A microorganism, or microbe, is an organism of microscopic 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 antiquity, with an early attestation in Jain literature authored in 6th-century BC India. The scientific study of microorganisms began with their observation under the microscope in the 1670s by Anton van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s, Robert Koch discovered that microorganisms caused the diseases tuberculosis, cholera, diphtheria, and anthrax.
Microorganisms are extremely diverse, representing most unicellular organisms in all three domains of life: two of the three domains, Archaea and Bacteria, only contain microorganisms. The third domain, Eukaryota, includes all multicellular organisms as well as many unicellular and that are microbes. Some protists are related to animals and some to green plants. Many multicellular organisms are also microscopic, namely , some fungus, and some algae.
Microorganisms can have very different , and live everywhere from the Geographic pole to the equator, in , , rocks, and the deep sea. Some are extremophiles such as very hot or psychrophile, others to piezophile, and a few, such as Deinococcus radiodurans, to radioresistance environments. Microorganisms also make up the microbiota found in and on all multicellular organisms. There is evidence that 3.45-billion-year-old Australian rocks once contained microorganisms, the earliest direct evidence of life on Earth.
Microbes are important in human culture and health in many ways, serving to ferment foods and sewage treatment, and to algae fuel, , and other bioactive compounds. Microbes are essential tools in biology as and have been put to use in biological warfare and bioterrorism. Microbes are a vital component of fertile soil. In the human body, microorganisms make up the human microbiota, including the essential gut flora. The responsible for many infectious diseases are microbes and, as such, are the target of hygiene.
Discovery
Ancient precursors
The possible existence of microscopic organisms was discussed for many centuries before their discovery in the 17th century. By the 6th century BC, the
of present-day India postulated the existence of tiny organisms called
.
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
Mahavira, the 24th preacher of Jainism, 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 scholar Marcus Terentius Varro in a 1st-century BC book entitled On Agriculture in which he called the unseen creatures animalia minuta, and warns against locating a homestead near a swamp:[ Varro on Agriculture 1, xii Loeb]
In The Canon of Medicine (1020), Avicenna suggested that tuberculosis and other diseases might be contagious.
Early modern
Turkish scientist
Akshamsaddin mentioned the microbe in his work (The Material of Life) about two centuries prior to Leeuwenhoek's experimental discovery:
In 1546, Girolamo Fracastoro proposed that epidemic diseases 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 is considered to be one of the fathers of microbiology. He was the first in 1673 to discover and conduct scientific experiments with microorganisms, using simple single-lensed Microscope of his own design.[Payne, A. S. The Cleere Observer: A Biography of Antoni Van Leeuwenhoek, p. 13, Macmillan, 1970] Robert Hooke, a contemporary of Leeuwenhoek, also used microscopy to observe microbial life in the form of the fruiting bodies of moulds. In his 1665 book Micrographia, he made drawings of studies, and he coined the term cell.
19th century
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, 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
on dust, rather than spontaneously generated within the broth. Thus, Pasteur refuted the theory of spontaneous generation and supported the germ theory of disease.
In 1876, Robert Koch (1843–1910) established that microorganisms can cause disease. He found that the blood of cattle that were infected with anthrax always had large numbers of 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.[ The Nobel Prize in Physiology or Medicine 1905 Nobelprize.org Accessed 22 November 2006.] 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 that did not fit into either the animal or plant kingdoms, since they were photosynthetic like plants, but 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 named it the Protista.
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 and Sergei Winogradsky in the late 19th century that the true breadth of microbiology was revealed.[ Retrieved from Internet Archive 12 January 2014.] While his work on the 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.
Classification and structure
Microorganisms can be found almost anywhere on
Earth.
Bacteria and
archaea are almost always microscopic, while a number of
are also microscopic, including most
Protista, some
fungus, as well as some
and plants.
are generally regarded as not living and therefore not considered to be microorganisms, although a subfield of
microbiology is
virology, the study of viruses.
Evolution
Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3.5 billion years ago.
Further evolution was slow,
and for about 3 billion years in the
Precambrian eon, (much of the history of
life), all
were microorganisms.
Bacteria, algae and fungi have been identified in
amber that is 220 million years old, which shows that the morphology of microorganisms has changed little since at least the
Triassic period.
The newly discovered biological role played by nickel, however – especially that brought about by volcanic eruptions from the
Siberian Traps – may have accelerated the evolution of
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 conjugation, transformation and transduction, even between widely divergent species.
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 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-
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 are made from phospholipid with ester bonds, Achaean membranes are made of .
The combined domains of archaea and bacteria make up the most diverse and abundant group of on Earth and inhabit practically all environments where the temperature is below +. They are found in water, soil, air, as the microbiome of an organism, and even deep beneath the Earth's crust in rocks.
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.[Bernstein H, Bernstein C. Sexual communication in archaea, the precursor to meiosis. pp. 103–117 in Biocommunication of Archaea (Guenther Witzany, ed.) 2017. Springer International Publishing DOI 10.1007/978-3-319-65536-9]
Bacteria
Like archaea, bacteria 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 colonies.
