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A microorganism, or microbe,, ''mikros'', "small") and ''
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes
hyphen The hyphen is a punctuation mark used to join word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (li ...
ated (''micro-organism''), especially in older texts. The informal synonym ''microbe'' () comes from μικρός, mikrós, "small" and βίος, bíos, "
life Life is a characteristic that distinguishes physical entities that have biological processes, such as Cell signaling, signaling and self-sustaining processes, from those that do not, either because such functions have ceased (they have Death ...

life
". is an
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
of
microscopic The microscopic scale (from , ''mikrós'', "small" and σκοπέω, ''skopéō'' "look") is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In phys ...
size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from ancient times, such as in
Jain scriptures Jain literature refers to the literature of the Jain religion. It is a vast and ancient literary tradition, which was initially transmitted orally. The oldest surviving material is contained in the canonical ''Jain Agamas,'' which are written ...
from sixth century BC India. The scientific study of microorganisms began with their observation under the
microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a laboratory instrument used to examine objects that are too small to be seen by the naked eye Naked eye, also called bare ...

microscope
in the 1670s by
Anton van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch businessman and scientist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the Father ...

Anton van Leeuwenhoek
. In the 1850s,
Louis Pasteur Louis Pasteur (, ; December 27, 1822 – September 28, 1895) was a French biologist, microbiologist, and chemist renowned for his discoveries of the principles of vaccination, microbial fermentation, and pasteurization. He is remembered ...

Louis Pasteur
found that microorganisms caused
food spoilage Food spoilage is the process where a food product becomes unsuitable to ingest by the consumer. The cause of such a process is due to many outside factors as a side-effect of the type of product it is, as well as how the product is packaged and stor ...
, debunking the theory of
spontaneous generation Spontaneous generation is a body of thought on the ordinary formation of living organisms without descent from similar organisms. The theory of spontaneous generation held that living creatures could arise from nonliving matter and that such process ...
. In the 1880s,
Robert Koch Heinrich Hermann Robert Koch (; ; 11 December 1843 – 27 May 1910) was a German physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medical doctor, or simply docto ...

Robert Koch
discovered that microorganisms caused the diseases
tuberculosis Tuberculosis (TB) is an infectious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but can also affect other parts of the body. Most infections show no symptoms, in which case ...

tuberculosis
,
cholera Cholera is an infection of the small intestine by some strain (biology), strains of the Bacteria, bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts ...

cholera
,
diphtheria Diphtheria is an infection An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and the reaction of host (biology), host tissues to the infectious agents and ...
, and
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
. Because microorganisms include most
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, p ...
s from all three domains of life they can be extremely diverse. Two of the three domains
Archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...
and
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a Bacte ...
, only contain microorganisms. The third domain
Eukaryota Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...
includes all
multicellular organism A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to a unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
s as well as many unicellular
protist A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukaryo ...
s and
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

protozoa
ns that are microbes. Some protists are related to
animals Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...
and some to
green plants Viridiplantae (literally "green plants") are a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ...

green plants
. There are also many multicellular organisms that are microscopic, namely
micro-animal 210px, A microscopic arachnid ''Lorryia formosa''">Lorryia_formosa.html" ;"title="arachnid ''Lorryia formosa">arachnid ''Lorryia formosa'' Micro-animals are animals so small that they can be visually observed only under a microscope. Unlike most ...
s, some
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
, and some
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cel ...
, but these are generally not considered microorganisms. Microorganisms can have very different
habitat Ibex in an alpine habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat c ...

habitat
s, and live everywhere from the
poles The Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a nation A nation is a community of people formed on the basis of a combination of shared features such as language, history, ethnicity, culture ...
to the
equator The Earth's equator is an imaginary planetary line that is about long in circumference. The equator divides the planet into the Northern Hemisphere and Southern Hemisphere and is located at 0 degrees latitude In geography, latitude is ...

equator
,
desert upright=1.5, alt=see caption, Sand dunes in the Rub' al Khali ("Empty quarter") in the United Arab Emirates">Rub'_al_Khali.html" ;"title="Sand dunes in the Rub' al Khali">Sand dunes in the Rub' al Khali ("Empty quarter") in the United Arab ...
s,
geyser A geyser (, ) is a spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditions that exist only i ...

geyser
s,
rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in which it is formed. Rocks form the Earth's oute ...
, and the
deep sea The deep sea or deep layer is the lowest layer in the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
. Some are adapted to extremes such as very hot or very cold conditions, others to
high pressure In science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictions ...
, and a few, such as ''
Deinococcus radiodurans ''Deinococcus radiodurans'' is an extremophilic bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms ...

