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Size and Shape
of bacteria

H err Gustav Druer, the Brno wine merchant has a problem and Brother Gregory has been asked to help.

You are to become his research assisitants and help him carry out a research investigation into the properties of microbes.

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Brother Gregory has been given a series of the microbes and been asked to determine the identity of each species so he can learn more about them.

He wants you, his research assistants, to try and determine two important properties of all bacteria and microbes; their shape and size.

This investigation involves determining the shape of the cells growing in microbial cultures, and then measuring the size of some of their individual members.

Background
Bacteria and archaea are prokaryotic cells. Most of these cells are very small, ranging from less that one to over tem micrometers (µm) in length. This makes it difficult to observe bacteria under the light microscope, and it is rare to be able to detect any of their internal contents this way.

Small size has other consequences as well. Each cell has a large surface area to volume ratio, and this determines how fast substances can enter and leave. Nutrients can enter, and wastes can leave these tiny cells very rapidly. Thus bacteria can, under appropriate circumstances, grow faster than they can replicate their DNA!

The shape of each cell is determined by the rigid outer covering called the envelope. This is a three layered structure consisting of (from outside to inside) an outer membrane, a cell wall, and a cytoplasmic, inner membrane (plasma membrane).

In some species thin appendages project beyond the surface of the cell, and there can also be a surrounding capsule. The three major groups of bacteria have very different envelopes. But it is the cell wall, made of peptidoglycan, that gives each species its characteristic shape.

Shape
The cell wall, consisting of a material that is part protein and part polysaccharide (peptido - and - glycan), called murein in bacteria, varies a lot from species to species. It functions to both resist the turgor pressure of the water and also to give the cell its shape.

If this cell wall is removed by digesting the peptidoglycan with the enzyme lysozyme, all cells, regardless of their original shape, become spheres.

Although bacteria come in many different shapes, some of the most easily identifiable are:

  1. Common Shapes:
    • spheres - (cocci).
    • rods - (bacilli).
    • spirals - (spirilla).

  2. Intermediate Shapes:
    • short rods - (coccobacilli).
    • commas - (vibrii).

  3. Rare shapes:
    • squares
    • stars
    • irregular

Bacteria are single celled organisms, but individuals of some species stick together to form groups or clusters of cells that are quite distinctive.

Cocci can divide to form chains (streptococci), groups of 4 (tetrads) or irregular clusters (staphylococci). Bacilli can divide to form chains (streptobacilli), but spiral bacteria normally remain as separate individuals.

Size
The average microbe is a million time smaller than the average human. Such sizes are not easy to measure using scales designed to determine the size of a house, or the height of a person.

Common, human sized standards of measure include the yard, the inch, the ounce and the pint or quart, but most scientific scales are based on the metric system, which has other standards.

A meter, a metric standard for measuring length, is approximately equivalent to about 39 inches, but this standard is far too large to be used to measure bacteria. A suitable standard for measuring microbes is the micrometer which is six times smaller than a meter (one-millionth of a meter).

There are 106 µmeters in one meter, and it is these units that are used to measure the size of bacteria. Typically, bacteria range from about 1 µm to about 5 µms.

Tools of the Trade
In these investigations, a tiny group of each microbe species are placed into a drop of water and placed on a microscope slide. A powerful microscope is then used to enlarge the image of the microbes. The objective lens magnifies 100X, and the eye piece or occular lens magnifies 10X. This gives a total magnification of 1000X for this oil immersion lens system - one of the most powerful for a light microscope.

The easiest way to see growing microbes is to use a wet mount. One way of doing this is to place a drop of bacteria in water on the surface of a microscope slide and carefully protecting it with a coverslip.

Alternatively, a drop of bacteria containing liquid can be placed in the middle of a coverslip, surrounded by a ring of petroleum jelly, and then inverted onto a microscope slide that has a slight depression at it's center. In this way the culture is hanging as a drop, upside down.

To make the cells more visible to the human eye, special dyes can be used to increase the contrast between light and dark.

These staining dyes bind to certain parts of the cells, increase the difference in color or contrast between them and the surrounding material, and thus make it easier to see many of the details of the structure or structures.

Certain vital stains can be used directly on wet mounts and they will stain living cells. But most other stains can only be used after the bacteria have been killed and attached firmly to the glass surface of the microscope slide (this is called, beingfixed).

Recording Results

CLICK HERE,
print out, and use this
Shape Table
Size Table
to record your data

The results of each of your investigations should be recorded as a table (a Table of Results). In these tables you should indicate the name of the microbe being studied, and either the shape of the cell type, and/or the sizes of the individual cells you have measured.

Graphs

CLICK HERE,
print out, and use this
Presenting the Results
sheet to graph your data

Some of the results of your investigations should be presented as a graph.

The horizontal axis of the graph should be the size intervals you have found and measured. The vertical axis should represent the number of cells you have found with that particular size.

The shape of these graphs or plots is characteristic for each species of microbe.

Instructions

  1. Select the microbial cultures from the ones given below.
  2. Print out the image that you see.
  3. Cut off and use the ruler at the bottom of each image.
  4. Note and use the scale given at the top of each image Note: they are NOT all the same!
  5. Measure all the cells and record your results in the appropriate table of results.
  6. Prepare graphs of your data as shown, and determine the average size of the cells in your culture.
------ use this application to
find the shape of cells.

----- find the size of cells.

  1. Streptococcus
  2. Staphylococcus
  3. Sarcinia
  4. Staphylococcus aureus
  5. Vibrio
  6. Sphaerotilus
  7. Aquaspirillum
  8. Fusobacterium

Conditions
of the investigation.
Each investigation is to carried out following the guidelines given to you by your instructor.

The shapes of each species of microbe should be made into a table (see above).

The sizes of each culture of microbes should be recorded first as a table, and then plotted as a graph.


Science at a Distance
© 2000, Professor John Blamire