Download Identification of Bacteria

Identification of Bacteria
State Standard 1.c. – Students know how prokaryotic cells, eukaryotic cells, and
viruses differ in complexity and general structure.
Introduction:
There are many different types of bacteria. They differ in the structure of
their cells, their metabolism and chemistry, and the structure of their cell walls.
The differences among bacteria are used to classify them and to identify a
species. Bacteria come in a number of shapes. Most, however, are cocci (round),
bacilli (rod-shaped), or spirilla (spirals). The way these individual cells are
arranged is also variable among bacterial species. Although some species exist
singularly, bacteria can be linked together in a long chain (strepto-), clumped like
grapes (staphylo-), paired (diplo-), and can exist in other arrangements as well.
The differences in the cell wall structure of bacteria can be used to identify
them. Using the Gram stain bacteria can be identified as Gram positive or Gram
negative. Bacteria that give a positive Gram reaction have thick cell walls
composed of peptidoglycan (consists of polymers of modified sugars cross linked
by short polypeptides that vary), retain the initial stain and appear a violet color
uder a microscope. Bacteria with a negative Gram reaction have a thin
peptidoglycan ceall wall and have an additional outer member of
lipopolysaccharides (carbohydrates bonded to lipids). This structure causes the
cell to be easily decolorized and the bacteria will appear reddish pink, the color of
the counter stain.
Part A – Classifying bacteria by their shape.
Purpose: To classify bacteria as cocci, spirilla, or bacilli.
Procedures:
1. Obtain a slide of Rhodospirrum rubrum and Micrococcus luteus.
2. Draw several examples of each type of bacteria using high power and identify
whether they are cocci, spirilla or bacilli.
3. Estimate the size of each type of bacteria. To do this divide the number of
bacteria that would fit across the field of view by the field of view:
Number of cells that fit across the field of view
Field of view
4. Prepare a wet mount slide of the plain yogurt.
5. Draw and identify the various bacteria that you observe.
Part B – Staining and observing Bacillus subtilis
Purpose:
To be able to view the strain of Bacteria found in peppercorn infusion.
PROCEDURE
Preparing the peppercorn slide.
1. Fill a 600 ml beaker full of tap water, and place it in the sink. It will be used to
wash the stain off the sides.
2. Using a cotton swab, draw a very small amount of the scum on the surface of
the peppercorn infusion water and place it on a microscope slide. Spread it out
throughout the center portion of the slide. Allow the slide to dry thoroughly.
3. Heat, fix the specimen to the slide by passing it (specimen side up) quickly
through the flame of a Bunsen burner three times. Be sure to hold the slide with
the clothespin while passing it through the flame.
4. After the slide has cooled, take the slide to the sink. Holding the slide with a
clothespin put three to four drops of crystal violet stain on the specimen. Allow it
to sit for approximately 2 minutes.
5. Pour off any excess stain, and gently rinse the slide off by dipping it several
times into the 600mL beaker of tap water, and put the slide down on a paper
towel allowing it to sit for approximately 30 seconds.
6. Once dry, examine the slide under a microscope using the high power
objective. Identify each type of bacteria found with respect to its cell morphology
and arrangement.
7. Record your observations and make drawings of what you see under the
microscope.