Deinococcus radiodurans
'', to high radiation environments. Microorganisms also make up the
microbiota Microbiota are "ecological communities of commensal, symbiotic and pathogenic microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usu ...
found in and on all multicellular organisms. There is evidence that 3.45-billion-year-old
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, smal ...

Australia
n rocks once contained microorganisms, the earliest direct evidence of life on Earth. Microbes are important in human culture and
health Health, according to the World Health Organization, is "a state of complete physical, Mental health, mental and social well-being and not merely the absence of disease and infirmity".World Health Organization. (2006)''Constitution of the World He ...

health
in many ways, serving to ferment foods and treat sewage, and to produce fuel,
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates int ...

enzyme
s, and other
bioactive compound Phytochemistry is the study of phytochemicals, which are chemicals derived from plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy ...
s. Microbes are essential tools in
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Developmenta ...
as
model organism A model organism (often shortened to model) is a non-human species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molec ...
s and have been put to use in
biological warfare A culture of '' anthrax.html"_;"title="Bacillus_anthracis'',_the_causative_agent_of_anthrax">Bacillus_anthracis'',_the_causative_agent_of_anthrax,_which_was_repeatedly_employed_for_biological_warfare. Biological_warfare,_also_known_as_germ_wa ...
and
bioterrorism Bioterrorism is terrorism Terrorism is, in the broadest sense, the use of intentional violence to achieve political aims. It is used in this regard primarily to refer to violence during peacetime or in the context of war against non-combata ...
. Microbes are a vital component of fertile soil. In the
human body The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabil ...

human body
, microorganisms make up the
human microbiota'' spp. anaerobically cultured in blood agar medium '' colonies growing on XLD agar plate An agar plate is a Petri dish that contains a growth medium solidified with agar, used to Microbiological culture, culture microorganisms. Sometimes selecti ...
, including the essential
gut flora Gut microbiota, gut flora, or microbiome are the microorganisms including bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of p ...
. The
pathogen In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
s responsible for many
infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and the reaction of host (biology), host tissues to the infectious agents and the toxins they produce. An inf ...
s are microbes and, as such, are the target of
hygiene measures
hygiene measures
.


Discovery

showed that boiling a broth stopped it from decaying.


Ancient precursors

File:Mahaveer swami.jpg, upMahavira, Vardhmana Mahavira postulated the existence of microscopic creatures in the sixth century BC. The possible existence of microscopic organisms was discussed for many centuries before their discovery in the seventeenth century. By the fifth century BC, the Jains of present-day India postulated the existence of tiny organisms called
nigoda In Jainism Jainism (), traditionally known as ''Jain Dharma'', is an ancient Indian religion and the method of acquiring perfect knowledge of self and universe and perfect joy through Extrasensory perception, extrasensory means as empiric ...
s. These nigodas are said to be born in clusters; they live everywhere, including the bodies of plants, animals, and people; and their life lasts only for a fraction of a second. According to the Jain leader Mahavira, the humans destroy these nigodas on a massive scale, when they eat, breathe, sit, and move. Many modern Jains assert that Mahavira's teachings presage the existence of microorganisms as discovered by modern science. The earliest known idea to indicate the possibility of diseases spreading by yet unseen organisms was that of the
Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter in ...
scholar
Marcus Terentius Varro Marcus Terentius Varro (; 116–27 BC) was one of ancient Rome's greatest scholars and a prolific author. He is sometimes called Varro Reatinus to distinguish him from his younger contemporary Varro Atacinus. Biography Varro was born in or near ...
in a first-century BC book entitled ''On Agriculture'' in which he called the unseen creatures
animalcule Animalcule ('little animal', from Latin ''animal'' + the diminutive suffix ''-culum'') is an old term for microorganism, microscopic organisms that included bacteria, protozoans, and very small animals. The word was invented by 17th-century Dut ...
s, and warns against locating a homestead near a swamp:''Varro on Agriculture'' 1, xii Loeb In ''
The Canon of Medicine ''The Canon of Medicine'' ( ar, القانون في الطب, italic=yes ''al-Qānūn fī al-Ṭibb''; fa, قانون در طب, italic=yes, ''Qanun-e dâr Tâb'') is an encyclopedia of medicine in five books compiled by Persian physician-phil ...

The Canon of Medicine
'' (1020),
Avicenna Ibn Sina ( fa, ابن سینا), also known as Abu Ali Sina (), Pur Sina (), and often known in the West as Avicenna (;  – June 1037), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, t ...

Avicenna
suggested that
tuberculosis Tuberculosis (TB) is an infectious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but can also affect other parts of the body. Most infections show no symptoms, in which case ...

tuberculosis
and other diseases might be contagious.


