Taxonomic Information

Salmonella Bacteria Diagram

Bacteria / October 13, 2019


Microorganisms or microbes are tiny living things that are found all around us. They are usually too small to be seen with the naked wilie eye. Three of the main groups of microorganisms are:


Electron micrograph of H. pylori possessing multiple flagella (negative staining)

Bacteria (singular: bacterium) are a large group of unicellular microorganisms. They are extremely tiny thus they can not be seen individually unless viewed through an electron microscope. When cultured on agar, the bacteria grow as colonies that contain many individual cells. These colonies appear as spots of varying size, shape and colour, depending on the microorganism.

Bacterium typically occur in the following forms:

  • cocci (spherical)
  • bacillus (rod)
  • spirillum (spiral)

Bacteria are commonly rod-shaped or spherical (cocci). An example of the structure of a bacillus (rod-shaped) bacterium is shown on the right.

  • Nuclear material (DNA) - controls the cell's processes
  • Cytoplasm - fluids that hold the internal structure
  • Cell membrane - controls the movement of substances in and out of the cell
  • Cell wall - maintains the shape of the bacterium

Some bacteria have the following additional parts:

  • capsule - a hard and rigid protective layer
  • slime layer - a soft and slimy layer
  • flagella (singular: flagellum) - helps with movement of the bacterium.

Many bacteria do not exist alone. They will exist in chains or groups – many individuals joined together.


There are a variety of fungi:

  • yeast - tiny, single celled fungi used in bread-making due the CO2 byproduct of respiration
  • mould - A fungi made of many threads. Grows on exposed food sources
  • mushrooms and - a large fungi that grows in soil

It is composed of the following structures:

  • sporangium- part of a fungus that produces spores
  • spores - reproductive cells produced by a fungus
  • hyphae - fine root-like threads that break down and absorb food.


Viruses are extremely tiny and are much smaller than a bacterium cell. There are a variety of different types of viruses. In year 11, we will learn about the phage virus. This type of virus infects the E. Coli bacterium which is a usually harmless. The bacterium can be found in the lower intestine of animals.

The phage virus composes of the following parts:

  • Genetic material (DNA or RNA) - controls the virus' processes
  • Protein case - protects the virus
  • Tail fibre - a hollow tube which genetic material passes through during infection
  • Sheath - irreversibly binds the virus onto the bacterium resulting in its contraction and the tail fibre being pushed into the bacterium

Culturing Microorganisms[edit]

When culturing or growing microorganisms in a laboratory, a petri dish can be used. An agar plate containing nutrients necessary for growth is placed inside this petri dish. Agar is a jelly-like substance derived from seaweed.

Only two out of the three main groups of microorganisms can be cultured:

Viruses can not be cultured on an agar plate because they need a live host. Exposure to the virus once it has reached high levels can be extremely dangerous as all viruses are pathogenic.


Colonies of bacteria on a petri dish

Steps to culturing bacteria:

1. Sterilise a petri dish by using boiling water or a disinfectant. 2. Add a food source to boiling agar and pour the agar into the petri dish. It will eventually set into a jelly-like substance. Close the plate to prevent other microbes from contaminating it. 3. Repeat steps 1 to 3 for another petri dish to act as a control to confirm that the dish was sterile. 3. Test whether a surface contains bacterium by rubbing a cotton bud or an inoculating loop onto a surface and gently wiping it onto the sterile agar plate. This is called inoculation. 4. Seal the petri dish with sellotape to prevent contamination. 5. Place the dish upside down into an incubator at around 30-40°C for 3-5 days to incubate the bacteria. Ensure that it remains upside down to prevent condensation build up.

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