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.
History Further information: Timeline of evolution Single-celled microorganisms were the
first forms of life to develop on earth, approximately
3—4 billion years ago.
Evolution 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.
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.
Classification 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.
Prokaryotes 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
Importance 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 Biology Microbial intelligence Nanobacterium Petri dish Prokaryote Soil contamination Staining
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