Early modern

Akshamsaddin Akshamsaddin (Muhammad Shams al-Din bin Hamzah, tr, Ak Şemsettin) (1389, Damascus )), is an adjective which means "spacious". , motto = , image_seal = Flag_of_Damascus.png , seal_type ...
(Turkish scientist) mentioned the microbe in his work ''Maddat ul-Hayat'' (The Material of Life) about two centuries prior to
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch Republic, Dutch businessman and scientist in the Dutch Golden Age, Golden Age of Dutch science and technology. A largely self-taught man in science, he is ...
's discovery through experimentation: In
1546 Year 1546 ( MDXLVI) was a common year starting on Friday A common year starting on Friday is any non-leap year A leap year (also known as an intercalary year or bissextile year) is a calendar year that contains an additional day (or, in the ...

1546
,
Girolamo Fracastoro Girolamo Fracastoro ( la, Hieronymus Fracastorius; c. 1476/86 August 1553) was an Italian physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medical doctor, or simp ...

Girolamo Fracastoro
proposed that
epidemic An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of people in a given population within a short period of time. For example, in meningococca ...
diseases A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not due to any immediate external injury. Diseases are often known to be medical conditions that ar ...
were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or even without contact over long distances.
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch Republic, Dutch businessman and scientist in the Dutch Golden Age, Golden Age of Dutch science and technology. A largely self-taught man in science, he is ...
is considered to be the father of microbiology. He was the first in
1673 Events January–June * January 22 Events Pre-1600 * 613 – Eight-month-old Constantine is crowned as co-emperor (''Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman general and statesman w ...
to discover and conduct scientific experiments with microorganisms, using simple single-lensed microscopes of his own design.
Robert Hooke Robert Hooke FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resources ...
, a contemporary of Leeuwenhoek, also used
microscopy Microscopy is the technical field of using microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a laboratory instrument used to examine objects that are too small to b ...

microscopy
to observe microbial life in the form of the fruiting bodies of moulds. In his
1665 Events January–June * January 5 Events Pre-1600 * 1477 – Battle of Nancy: Charles the Bold is defeated and killed in a conflict with René II, Duke of Lorraine; the Duchy of Burgundy, Burgundy subsequently becomes part o ...
book ''
Micrographia ''Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. With Observations and Inquiries Thereupon.'' is a historically significant book by Robert Hooke Robert Hooke FRS (;  – 3 March 1703) wa ...

Micrographia
'', he made drawings of studies, and he coined the term ''
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
''.


19th century

Louis Pasteur Louis Pasteur (, ; December 27, 1822 – September 28, 1895) was a French biologist, microbiologist, and chemist renowned for his discoveries of the principles of vaccination, microbial fermentation, and pasteurization. He is remembered ...

Louis Pasteur
(1822–1895) exposed boiled broths to the air, in vessels that contained a filter to prevent particles from passing through to the
growth medium A growth medium or culture medium is a solid, liquid or semi-solid designed to support the growth of a population of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism (from Ancient ...
, and also in vessels without a filter, but with air allowed in via a curved tube so dust particles would settle and not come in contact with the broth. By boiling the broth beforehand, Pasteur ensured that no microorganisms survived within the broths at the beginning of his experiment. Nothing grew in the broths in the course of Pasteur's experiment. This meant that the living organisms that grew in such broths came from outside, as
spore )'', growing on a thinned hybrid black poplar ''(Populus x canadensis)''. The last stage of the moss lifecycle is shown, where the sporophytes are visible before dispersion of their spores: the calyptra (1) is still attached to the capsule ( ...
s on dust, rather than spontaneously generated within the broth. Thus, Pasteur refuted the theory of
spontaneous generation Spontaneous generation is a body of thought on the ordinary formation of living organisms without descent from similar organisms. The theory of spontaneous generation held that living creatures could arise from nonliving matter and that such process ...
and supported the
germ theory of disease The germ theory of disease is the currently accepted scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method ...
. In 1876,
Robert Koch Heinrich Hermann Robert Koch (; ; 11 December 1843 – 27 May 1910) was a German physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medical doctor, or simply docto ...

Robert Koch
(1843–1910) established that microorganisms can cause disease. He found that the blood of cattle that were infected with
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
always had large numbers of ''
Bacillus anthracis ''Bacillus anthracis'' is a Gram-positive and rod-shaped bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic micro ...

