A microorganism (from the Greek: μικρός, mikrós, "small" and ὀργανισμός, organismós, "organism") or microbe is a microscopic organism, which may be a single cell or multicellular organism. The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design.
Microorganisms are very diverse; they include all of the prokaryotes, namely the bacteria and archaea; and various forms of eukaryotes, comprising the protozoa, fungi, algae, microscopic plants (green algae), and animals such as rotifers and planarians. Some microbiologists also classify viruses as microorganisms, but others consider these as nonliving. Most microorganisms are microscopic, but there are some like Thiomargarita namibiensis, which are macroscopic and visible to the naked eye.
Microorganisms live in every part of the biosphere including soil, hot springs, on the ocean floor, high in the atmosphere and deep inside rocks within the Earth's crust (see also endolith). Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can fix nitrogen, they are a vital part of the nitrogen cycle, and recent studies indicate that airborne microbes may play a role in precipitation and weather.
On 17 March 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench. the deepest spot on the Earth. Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet (580 metres) below the sea floor under 8500 feet (2590 metres) of ocean off the coast of the northwestern United States. According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."
Microbes are also exploited by people in biotechnology, both in traditional food and beverage preparation, and in modern technologies based on genetic engineering. However there are many pathogenic microbes which are harmful and can even cause death in plants and animals.
Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3–4 billion years ago. Further evolution was slow, and for about 3 billion years in the Precambrian eon, all organisms were microscopic. So, for most of the history of life on Earth the only forms of life 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 the Triassic period.
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. This horizontal gene transfer, coupled with a high mutation rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via natural selection) to survive in new environments and respond to environmental stresses. This rapid evolution is important in medicine, as it has led to the recent development of "super-bugs", pathogenic bacteria that are resistant to modern antibiotics.
The possibility that microorganisms exist was discussed for many centuries before their actual discovery in the 17th century. The existence of unseen microbiological life was postulated by Jainism, which is based on Mahavira's teachings as early as 6th century BCE. Paul Dundas notes that Mahavira asserted existence of unseen microbiological creatures living in earth, water, air and fire. Jain scriptures also describe nigodas, which are sub-microscopic creatures living in large clusters and having a very short life and are said to pervade each and every part of universe, even in tissues of plants and flesh of animals. However, 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 titled On Agriculture in which he warns against locating a homestead near swamps:
… 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.
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.
All these early claims about the existence of microorganisms were speculative and were not based on any data or science. Microorganisms were neither proven, observed, nor correctly and accurately described until the 17th century. The reason for this was that all these early studies lacked the microscope.
History of microorganisms' discovery 
Antonie Van Leeuwenhoek (1632–1723) was one of the first people to observe microorganisms, using a microscope of his own design, thereby making one of the most important contributions to biology. Robert Hooke was the first to use a microscope to observe living things; his 1665 book Micrographia contained descriptions of plant cells.
Before Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery why grapes could be turned into wine, milk into cheese, or why food would spoil. Leeuwenhoek did not make the connection between these processes and microorganisms, but using a microscope, he did establish that there were forms of life that were not visible to the naked eye. Leeuwenhoek's discovery, along with subsequent observations by Spallanzani and Pasteur, ended the long-held belief that life spontaneously appeared from non-living substances during the process of spoilage.
Lazzaro Spallanzani (1729–1799) found that boiling broth would sterilise it, killing any microorganisms in it. He also found that new microorganisms could only settle in a broth if the broth was exposed to air.
Louis Pasteur (1822–1895) expanded upon Spallanzani's findings by exposing boiled broths to the air, in vessels that contained a filter to prevent all particles from passing through to the growth medium, and also in vessels with no filter at all, with air being admitted via a curved tube that would not allow dust particles to 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 dealt the death blow to the theory of spontaneous generation and supported germ theory.
In 1876, Robert Koch (1843–1910) established that microbes can cause disease. He found that the blood of cattle who 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 microbe and a disease and these are now known as 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.
Classification and structure 
Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet. Bacteria and archaea are almost always microscopic, while a number of eukaryotes are also microscopic, including most protists, some fungi, as well as some animals and plants. Viruses are generally regarded as not living and therefore not considered as microbes, although the field of microbiology also encompasses the study of viruses.
Prokaryotes are organisms that lack a cell nucleus and the other membrane bound organelles. They are almost always unicellular, although some species such as myxobacteria can aggregate into complex structures as part of their life cycle.
Consisting of two domains, bacteria and archaea, the prokaryotes are the most diverse and abundant group of organisms on Earth and inhabit practically all environments where the temperature is below +140 °C. They are found in water, soil, air, animals' gastrointestinal tracts, hot springs and even deep beneath the Earth's crust in rocks. Practically all surfaces that have not been specially sterilized are covered by prokaryotes. The number of prokaryotes on Earth is estimated to be around five million trillion trillion, or 5 × 1030, accounting for at least half the biomass on Earth.
