Bacteria are single-cell organisms. They are neither animals, plants nor fungi but belong in a group all of their own. They are not the same as viruses which are considerably smaller and simpler structures and, unlike bacteria, are unable to reproduce on their own. Bacterium is the singular of the word and bacteria is the plural - one bacterium and many bacteria.
Bacteria are best known for the diseases that they cause but most bacteria do not cause diseases or depend on other organisms for their existence. Bacteria were the first organisms to evolve on the planet, around three thousand million years before there were any multi-celled animals.
One of the defining features of bacteria is that their DNA is not enclosed in a nucleus like plants and animals. Bacteria reproduce asexually by simple cell division called mitosis though they have other ways of exchanging DNA material. This enables them to reproduce very quickly and one bacterium can become several million bacteria in a very short space of time if the conditions are right.
Some bacteria contain chlorophyll and can synthesise their own sugars from sunlight and carbon dioxide much like plants.
Some bacteria live in the guts of large animals, including humans, and are essential to that animals digestive processes.
Some bacteria can survive in boiling water and others can survive radiation more a thousand time that which would kill people.
A number of bacterial species cause diseases in animals. A variety of the Chlamydia bacteria, C. Pneumonia which can cause pneumonia in weakened individuals has been linked with multiple sclerosis. This link has been challenged by many other researchers.
What are bacteria?
Virtual Bacteria Museum
Introduction to the Bacteria
Bacteria (Eubacteria, or Typical Bacteria)
Pneumonia Organism May Play Role in MS Development
Chlamydia not a cause of multiple sclerosis after all (2nd feature)