Bacillus anthracis
''. Koch found that he could transmit anthrax from one animal to another by taking a small sample of blood from the infected animal and injecting it into a healthy one, and this caused the healthy animal to become sick. He also found that he could grow the bacteria in a nutrient broth, then inject it into a healthy animal, and cause illness. Based on these experiments, he devised criteria for establishing a causal link between a microorganism and a disease and these are now known as
Koch's postulates 200px, Robert Hermann Koch (11 December 1843 – 27 May 1910) was a German physician who developed Koch's postulates. Koch's postulates ()
. Although these postulates cannot be applied in all cases, they do retain historical importance to the development of scientific thought and are still being used today. The discovery of microorganisms such as ''
Euglena ''Euglena'' is a genus of Unicellular organism, single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 800 species. Species of ''Eugl ...

Euglena
'' that did not fit into either the
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
or
plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions o ...

plant
kingdoms, since they were
photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's metabolic activities. This chemi ...
like plants, but
motile Motility is the ability of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for ...

motile
like animals, led to the naming of a third kingdom in the 1860s. In 1860 John Hogg called this the Protoctista, and in 1866
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, natural history, naturalist, eugenics, eugenicist, philosopher, physician, professor, marine biologist, and artist who discovered, described and ...

Ernst Haeckel
named it the
Protista A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
. The work of Pasteur and Koch did not accurately reflect the true diversity of the microbial world because of their exclusive focus on microorganisms having direct medical relevance. It was not until the work of
Martinus Beijerinck The Laboratory of Microbiology in Delft, where Beijerinck worked from 1897 to 1921. Martinus Willem Beijerinck (, 16 March 1851 – 1 January 1931) was a Dutch microbiologist and botanist who was one of the founders of virology Virology i ...

Martinus Beijerinck
and
Sergei Winogradsky Sergei Nikolaievich Winogradsky (or Vinogradskiy; ukr, Сергій Миколайович Виноградський; 1 September 1856 – 25 February 1953) was a Russian microbiologist, ecologist Ecology (from el, οἶκος, "house" ...

Sergei Winogradsky
late in the nineteenth century that the true breadth of microbiology was revealed. Beijerinck made two major contributions to microbiology: the discovery of
virus A virus is a submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacte ...

virus
es and the development of
enrichment cultureEnrichment culture is the use of certain growth media to favor the growth of a particular microorganism over others, enriching a sample for the microorganism of interest. This is generally done by introducing nutrients or environmental conditions t ...
techniques. While his work on the
tobacco mosaic virus ''Tobacco mosaic virus'' (TMV) is a positive-sense single-stranded RNA virus virus species, species in the genus ''Tobamovirus'' that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection cau ...

tobacco mosaic virus
established the basic principles of virology, it was his development of enrichment culturing that had the most immediate impact on microbiology by allowing for the cultivation of a wide range of microbes with wildly different physiologies. Winogradsky was the first to develop the concept of chemolithotrophy and to thereby reveal the essential role played by microorganisms in geochemical processes. He was responsible for the first isolation and description of both nitrifying and nitrogen-fixing bacteria. French-Canadian microbiologist Felix d'Herelle co-discovered
bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a virus A virus is a submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all t ...
s and was one of the earliest applied microbiologists.


Classification and structure

Microorganisms can be found almost anywhere on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...
.
Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a Bacte ...
and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...
are almost always microscopic, while a number of
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...

eukaryote
s are also microscopic, including most
protists A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
, some
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
, as well as some
micro-animal 210px, A microscopic arachnid ''Lorryia formosa''">Lorryia_formosa.html" ;"title="arachnid ''Lorryia formosa">arachnid ''Lorryia formosa'' Micro-animals are animals so small that they can be visually observed only under a microscope. Unlike most ...
s and plants.
Virus A virus is a submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacte ...

Virus
es are generally regarded as not living and therefore not considered as microorganisms, although a subfield of
microbiology Microbiology (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mi ...

microbiology
is
virology Virology is the Scientific method, scientific study of virusessubmicroscopic, parasitic particles of genetic material contained in a protein coatand virus-like agents. It focuses on the following aspects of viruses: their structure, classificatio ...
, the study of viruses.


Evolution

Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3.5
billion years A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) ...
ago. Further evolution was slow, and for about 3 billion years in the
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologic ...

Precambrian
eon, (much of the history of ), all
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
s were microorganisms. Bacteria, algae and fungi have been identified in
amber Amber is fossilized tree resin that has been appreciated for its color and natural beauty since Neolithic times. Much valued from antiquity to the present as a gemstone, amber is made into a variety of decorative objects."Amber" (2004). In ...
that is 220 million years old, which shows that the
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
of microorganisms has changed little since at least the
Triassic The Triassic ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions int ...

