Bacterial Cultures
Background Information:
Bacteria are single-celled organisms that are too small to be seen without a microscope. In
order to grow bacteria, known as culturing, the bacteria are spread onto the surface of agar
contained within a petri dish. This agar is gel-like and contains all the food and nutrients that
the bacteria need to grow. As bacteria consume the nutrients, they begin to grow and multiply
which generates thousands to billions of cells that begin to piles up becoming visible to the
naked eye. This pile of cells is called a bacterial colony. Each species of bacteria produces a
colony that looks different from the colonies produced by the other species of bacteria. The
examination of the form and structure of bacterial colonies called colony morphology.
A microbiological culture (bacterial culture) is a method of multiplying microbial organisms by
letting them reproduce in a predetermined culture media under controlled laboratory
conditions. Microbial cultures are used to determine the type of organism, its abundance in the
sample being tested, or both. This method can be used as a tool to determine the cause of
infectious diseases by letting the agent to multiply in a predetermined medium. When testing
for strep throat (Streptococcus pyogenes), a throat culture is taken by scraping the lining of
tissue in the back of the throat and blotting the sample into the agar medium to screen for
harmful microorganisms.
Materials:
2 Petri Dishes
60 mL water
1.2 grams Nutrient agar
Large beaker
Cotton swabs
Hot plate
Thermal Gloves
Stirring Rod
Zipper lock bags
Procedure:
Part I: Preparing the Nutrient Agar
1. Use a large beaker to mix 1.2 grams of nutrient agar in 60 mL of water.
2. Place beaker of agar mixture on the hot plate and bring to a boil for 3 minutes or until the
mixture is clear with no particles floating.
3. Remove the mixture from the hot plate with the thermal gloves and set on table for 3-5
minutes until cool to touch.
4. Use a marker to section the petri dishes into quarters by drawing a cross on the bottom of
the dish (shown below).
5. Pour the agar mixture in the petri dishes to carefully cover the bottom half of the petri dish.
6. Place the lid on and tape the sides. Place the petri dishes upside down in the fridge to cool.
Part II: Collecting Bacteria
1. Use the ends of a cotton swab to collect bacteria from different surfaces (e.g. tables, toilets,
cell phones, etc.) Do NOT swab any body fluids, body parts, animal fluids, or animal parts!
a) Label each section of your petri dish 1-4.
b) Write down where each section of bacteria came from (e.g. toilet handle in girls bathroom in
G-building) in Table 1.
2. When the bacteria is collected on cotton swab, lightly streak the culture plate with the
cotton swap. Make sure not to scrape up any agar.
3. tape the dish closed and place the Petri dish upside down in the incubator at 37C for a
several days.
4. Check each day for bacterial colonies and count the colonies and record in Table 2.
Results:
Table 1: Location of the bacteria.
Section
Location
1
2
3
4
Table 2: Results of bacterial culture.
Time
Section 1
Section 2
Section 3
Section 4
Day 1
# of
Color of # of
Color of # of
Color of # of
Color of
colonies colonies colonies colonies colonies colonies colonies colonies
Day 2
Day 3
Day 4
Day 5
Clean up:
Submerge bacterial cultures in bleach water to kill off bacteria and throw the petri dishes in a
trash bag.