbers and names of those you contact:
1 Your name
3 Your I.D. #
I.D. Number and Name of those infected Round 1 Round 2 Round 3 Round 4