Triassic
period. The newly discovered biological role played by nickel, however – especially that brought about by
volcanic eruptions Several types of volcanic eruptions—during which lava of pāhoehoe lava, Hawaii, United States , Iceland in 1984 Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) ...
from the Siberian Traps – may have accelerated the evolution of methanogens towards the end of the Permian–Triassic extinction event. Microorganisms tend to have a relatively fast rate of evolution. Most microorganisms can reproduce rapidly, and bacteria are also able to freely exchange genes through Bacterial conjugation, conjugation, Transformation (genetics), transformation and Transduction (genetics), transduction, even between widely divergent species. This horizontal gene transfer, coupled with a high mutation rate and other means of transformation, allows microorganisms to swiftly biological evolution, evolve (via natural selection) to survive in new environments and respond to stressors, environmental stresses. This rapid evolution is important in medicine, as it has led to the development of multidrug resistance, multidrug resistant pathogenic bacteria, ''superbugs'', that are antimicrobial resistance, resistant to antibiotics. A possible transitional form of microorganism between a prokaryote and a eukaryote was discovered in 2012 by Japanese scientists. ''Parakaryon myojinensis'' is a unique microorganism larger than a typical prokaryote, but with nuclear material enclosed in a membrane as in a eukaryote, and the presence of endosymbionts. This is seen to be the first plausible evolutionary form of microorganism, showing a stage of development from the prokaryote to the eukaryote.


Archaea

Archaea are prokaryote, prokaryotic unicellular organisms, and form the first domain of life, in Carl Woese's three-domain system. A prokaryote is defined as having no cell nucleus or other lipid bilayer, membrane bound-organelle. Archaea share this defining feature with the bacteria with which they were once grouped. In 1990 the microbiologist Woese proposed the three-domain system that divided living things into bacteria, archaea and eukaryotes, and thereby split the prokaryote domain. Archaea differ from bacteria in both their genetics and biochemistry. For example, while bacterial cell membranes are made from phospholipid, phosphoglycerides with ester bonds, archaean membranes are made of ether lipids. Archaea were originally described as extremophiles living in extreme environments, such as hot springs, but have since been found in all types of
habitat Ibex in an alpine habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat c ...

habitat
s. Only now are scientists beginning to realize how common archaea are in the environment, with Crenarchaeota being the most common form of life in the ocean, dominating ecosystems below 150 m in depth. These organisms are also common in soil and play a vital role in ammonia oxidation. The combined domains of archaea and bacteria make up the most diverse and abundant group of
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
s on Earth and inhabit practically all environments where the temperature is below +140 °C. They are found in water, soil, Earth's atmosphere, air, as the microbiota, microbiome of an organism, hot springs and even deep beneath the Earth's crust in
rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in which it is formed. Rocks form the Earth's oute ...
. The number of prokaryotes is estimated to be around five nonillion, or 5 × 1030, accounting for at least half the Biomass (ecology), biomass on Earth. The biodiversity of the prokaryotes is unknown, but may be very large. A May 2016 estimate, based on laws of scaling from known numbers of species against the size of organism, gives an estimate of perhaps 1 trillion species on the planet, of which most would be microorganisms. Currently, only one-thousandth of one percent of that total have been described. Archaea, Archael cells of some species aggregate and transfer DNA from one cell to another through direct contact, particularly under stressful environmental conditions that cause DNA damage (naturally occurring), DNA damage.


Bacteria

Bacteria like archaea are prokaryotic – unicellular, and having no cell nucleus or other membrane-bound organelle. Bacteria are microscopic, with a few extremely rare exceptions, such as ''Thiomargarita namibiensis''. Bacteria function and reproduce as individual cells, but they can often aggregate in multicellular Colony (biology)#Microbial colony, colonies. Some species such as myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as part of their Biological life cycle, life cycle, or form clusters in colony (biology), bacterial colonies such as ''E.coli''. Their genome is usually a circular bacterial chromosome – a single loop of DNA, although they can also harbor small pieces of DNA called plasmids. These plasmids can be transferred between cells through bacterial conjugation. Bacteria have an enclosing Bacterial cell structure#Cell wall, cell wall, which provides strength and rigidity to their cells. They reproduce by binary fission or sometimes by budding, but do not undergo Meiosis, meiotic sexual reproduction. However, many bacterial species can transfer DNA between individual cells by a horizontal gene transfer process referred to as natural Transformation (genetics), transformation. Some species form extraordinarily resilient endospore, spores, but for bacteria this is a mechanism for survival, not reproduction. Under optimal conditions bacteria can grow extremely rapidly and their numbers can double as quickly as every 20 minutes.


