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