Microorganism

Part of a series on
Biology
Science of life
Index Outline Glossary History (timeline)
Key components Cell theory Ecosystem Evolution Phylogeny Properties of life Adaptation Energy processing Growth Order Regulation Reproduction Response to environment Domains and Kingdoms of life Archaea Bacteria Protists )
Branches Abiogenesis Aerobiology Agronomy Agrostology Anatomy Astrobiology Bacteriology Biochemistry Biogeography Biogeology Bioinformatics Biological engineering Biomechanics Biophysics Biosemiotics Biostatistics Biotechnology Botany Cell biology Cellular microbiology Chemical biology Chronobiology Cognitive biology Computational biology Conservation biology Cryobiology Cytogenetics Dendrology Developmental biology Ecological genetics Ecology Embryology Epidemiology Epigenetics Evolutionary biology Freshwater biology Generative biology Genetics Genomics Geobiology Gerontology Herpetology Histology Human biology Ichthyology Immunology Lipidology Mammalogy Marine biology Mathematical biology Microbiology Molecular biology Mycology Neontology Neuroscience Nutrition Ornithology Osteology Paleontology Parasitology Pathology Pharmacology Photobiology Phycology Phylogenetics Physiology Pomology Primatology Proteomics Protistology Quantum biology Relational biology Reproductive biology Sociobiology Structural biology Synthetic biology Systematics Systems biology Taxonomy Teratology Toxicology Virology Virophysics Xenobiology Zoology
Research Biologist (list) List of biology awards List of journals List of research methods List of unsolved problems
Applications Agricultural science Biomedical sciences Health technology Pharming
Biology portal  Category
v t e

A microorganism, or microbe,^[a] 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 ancient times, such as in

Ancient precursors

The possible existence of microscopic organisms was discussed for many centuries before their discovery in the seventeenth century. By the

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 first-century BC book entitled On Agriculture in which he called the unseen creatures animalia minuta, and warns against locating a homestead near a swamp:^[6]

… and because there are bred certain minute creatures that cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and they cause serious diseases.^[6]

In The Canon of Medicine (1020), Avicenna suggested that tuberculosis and other diseases might be contagious.^[7][8]

Early modern

Turkish scientist Akshamsaddin mentioned the microbe in his work Maddat ul-Hayat (The Material of Life) about two centuries prior to Antonie van Leeuwenhoek's discovery through experimentation:

It is incorrect to assume that diseases appear one by one in humans. Disease infects by spreading from one person to another. This infection occurs through seeds that are so small they cannot be seen but are alive.^[9][10]

In

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 spores on dust, rather than spontaneously generated within the broth. Thus, Pasteur refuted the theory of spontaneous generation and supported the germ theory of disease.^[17]

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.^[18] 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.^[19]

The discovery of microorganisms such as

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

Felix d'Herelle co-discovered bacteriophages and was one of the earliest applied microbiologists.^[26]

Classification and structure

Microorganisms can be found almost anywhere on

micro-animals and plants. Viruses are generally regarded as not living and therefore not considered to be microorganisms, although a subfield of microbiology is virology, the study of viruses.^[27][28][29]

Evolution

Further information: Timeline of the evolutionary history of life and Earliest known life forms

Single-celled microorganisms were the

volcanic eruptions from the Siberian Traps – may have accelerated the evolution of methanogens towards the end of the Permian–Triassic extinction event.^[37]

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

multidrug resistant pathogenic bacteria, superbugs, that are resistant to antibiotics.^[39]

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.^[40][41]

Archaea

Main article: Archaea

Further information: Prokaryote

Archaea are 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 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,^[42] 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 phosphoglycerides with ester bonds, archaean membranes are made of ether lipids.^[43] Archaea were originally described as extremophiles living in extreme environments, such as hot springs, but have since been found in all types of habitats.^[44] Only now are scientists beginning to realize how common archaea are in the environment, with Thermoproteota (formerly Crenarchaeota) being the most common form of life in the ocean, dominating ecosystems below 150 metres (490 ft) in depth.^[45][46] These organisms are also common in soil and play a vital role in ammonia oxidation.^[47]

The combined domains of archaea and bacteria make up the most diverse and abundant group of

air, as the microbiome of an organism, hot springs and even deep beneath the Earth's crust in rocks.^[48] The number of prokaryotes is estimated to be around five nonillion, or 5 × 10³⁰, accounting for at least half the biomass on Earth.^[49]

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.[50] 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.^[51][52]

Bacteria

Main article: 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

E.coli

.

Their

binary fission or sometimes by budding, but do not undergo meiotic sexual reproduction. However, many bacterial species can transfer DNA between individual cells by a horizontal gene transfer process referred to as natural transformation.^[56] Some species form extraordinarily resilient 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.^[57]

Eukaryotes

Main article: Eukaryote

Most living things that are visible to the naked eye in their adult form are eukaryotes, including humans. However, many eukaryotes are also microorganisms. Unlike bacteria and archaea, eukaryotes contain organelles such as the cell nucleus, the Golgi apparatus and mitochondria 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 chromosomes.^[58] Mitochondria are organelles vital in

symbiotic bacteria and retain a remnant genome.^[59] 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.^[59]

Unicellular eukaryotes consist of a single

diploid, and some organisms have multiple cell nuclei.^[60]

Unicellular eukaryotes usually reproduce asexually by

syngamy.^[61]

Protists

Main article:

Protista

photosynthetic flagellate

Of

multicellular protists, and slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.^[64] 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.^[65][66]

Fungi

Main article: Fungus

The 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 filamentous hyphae in others.^[67]

Plants

Main article: Plant

The green algae are a large group of photosynthetic eukaryotes that include many microscopic organisms. Although some green algae are classified as protists, 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 flagella per cell, as well as various colonial, 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.^[68]

Ecology

Main article: Microbial ecology

Microorganisms are found in almost every

radiation, which may even allow them to survive in space.^[70] 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

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.^[71]

Bacteria use regulatory networks that allow them to adapt to almost every environmental niche on earth.^[72][73] 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.^[74] A complex organization of networks permits the microorganism to coordinate and integrate multiple environmental signals.^[72]

Extremophiles

Main article: Extremophile

Further information: List of microorganisms tested in outer space

radioresistant extremophile

bacterium

atm, down to 0 atm as in a vacuum of space.^[b] A few extremophiles such as Deinococcus radiodurans are radioresistant,^[81] resisting radiation exposure of up to 5k Gy. Extremophiles are significant in different ways. They extend terrestrial life into much of the Earth's hydrosphere, 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.^[82]

Plants and soil

Main article: Soil biology

The

The roots of plants create a narrow region known as the rhizosphere that supports many microorganisms known as the root microbiome.^[84]

These microorganisms in the

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 organisms. They have been weaponised and sometimes used in warfare and bioterrorism. They are vital to agriculture through their roles in maintaining soil fertility and in decomposing organic matter.

Food production

Microorganisms are used in a

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.^[90] Anaerobic digestion by methanogens generate useful methane gas as a by-product.^[91]

Energy

Microorganisms are used in fermentation to produce ethanol,^[92] and in biogas reactors to produce methane.^[93] Scientists are researching the use of algae to produce liquid fuels,^[94] and bacteria to convert various forms of agricultural and urban waste into usable fuels.^[95]

Chemicals, enzymes

Microorganisms are used to produce many commercial and industrial chemicals,

Microorganisms are essential tools in

In the Middle Ages, as an early example of biological warfare, 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.^[105]

In modern times, bioterrorism has included the 1984 Rajneeshee bioterror attack^[106] and the 1993 release of anthrax by Aum Shinrikyo in Tokyo.^[107]

Soil