Eukaryotes

Most living things that are visible to the naked eye in their adult form are
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...

eukaryote
s, including humans. However, many eukaryotes are also microorganisms. Unlike bacteria and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...
, eukaryotes contain organelles such as the cell nucleus, the Golgi apparatus and mitochondrion, mitochondria in their cell (biology), cells. The nucleus is an organelle that houses the DNA that makes up a cell's genome. DNA (Deoxyribonucleic acid) itself is arranged in complex chromosomes. Mitochondria are organelles vital in metabolism as they are the site of the citric acid cycle and oxidative phosphorylation. They evolved from symbiotic bacteria and retain a remnant genome. Like bacteria, plant cells have cell walls, and contain organelles such as chloroplasts in addition to the organelles in other eukaryotes. Chloroplasts produce energy from light by photosynthesis, and were also originally symbiotic bacteria. Unicellular eukaryotes consist of a single
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
throughout their life cycle. This qualification is significant since most multicellular organism, multicellular eukaryotes consist of a single cell called a zygote only at the beginning of their life cycles. Microbial eukaryotes can be either haploid or diploid, and some organisms have multiple cell nucleus, cell nuclei. Unicellular eukaryotes usually reproduce asexually by mitosis under favorable conditions. However, under stressful conditions such as nutrient limitations and other conditions associated with DNA damage, they tend to reproduce sexually by meiosis and Fertilization, syngamy.


Protists

Of Eukaryote, eukaryotic groups, the protists are most commonly unicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify. Several
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cel ...
species are multicellular protists, and slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms. The number of species of protists is unknown since only a small proportion has been identified. Protist diversity is high in oceans, deep sea-vents, river sediment and an acidic river, suggesting that many eukaryotic microbial communities may yet be discovered.


Fungi

The
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
have several unicellular species, such as baker's yeast (''Saccharomyces cerevisiae'') and fission yeast (''Schizosaccharomyces pombe''). Some fungi, such as the pathogenic yeast ''Candida albicans'', can undergo phenotypic switching and grow as single cells in some environments, and Hypha, filamentous hyphae in others.


Plants

The green algae are a large group of photosynthetic eukaryotes that include many microscopic organisms. Although some green algae are classified as
protist A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukaryo ...
s, others such as charophyta are classified with embryophyte plants, which are the most familiar group of land plants. Algae can grow as single cells, or in long chains of cells. The green algae include unicellular and colonial flagellates, usually but not always with two flagellum, flagella per cell, as well as various colonial, Chlorococcales, coccoid, and filamentous forms. In the Charales, which are the algae most closely related to higher plants, cells differentiate into several distinct tissues within the organism. There are about 6000 species of green algae.


Ecology

Microorganisms are found in almost every Habitat (ecology), habitat present in nature, including hostile environments such as the Geographic poles, North and South poles,
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s,
geyser A geyser (, ) is a spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditions that exist only i ...

geyser
s, and
rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in which it is formed. Rocks form the Earth's oute ...
. They also include all the marine microorganisms of the World ocean, oceans and
deep sea The deep sea or deep layer is the lowest layer in the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
. Some types of microorganisms have adapted to extreme environments 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, and it has been suggested that the amount of organisms living below the Earth's surface is comparable with the amount of life on or above the surface. Extremophiles have been known to survive for a prolonged time in a vacuum, and can be highly resistant to ultraviolet radiation, radiation, which may even allow them to survive in space. Many types of microorganisms have intimate symbiosis, symbiotic relationships with other larger organisms; some of which are mutually beneficial (Mutualism (biology), mutualism), while others can be damaging to the host (biology), host organism (parasitism). If microorganisms can cause disease in a host they are known as
pathogen In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
s and then they are sometimes referred to as ''microbes''. Microorganisms play critical roles in Earth's biogeochemical cycles as they are responsible for decomposition and nitrogen fixation. Bacteria use gene regulatory network, regulatory networks that allow them to adapt to almost every environmental niche on earth. A network of interactions among diverse types of molecules including DNA, RNA, proteins and metabolites, is utilised by the bacteria to achieve regulation of gene expression. In bacteria, the principal function of regulatory networks is to control the response to environmental changes, for example nutritional status and environmental stress. A complex organization of networks permits the microorganism to coordinate and integrate multiple environmental signals.


Extremophiles

file:Deinococcus radiodurans.jpg, upA tetrad of ''
Deinococcus radiodurans ''Deinococcus radiodurans'' is an extremophilic bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms ...

Deinococcus radiodurans
'', a radioresistant extremophile bacterium Extremophiles are microorganisms that have adapted so that they can survive and even thrive in extreme environments that are normally fatal to most life-forms. Thermophiles and hyperthermophiles thrive in high temperatures. Psychrophiles thrive in extremely low temperatures. – Temperatures as high as , as low as Halophiles such as ''Halobacterium salinarum'' (an archaean) thrive in high Salinity, salt conditions, up to saturation. Alkaliphiles thrive in an alkaline pH of about 8.5–11. Acidophiles can thrive in a pH of 2.0 or less. Piezophiles thrive at very high pressures: up to 1,000–2,000 Atmosphere (unit), atm, down to 0 atm as in a vacuum of Outer space, space. A few extremophiles such as ''
Deinococcus radiodurans ''Deinococcus radiodurans'' is an extremophilic bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms ...

