Download Environmental Plate for Growth of Bacteria

Environmental Plate for Growth of Bacteria (Ubiquity of Microorganisms)
In this exercise, you will be investigating bacterial growth (and perhaps some other
microorganisms as well, e.g. fungi) from different environmental areas. Some of the suggested
areas to be cultured will be found in this laboratory. Unwanted microorganisms which can get onto
culture media and grow are call contaminants. Care must be taken throughout subsequent
laboratory exercises to avoid contamination. In this exercise, you will see the many places from
which contamination can come, as well as seeing where microorganisms can be found naturally.
The Petri dish you will be using contains a solid nutrient medium (food source) for growing
bacteria. The bacteria will grow on this plate in the form of colonies. Colonies of bacteria are
discrete circular areas which develop from a single cell or spore. Each single cell grows into many
cells which can be viewed macroscopically (that is, with the naked eye rather than a microscope).
Because each colony grows from a single cell or spore, it represents a pure culture (cells of a single
type = clones).
Slightly raised colonies are usually bacteria.
Materials:
1 Trypticase Soy Agar plate (TSA)
1 sterile swab (for treatments c, d, e, f)
1 wax pencil or Sharpie
Very raised colonies are often yeasts (fungi).
Procedure:
Fuzzy, filamentous colonies are molds (fungi).
1.
Each of the six students at each table will perform ONE of the following treatments:
a. Place your fingers on the Petri dish and rub them around. Do not cut into the agar surface.
b. Leave a plate open to the air without the cover for at least ½ hour or until the end of class.
c. Swab the surface of your teeth and gums and then gently swab the surface of the agar
plate moving the swab from side to side, top to bottom. Do not cut into the agar surface.
d. Dip a sterile swab into the beaker of soil and then gently swab the surface of the agar plate
moving the swab from side to side, top to bottom. Do not cut into the agar surface. Discard
swab into red biohazard bag.
e. Dip a sterile swab into the sample of aquarium water and then gently swab the surface of
the agar plate moving the swab from side to side, top to bottom. Do not cut into the agar
surface. Discard swab into red biohazard bag.
f. Dip the swab into a tube of sterile water and then rub the desktop vigorously. Gently swab
the surface of the agar plate moving the swab from side to side, top to bottom. Do not cut into
the agar surface. Discard the tube of sterile water into the baskets on the cart. Discard swab
into red biohazard bag.
2.
Each student should label the bottom of the Petri plate with your name and the treatment
(type of specimen) using a wax pencil or Sharpie.
3.
Place the Petri dish upside (top side) down into the plastic box designated by your instructor
for incubation at 37°C. Observe and record the results in the next lab period.
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Next Lab Period:
1.
Retrieve your incubated Petri dish. Carefully remove the lid and observe the growth. Do not
put the plate up to your face!
2.
Record your observations below.
3.
Save all plates for future staining.
Results/Observations:
Type of specimen used (treatment):____________________________
Relative amount of growth: ___________________________________
- None
- light growth = few colonies
- moderate growth = ~1/2 plate covered with many colonies
- heavy growth = most of plate covered with very many colonies or confluent growth (colonies cannot be seen as
separate)
Note: one large colony that covers half of the plate is still just one colony and would be considered light growth.
Description of colonies on your plate (shape, size, color, texture):
See diagrams in Ex. 2-2 in lab manual.
Compare your plate with all of the other plates at your table:
1. Which type of specimen/treatment had the most growth/colonies? _________________________
2. Which had the least amount of growth/fewest colonies? _________________________________
Questions:
1. Where are you likely to find bacterial and other microorganisms?
2. What is a colony?
3. Why would the number and type of colonies vary on plates from different sources?
4. What is considered contamination?
5. Where can contamination come from?
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