DEMONSTRATING AN EPIDEMIC
M. Beth Powel
1993 Woodrow Wilson Biology Institute
Epidemiology is the study of disease origin and transmission. This experiment allows students to experience a small scale "epidemic," demonstrating the ease with which disease organisms are spread, and enables the student to determine the originator of the "epidemic." Students will transfer live bacteria by hand contact, then transfer an inoculum to a nutrient agar plate for 24 hour incubation. After incubation, plates are observed for growth of the microbial agent. By arranging the plates in the order of hand contact, it can be determined what individual received the original contaminant and started the "epidemic," which individuals transferred the organism yet did not grow it out (carriers), and how dosage, or amount of contamination, affects getting a disease. It must be pointed out that in an actual epidemic a contaminated individual could feasibly contact many others and not just one as demonstrated in this procedure and that the organism will multiply in each host before being passed on. Other means of microbial transmission (air, water, body fluids, fomites) may also be discussed. (Fomites are inanimate objects such as combs, pencils, etc. which may carry microbes on the surface.)
Regular to Advanced Biology Students
CLASS TIME REQUIRED:
2 thirty minute periods
(prices and item numbers are from Carolina Biological 93/94)
All microbes should be handled as pathogens (disease causing) and therefore appropriate aseptic techniques should be followed. In particular, drips of candy liquid on any surface should be disinfected and all swabs, gloves, reusable items and used plates should be properly sterilized before placing in garbage cans or containers to be washed.
More on Safety:
- Broth culture: Both Serratia marcescens (15-
5450 $6.95) and Micrococcus roseus (15-5160
$6.95) are provided as aga/cultures from which
a broth culture must be made. Sterile nutrient
broth tubes (82-6120 $10.60/10) are available
but it is much cheaper if you can prepare your
own. To prepare a broth culture you will need a
sterile tube of nutrient broth, a stock plate
culture of Micrococcus roseus or Serratia
marcescens, an inoculating loop and a Bunsen
burner, alcohol burner or incinerator.
- Unknown candy samples: Prepare at least 30
minutes prior to class to allow sufficient
dissolving to occur.
- Data collection: On the day of the experiment
record the number of the students as they are
called to shake hands. On the second day each
student will record the number of red colonies
counted on that plate so that the entire class
may share data.
- Sterilization of contaminated materials: All
contaminated materials must be sterilized
before discarding. This may be accomplished by
steam sterilization at 15 psi for 15 minutes.
- Obtain all materials. Lab coats must be worn
whenever dealing with microbes. Label the
sterile nutrient agar plate on the bottom portion
with the experimenter's name, date, and the
number indicated on the candy dish.
- If gloves are being used, place glove on left
hand. Alternately, spray both hands with
disinfectant spray and wait one minute. Then
wash hands with disinfectant soap and rinse in
sterile water. Do not dry. Keep clean or gloved
hand slightly closed to avoid outside
- Carefully pour the liquid and the candy into the
left hand. Roll the candy around until the hand
is thoroughly moistened. Avoid allowing any
liquid to escape. Return the candy to the
original dish and close the hand.
- When all experimenters are similarly exposed
the instructor will direct a sequence for shaking
left hands based on the numbered dishes.
Making firm hand contact is essential to the
success of the experiment. The first individual
will shake hands with the next one called, then
return to inoculate the nutrient agar plate. The
second individual called will then shake hands
with the third before inoculating. Each
individual, except the first and last called, will
shake with two others before inoculation.
- Remove a sterile cotton swab from its package
with the right hand. Thoroughly moisten the
swab with candy liquid by rolling the cotton tip
across the contaminated hand. Cover all of the
- Carefully lift the lid of the agar plate to limit
outside exposure. Using a rolling motion, gently
rub the swab over the surface of the agar in a
close zig-zag pattern; completely cover the face
of the agar. Close the lid of the dish. Return the
swab to the empty package and discard to be
- If a glove was used, place it in a container to be
sterilized. If not, repeat the disinfection
procedure described earlier.
- Place the inverted agar plate into the incubator
for 24 hours. Disinfect the areas used, taking
particular care to clean any drops of liquid on
tables or the floor. Remove lab coats. Record the
sequence of contact for the entire group.
- After 24 hour incubation, remove the plate from
the incubator. Count the number of red colonies
on the plate and record with class data. If there
are a large number of red colonies, divide the
bottom of the plate into 6 or 8 equal wedges.
Count one half of the wedges, choosing those
that have a representative distribution and
multiply the total number counted by two.
Arrange the plate with those of the other
experimenters so that they represent the contact
sequence and observe the general trend in
numbers of red colonies. Prepare a data chart
indicating the sequence of contact and the
number of red colonies per plate for the entire
class. Indicate those individuals who got the
"disease" by a + and those who were disease
free by a -. Place a * by the individual who
started the epidemic.
- Discard plates in sterilization containers and
disinfect all areas used.
- Which individual began the "epidemic? How can
this be determined from the class data?
- Were there individuals who appear to have
passed on the organism without having the
disease? How does this appear in the data?
Does this occur in an actual epidemic?
- Describe at least two ways in which this model
epidemic differs from a real one.
This laboratory experiment was adapted from A Laboratory Manual for Microbiology by Richard A. Mangino.