Bacteria are living things that are neither plants nor animals, but belong to a group all by themselves. They are very small individually not more than one single cell, however there are normally millions of them together, for they can multiply really fast. A few beneficial functions of bacteria are the production of food products including dairy products, such as yogurt, sauerkraut, fermented meat products such as summer sausage, and vinegar. Bacteria also help fix nitrogen in the soil and are responsible for decomposing organic materials, which returns important nutrients back to the soil. The beneficial aspects of microorganisms far outweigh their harmful effects.
Viruses occupy a special taxonomic position. they are not plants, animals, or prokaryotic bacteria (single cell organisms without defined nuclei), and they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms because they are not free living; for example, they cannot reproduce and carry on metabolic processes without a host cell.
A prion is a microscopic protein particle similar to a virus but lacking
nucleic acid, thought to be the infectious agent responsible for scrapie and certain other degenerative diseases of the nervous system. Prion illnesses affect the brain and cause extensive damage to brain tissue, causing it to become spongy in appearance. Though prion illnesses are not contagious in typical fashion, introduction of infectious prions into healthy brain tissue transmits the infection.
Protists are organisms in the kingdom Protista. These organisms are eukaryotes, meaning they are made up of single or multiple cells which all contain a nucleus enclosed by a membrane. The protists are a diverse group of eukaryotes that cannot be classified as animals, plants, or fungi. Organisms in the Protista kingdom include amoebae, red algae, dinoflagellates, diatoms, euglena and slime molds.
http://www.funsci.com/fun3_en/protists/introduction.htm
http://www.ucmp.berkeley.edu/bacteria/bacteria.html
http://www.bacteriamuseum.org/cms/Bacteria/what-are-bacteria.html
http://www.news-medical.net/health/What-is-a-Virus.aspx
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