Deinococcus radiodurans
'' are radioresistance, radioresistant, resisting Ionizing radiation, radiation exposure of up to 5k Gray (unit), Gy. Extremophiles are significant in different ways. They extend terrestrial life into much of the Earth's hydrosphere, Crust (geology), crust and atmosphere, their specific evolutionary adaptation mechanisms to their extreme environment can be exploited in biotechnology, and their very existence under such extreme conditions increases the potential for extraterrestrial life.


Plants and Soil

The nitrogen cycle in soils depends on the nitrogen fixation, fixation of atmospheric nitrogen. This is achieved by a number of diazotrophs. One way this can occur is in the root nodules of legumes that contain symbiotic bacteria of the genera ''Rhizobium'', ''Mesorhizobium'', ''Sinorhizobium'', ''Bradyrhizobium'', and ''Azorhizobium''. The roots of plants create a narrow region known as the rhizosphere that supports many microorganisms known as the root microbiome. These microorganisms in the root microbiome are able to interact with each other and surrounding plants through signals and cues. For example, mycorrhizal fungi are able to communicate with the root systems of many plants through chemical signals between both the
plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions o ...

plant
and fungi. This results in a mutualistic symbiosis between the two. However, these signals can be eavesdropped by other microorganisms, such as the soil bacteria, ''Myxococcus xanthus'', which preys on other bacteria. Eavesdropping, or the interception of signals from unintended receivers, such as plants and microorganisms, can lead to large-scale, evolutionary consequences. For example, signaler-receiver pairs, like plant-microorganism pairs, may lose the ability to communicate with neighboring populations because of variability in eavesdroppers. In adapting to avoid local eavesdroppers, signal divergence could occur and thus, lead to the isolation of plants and microorganisms from the inability to communicate with other populations.


Symbiosis

A lichen is a symbiosis of a macroscopic fungus with
photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's metabolic activities. This chemi ...
microbial algae or cyanobacteria.


Applications

Microorganisms are useful in producing foods, treating waste water, creating biofuels and a wide range of chemicals and enzymes. They are invaluable in research as
model organism A model organism (often shortened to model) is a non-human species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molec ...
s. They have been Biological agent, weaponised and sometimes used in Biological warfare, warfare and
bioterrorism Bioterrorism is terrorism Terrorism is, in the broadest sense, the use of intentional violence to achieve political aims. It is used in this regard primarily to refer to violence during peacetime or in the context of war against non-combata ...
. They are vital to agriculture through their roles in maintaining soil fertility and in decomposing organic matter.


Food production

Microorganisms are used in a Fermentation (food), fermentation process to make yoghurt, cheese, curd, kefir, ayran, fermented milk products, xynogala, and other types of food. Fermentation cultures provide flavour and aroma, and inhibit undesirable organisms. They are used to leavening agent, leaven bread, and to convert sugars to ethanol, alcohol in wine and beer. Microorganisms are used in brewing, wine making, baking, pickling and other food-making processes. Some industrial uses of Microorganisms:


Water treatment

These depend for their ability to clean up water contaminated with organic material on microorganisms that can respire dissolved substances. Respiration may be aerobic, with a well-oxygenated filter bed such as a slow sand filter. Anaerobic digestion by methanogens generate useful methane gas as a by-product.'


Energy

Microorganisms are used in Ethanol fermentation, fermentation to produce ethanol, and in biogas reactors to produce methane. Scientists are researching the use of algae fuel, algae to produce liquid fuels, and bacteria to convert various forms of agricultural and urban waste into cellulosic ethanol, usable fuels.


Chemicals, enzymes

Microorganisms are used to produce many commercial and industrial chemicals, enzymes and other bioactive molecules. Organic acids produced on a large industrial scale by microbial fermentation include acetic acid produced by acetic acid bacteria such as ''Acetobacter aceti'', butyric acid made by the bacterium ''Clostridium butyricum'', lactic acid made by ''Lactobacillus'' and other lactic acid bacteria, and citric acid produced by the mould fungus ''Aspergillus niger''. Microorganisms are used to prepare bioactive molecules such as Streptokinase from the bacterium ''Streptococcus'', Cyclosporin A from the ascomycete fungus ''Tolypocladium inflatum'', and statins produced by the yeast ''Monascus purpureus''.


