Download Microorganisms everywhere

Microorganisms Everywhere
Copyright, 2003, Dr. Mecky Pohlschroder, Department of Biology, University of Pennsylvania
Although we can’t always see them, microorganisms are present in
almost all environments. Many can be grown in the laboratory on plates
containing a solid, nutrient-rich medium (agar plates). After several days, a
single bacterial cell grows into a population of bacterial cells that is visible
to the naked eye. This population of cells is called a microbial colony. In
this exercise we will test for the presence of microorganisms on our hands
and on objects we are in contact with.
Materials
™ Sterile agar plates (1 per student)
™ Objects of your choice to test for the
presence of microorganisms: a penny, keys,
pens, etc.
™ Q-tips (to get bacteria from teeth,
sandwiches, fruit juice, etc.)
™ Toothpicks
Instructions - First Week
1. Get an agar plate and label it on the bottom with your name and the date
and the objects that you are going to test for microorganisms. This agar
plate is sterile – it has been heated and no bacteria are on it. Try
dividing your plate into sections by drawing lines on the bottom of the
plate (on the outside) and trying different objects in each section.
2. Open the cover of the plate and gently press your object on the surface
of the agar. Be careful not to press too hard – you don’t want to press
through the agar. Make sure to label what object you used in each
section. If you want to try testing food, drinks, your teeth, etc., then
wet the Q-tip with the object, and then rub the Q-tip on the surface of
the agar. You can also use a toothpick.
3. Cover the plate as soon as you remove your object or the Q-tip or the
toothpick. You want to have the lid open for as little time as possible so
microorganisms from the air do not fall on the plate.
1
Your plate will be left at room temperature for a couple of days so any
bacteria that are present can grow into colonies. After colonies appear, the
instructor will keep it at 4oC in the refrigerator until the next class.
Instructions – Second Week
1. Record what happened on your plate.
1.1. Sketch your plate. Label your drawing so that you know what objects
the colonies came from.
1.2. Record size, color, texture, and shape of colonies.
2. Prepare samples of the microorganisms on your plate by using the methyl
blue staining method.
2.1. Place a few drops of stain on the slide.
2.2. Use a toothpick to smear some of the bacteria from one colony on
the slide and mix it into the stain. You should take only enough
bacteria to cover the tip of a toothpick – definitely not more! If you
take too many cells, you won’t be able to see them clearly under the
microscope.
2.3. Carefully place the cover slip over the bacteria.
2
3. For each sample, examine it under the microscope and draw the bacteria.
You will need to use the highest magnification on the microscope to see
the bacteria. Make sure you record which colony it came from. If you
find a well-stained sample, take it to the videomicroscope so that
everyone can see it.
4. On your plate, you used many different objects to grow bacteria. Do
some of the bacterial colonies look similar? If so, why do you think this
is?
5. If some bacterial colonies look very different from one another, why do
you think this is?
3