Wednesday, September 19, 2007

A microorganism (also spelt as microrganism) or microbe is an organism that is microscopic (too small to be seen by the human eye). The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists, but not viruses and prions, which are generally classified as non-living. Micro-organisms are generally single-celled, or unicellular organisms; however, there are exceptions as some unicellular protists are visible to the average human, and some multicellular species are microscopic.
Microorganisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the earth's crust. Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can also fix nitrogen, they are an important part of the nitrogen cycle. However, pathogenic microbes can invade other organisms and cause diseases that kill millions of people every year.

Further information: Timeline of evolution
Single-celled microorganisms were the first forms of life to develop on earth, approximately 3—4 billion years ago.

Prior to Anton van Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery as to why grapes could be turned into wine, milk into cheese, or why food would spoil. Leeuwenhoek did not make the connection between these processes and microorganisms, but he did establish that there were forms of life that were not visible to the naked eye.

Microbe Discovery
Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet. Bacteria and archaea are almost always microscopic, whilst a number of eukaryotes are also microscopic, including most protists and a number of fungi. Viruses are generally regarded as not living and therefore are not microbes, although the field of microbiology also encompasses the study of viruses.

Prokaryotes are organisms that lack a cell nucleus and the other organelles found in eukaryotes. Prokaryotes are almost always unicellular, although some such as myxobacteria can aggregate into complex structures as part of their life cycle. These organisms are divided into two groups, the archaea and the bacteria.


Main article: Bacteria Bacteria

Main article: Archaea Archaea

Main article: Eukaryote Eukaryotes

Main article: Protista Protists

Main article: Micro-animals Animals

Main article: Fungus Fungi

Main article: Plant Plants
Microorganisms are found in almost every habitat present in nature. Even in hostile environments such as the poles, deserts, geysers, rocks, and the deep sea, some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles. Extremophiles have been isolated from rocks as much as 7 kilometres below the earth's surface, Many types of microorganisms have intimate symbiotic relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to the host organism (parasitism). If microorganisms can cause disease in a host they are known as pathogens.

Habitats and ecology

Main article: Extremophile Extremophiles
The nitrogen cycle in soils depends on the fixation of atmospheric nitrogen. One way this can occur is in the nodules in the roots of legumes that contain symbiotic bacteria of the genera Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium.

Soil microbes
Symbiotic microbes


Main article: Fermentation (food) Use in food

Main article: Ethanol fermentation Use in energy
Microbes are also essential tools in biotechnology, biochemistry, genetics, and molecular biology. Microbes can be harnessed for uses such as creating steroids and treating skin diseases. Scientists are also considering using microbes for living fuel cells, and as a solution for pollution.

Use in science

Main article: Biological warfare Use in warfare

Importance in human health
Microorganisms can form an endosymbiotic relationship with other, larger, organisms. For example, the bacteria that live within the human digestive system contribute to gut immunity, synthesise vitamins such as folic acid and biotin, and ferment complex undigestible carbohydrates.

Human digestion
Microorganisms are the cause of many infectious diseases. The organisms involved include bacteria, causing diseases such as plague, tuberculosis and anthrax; protozoa, causing diseases such as malaria, sleeping sickness and toxoplasmosis; and also fungi causing diseases such as ringworm, candidiasis or histoplasmosis. However, other diseases such as influenza, yellow fever or AIDS are caused by viruses, which are not living organisms and are not therefore microorganisms. As of 2007, no clear examples of archaean pathogens are known,

Diseases and immunology

Main article: Hygiene Microorganisms in fiction

Biological warfare
Microbial intelligence
Petri dish
Soil contamination

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