Illustration of the ultrastructure of a single bacterium of the genus Salmonella
Rod-shaped Escherichia coli (E.coli) bacteria, commonly found in the intestines of animals

Bacteria are microscopic single-celled organisms. Examples of bacteria include:

  • Lactobacillus bulgaricus (a rod-shaped bacterium used to make yoghurt from milk)
  • Pneumococcus (a spherical bacterium that acts as the pathogen that causes pneumonia)

Bacterial cell structure

Bacterial cells have a cell wall made of polysaccharides and proteins. They do not have a nucleus, but instead they have a circular chromosome of DNA. They may also have small extra circles of DNA called plasmids.

The ultrastructure of a typical bacterial cell showing cell wall, the small ribosomes, plasmids with their figure of eights, cell membrane, large circular dna molecule and cytoplasm.

Other features of bacteria

Some bacteria can carry out photosynthesis, but most bacteria feed from other organisms (living or dead).

Some bacteria are pathogens, for example Pneumococcus (which causes pneumonia).