Science

Microorganisms are essential tools in biotechnology, biochemistry, genetics, and molecular biology. The yeasts ''Saccharomyces cerevisiae'' and ''Schizosaccharomyces pombe'' are important
model organism A model organism (often shortened to model) is a non-human species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molec ...
s in science, since they are simple eukaryotes that can be grown rapidly in large numbers and are easily manipulated. They are particularly valuable in genetics, genomics and proteomics. Microorganisms can be harnessed for uses such as creating steroids and treating skin diseases. Scientists are also considering using microorganisms for living fuel cells, and as a solution for pollution.


Warfare

In the Middle Ages, as an early example of
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, diseased corpses were thrown into castles during sieges using catapults or other siege engines. Individuals near the corpses were exposed to the pathogen and were likely to spread that pathogen to others. In modern times,
bioterrorism Bioterrorism is terrorism Terrorism is, in the broadest sense, the use of intentional violence to achieve political aims. It is used in this regard primarily to refer to violence during peacetime or in the context of war against non-combata ...
has included the 1984 Rajneeshee bioterror attack and the 1993 release of
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
by Aum Shinrikyo in Tokyo.


Soil

Microbes can make nutrients and minerals in the soil available to plants, produce hormones that spur growth, stimulate the plant immune system and trigger or dampen stress responses. In general a more diverse set of Soil biology, soil microbes results in fewer plant diseases and higher yield.


Human health


Human gut flora

Microorganisms can form an Endosymbiont, endosymbiotic relationship with other, larger organisms. For example, Microbial symbiosis and immunity, microbial symbiosis plays a crucial role in the immune system. The microorganisms that make up the
gut flora Gut microbiota, gut flora, or microbiome are the microorganisms including bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of p ...
in the gastrointestinal tract contribute to gut immunity, synthesize vitamins such as folic acid and biotin, and ferment complex indigestible carbohydrates. Some microorganisms that are seen to be beneficial to health are termed probiotics and are available as dietary supplements, or food additive#Fortifying agents, food additives.


Disease

Microorganisms are the causative agents (
pathogen In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
s) in many Infection, infectious diseases. The organisms involved include pathogenic bacteria, causing diseases such as bubonic plague, plague,
tuberculosis Tuberculosis (TB) is an infectious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but can also affect other parts of the body. Most infections show no symptoms, in which case ...

tuberculosis
and
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
;
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

protozoa
n parasites, causing diseases such as malaria, African trypanosomiasis, sleeping sickness, dysentery 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 pathogenic viruses, which are not usually classified as living organisms and are not, therefore, microorganisms by the strict definition. No clear examples of archaean pathogens are known, although a relationship has been proposed between the presence of some archaean methanogens and human periodontal disease. Numerous microbial pathogens are capable of sexual processes that appear to facilitate their survival in their infected host.


Hygiene

Hygiene is a set of practices to avoid infection or
food spoilage Food spoilage is the process where a food product becomes unsuitable to ingest by the consumer. The cause of such a process is due to many outside factors as a side-effect of the type of product it is, as well as how the product is packaged and stor ...
by eliminating microorganisms from the surroundings. As microorganisms, in particular bacteria, are found virtually everywhere, pathogen, harmful microorganisms may be reduced to acceptable levels rather than actually eliminated. In food preparation, microorganisms are reduced by Food preservation, preservation methods such as cooking, cleanliness of utensils, short storage periods, or by low temperatures. If complete sterility is needed, as with surgical equipment, an autoclave is used to kill microorganisms with heat and pressure.


In fiction

*''Osmosis Jones'', a 2001 film, and its show ''Ozzy & Drix'', set in a stylized version of the human body, featured anthropomorphic microorganisms.


See also

* Catalogue of Life * Impedance microbiology * Microbial biogeography * Microbial intelligence * Microbiological culture * Microbivory, an eating behavior of some animals feeding on living microbes * Nanobacterium * Nylon-eating bacteria * Petri dish * Staining (biology), Staining


Notes


References


External links


Microbes.info
is a microbiology information portal containing a vast collection of resources including articles, news, frequently asked questions, and links pertaining to the field of microbiology.
Our Microbial Planet
A free poster from the National Academy of Sciences about the positive roles of micro-organisms.
"Uncharted Microbial World: Microbes and Their Activities in the Environment"
Report from the American Academy of Microbiology
Understanding Our Microbial Planet: The New Science of Metagenomics
A 20-page educational booklet providing a basic overview of metagenomics and our microbial planet.
Tree of Life Eukaryotes

Microbe News from Genome News Network


On-line textbook
Through the microscope: A look at all things small
On-line microbiology textbook by Timothy Paustian and Gary Roberts, University of Wisconsin–Madison *
Methane-spewing microbe blamed in worst mass extinction. CBCNews
{{Authority control Microorganisms,