Bacteria

Structure of a salmonella bacterium cell

Bacteria aremicroscopic organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - 10 millionths of a metre.

Bacteria areliving cellsand, in favourable conditions, can reproduce rapidly. Bacteria reproduce by simply dividing in two (this is called binary fission).

Once inside the body, they release poisons or toxins that make us feel ill. Sometimes they can damage body cells directly.

Diseases caused by bacteria include:

  • food poisoning
  • cholera
  • typhoid
  • whooping cough

Pathogens - viruses

A hepatitis C virus showing DNA enclosed in a protein coat.

Viruses are many times smaller thanbacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.

Viruses can only reproduceinside host cells. They take over the cells of the body, damaging and destroying cells when they reproduce. A virus can get inside a cell and, once there, take over the body cell, and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open (killing the cell). The viruses are then passed out in the bloodstream, the airways, or by other routes.

Diseases caused by viruses include:

  • influenza - flu
  • colds
  • measles
  • mumps
  • rubella
  • chicken pox
  • AIDS

Microbes

Many living things are so small that they can only be seen through a microscope. These living things are calledmicroorganismsormicrobes. There are three main types of microbe:

  • Fungi (will not be discussed here)
  • bacteria
  • viruses

Bacteria

If there is just one of them, we call it abacterium. Bacteria have many different shapes. Some have 'tails' (called flagella) that let them swim.

Viruses

Viruses are the smallest type of microbe (much smaller than bacteria). As a virus can only reproduce inside a cell, some people are not convinced that viruses are really living things.

Differences between fungi, bacteria and viruses

The table shows some of the similarities and differences between the three types of microbe.

Feature / Fungi / Bacteria / Viruses
Cell membrane / / /
Cell wall /
(hard) /
(soft) /
(protein coat)
Cell nucleus / / (DNA circle) / (DNA strand)

Microbes - useful or not?

People often use the word germ instead of microbe, so you might think that microbes are all harmful. But some are useful to us.

Useful microbes

Yeast cells (fungi) are useful to bakers and brewers. Yeast cells can change sugar into carbon dioxide gas and alcohol. This is useful to bakers because the gas helps the bread rise, and it is useful to brewers because it adds the alcohol needed for their drinks.

Bacteria are also useful to us. For example, certain bacteria cause the changes needed in milk to make yogurt and cheese out of it. Many bacteria also live in our digestive system and help to keep it functioning properly.

Harmful microbes

Many microbes can cause diseases. For example here are some diseases caused by fungi:

  • athlete's foot
  • thrush

Here are some diseases caused by bacteria:

  • tuberculosis, TB (affects the lungs)
  • salmonella (causes food poisoning)
  • whooping cough (affects the lungs)

Here are some diseases caused by viruses:

  • chicken pox (affects skin and nerves)
  • common cold
  • influenza, flu
  • measles (affects skin and lungs)
  • mumps (affects salivary glands)
  • rubella, german measles

Microbes cause disease when they are able to reproduce in the body. They produce harmful substances calledtoxins, and damage tissues and organs. We say that someone who has harmful disease-causing microbes in them is infected.