Some species such as
myxobacteria can aggregate into complex
structures, operating as multicellular groups as part of their life cycle,
or form clusters in 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 . These plasmids can be transferred between cells through bacterial conjugation. Bacteria have an enclosing cell wall, which provides strength and rigidity to their cells. They reproduce by binary fission or sometimes by budding, but do not undergo Meiosis sexual reproduction. However, many bacterial species can transfer DNA between individual cells by a horizontal gene transfer process referred to as natural transformation.
Eukaryotes
Most living things that are visible to the naked eye in their adult form are
, including
. However, many eukaryotes are also microorganisms. Unlike
bacteria and
archaea, eukaryotes contain
such as the
cell nucleus, the
Golgi apparatus and
mitochondrion in their 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
.
[ Eukaryota: More on Morphology. (Retrieved 10 October 2006)]
Mitochondrion 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,
have
, and contain organelles such as
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 throughout their life cycle. This qualification is significant since most 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.
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.
Protists
Of Eukaryote groups, the protists are most commonly unicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify.
Fungi
The fungus 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 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 , 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 , usually but not always with two flagellum per cell, as well as various colonial, Chlorococcales, 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 present in nature, including hostile environments such as the Geographic poles, , , and rocks. They also include all the marine microorganisms of the World ocean and deep sea. 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,[ Extremophiles have been known to survive for a prolonged time in a vacuum, and can be highly resistant to radiation, which may even allow them to survive in space.]
Bacteria use regulatory networks that allow them to adapt to almost every environmental niche on earth.
Extremophiles
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. and hyperthermophiles thrive in high . thrive in extremely low temperatures. – Temperatures as high as ,[Strain 121, a Hyperthermophile archaea, has been shown to reproduce at , and survive at .[3]] as low as [Some Psychrophiles bacteria can grow at ),[4] and can survive near absolute zero).] such as Halobacterium salinarum (an archaean) thrive in high Salinity, up to saturation.[Dyall-Smith, Mike, HALOARCHAEA, University of Melbourne. See also Haloarchaea.] thrive in an alkaline pH of about 8.5–11.[Picrophilus can grow at pH −0.06.[6] ] thrive at very : up to 1,000–2,000 atm, down to 0 atm as in a vacuum of Outer space. A few extremophiles such as Deinococcus radiodurans are radioresistance,
Plants and soil
The nitrogen cycle in soils depends on the fixation of atmospheric nitrogen. This is achieved by a number of . One way this can occur is in the of legumes that contain symbiotic bacteria of the genera Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium.
The 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 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 microbial 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 . They have been Biological agent and sometimes used in warfare and bioterrorism. They are vital to agriculture through their roles in maintaining soil fertility and in decomposing organic matter. They also have applications in aquaculture, such as in biofloc technology.
Food production
Microorganisms are used in a fermentation process to make yoghurt, cheese, curd, kefir, ayran, xynogala, and other types of food. Fermentation cultures provide flavour and aroma, and inhibit undesirable organisms.
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.
Energy
Microorganisms are used in fermentation to produce ethanol,
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,[
]
Microorganisms are used to prepare bioactive molecules such as Streptokinase from the bacterium Streptococcus,
Science
Microorganisms are essential tools in biotechnology, biochemistry, genetics, and molecular biology. The Saccharomyces cerevisiae and Schizosaccharomyces pombe are important in science, since they are simple eukaryotes that can be grown rapidly in large numbers and are easily manipulated.
Warfare
In the Middle Ages, as an early example of biological warfare, diseased corpses were thrown into castles during using catapults or other . Individuals near the corpses were exposed to the pathogen and were likely to spread that pathogen to others.
In modern times, bioterrorism has included the 1984 Rajneeshee bioterror attack
Soil
Microbes can make 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 microbes results in fewer plant diseases and higher yield.
Human health
Human gut flora
Microorganisms can form an Endosymbiont relationship with other, larger organisms. For example, microbial symbiosis plays a crucial role in the immune system. The microorganisms that make up the gut flora in the gastrointestinal tract contribute to gut immunity, synthesize such as folic acid and biotin, and ferment complex indigestible .
Disease
Microorganisms are the causative agents () in many Infection. The organisms involved include pathogenic bacteria, causing diseases such as bubonic plague, tuberculosis and anthrax; , causing diseases such as malaria, 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,
Hygiene
Hygiene is a set of practices to avoid infection or food spoilage by eliminating microorganisms from the surroundings. As microorganisms, in particular bacteria, are found virtually everywhere, pathogen may be reduced to acceptable levels rather than actually eliminated. In food preparation, microorganisms are reduced by 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 Anthropomorphism microorganisms.
-
War of the Worlds (2005 film), when alien lifeforms attempt to conquer Earth, they are ultimately defeated by a common microbe to which humans are immune.
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
-
Budapest Treaty (Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure)
Notes
External links