Almost all bacteria are invisible to the naked eye, with a few extremely rare exceptions, such as Thiomargarita namibiensis. They lack a nucleus and other membrane-bound organelles, and can function and reproduce as individual cells, but often aggregate in multicellular colonies. Their genome is usually 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 are surrounded by a cell wall, which provides strength and rigidity to their cells. They reproduce by binary fission or sometimes by budding, but do not undergo sexual reproduction. 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 can double as quickly as every 20 minutes.
Archaea are also single-celled organisms that lack nuclei. In the past, the differences between bacteria and archaea were not recognised and archaea were classified with bacteria as part of the kingdom Monera. However, in 1990 the microbiologist Carl Woese proposed the three-domain system that divided living things into bacteria, archaea and eukaryotes. 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.
Archaea were originally described in extreme environments, such as hot springs, but have since been found in all types of habitats. 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.
Most living things that are visible to the naked eye in their adult form are eukaryotes, including humans. However, a large number of 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 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 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 nuclei (see coenocyte).
Of 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 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 we may have identified only a small proportion. Studies from 2001-2004 have shown that a large number of protist diversity exitsts in oceans, deep sea-vents, river sediment and an acidic river which suggests that a large number of eukaryotic microbial communities have yet to be discovered.
Most animals are multicellular, but some are too small to be seen by the naked eye. Microscopic arthropods include dust mites and spider mites. Microscopic crustaceans include copepods and the cladocera, while many nematodes are too small to be seen with the naked eye. Another particularly common group of microscopic animals are the rotifers, which are filter feeders that are usually found in fresh water. Micro-animals reproduce both sexually and asexually and may reach new habitats as some eggs can survive harsh environments that would kill the adult animal. However, some simple animals, such as rotifers and nematodes, can dry out completely and remain dormant for long periods of time.
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. Fungi reproduce both asexually, by budding or binary fission, as well by producing spores, which are called conidia when produced asexually, or basidiospores when produced sexually.
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.
Habitats and ecology 
Microorganisms are found in almost every habitat present in nature. Even in hostile environments such as the poles, deserts, geysers, rocks, and the deep sea. Some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles. Extremophiles have been isolated from rocks as much as 7 kilometres below the Earth's surface, and it has been suggested that the amount of living organisms below the Earth's surface may be 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 radiation, which may even allow them to survive in space. 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 usually referred to as microbes.
Extremophiles are microorganisms that have adapted so that they can survive and even thrive in conditions that are normally fatal to most life-forms. For example, some species have been found in the following extreme environments:
- Temperature: as high as 130 °C (266 °F), as low as −17 °C (1 °F)
- Acidity/alkalinity: less than pH 0, up to pH 11.5
- Salinity: up to saturation
- Pressure: up to 1,000-2,000 atm, down to 0 atm (e.g. vacuum of space)
- Radiation: up to 5kGy
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 bio-technology, and their very existence under such extreme conditions increases the potential for extraterrestrial life.
Soil microorganisms 
The nitrogen cycle in soils depends on the fixation of atmospheric nitrogen. One way this can occur is in the nodules in the roots of legumes that contain symbiotic bacteria of the genera Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium.
Symbiotic microorganisms 
Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms' dead remains and waste products through decomposition. Microorganisms also have an important place in most higher-order multicellular organisms as symbionts. Many blame the failure of Biosphere 2 on an improper balance of microorganisms.
Use in digestion 
Some forms of bacteria that live in animals' stomachs help in their digestion.For example,cows have a variety of different microbes in their stomachs that aid them in their digestion of grass and hay.
Use in food 
They are also used to control the fermentation process in the production of cultured dairy products such as yogurt and cheese. The cultures also provide flavour and aroma, and inhibit undesirable organisms.
Use in water treatment 
The majority of all oxidative sewage treatment processes rely on a large range of microorganisms to oxidise organic constituents which are not amenable to sedimentation or flotation. Anaerobic microorganisms are also used to reduce sludge solids producing methane gas (amongst other gases) and a sterile mineralised residue. In potable water treatment, one method, the slow sand filter, employs a complex gelatinous layer composed of a wide range of microorganisms to remove both dissolved and particulate material from raw water.
Use in energy 
Microbes are used in fermentation to produce ethanol, and in biogas reactors to produce methane. Scientists are researching the use of algae to produce liquid fuels, and bacteria to convert various forms of agricultural and urban waste into usable fuels.
Use in production of chemicals, enzymes etc. 
Many microbes are used for commercial and industrial production of chemicals, enzymes and other bioactive molecules.
Examples of organic acid produced include
- Acetic acid: Produced by the bacterium Acetobacter aceti and other acetic acid bacteria (AAB)
- Butyric acid (butanoic acid): Produced by the bacterium Clostridium butyricum
- Lactic acid: Lactobacillus and others commonly called as lactic acid bacteria (LAB)
- Citric acid: Produced by the fungus Aspergillus niger
Microbes are used for preparation of bioactive molecules and enzymes.
- Streptokinase produced by the bacterium Streptococcus and modified by genetic engineering is used as a clot buster for removing clots from the blood vessels of patients who have undergone myocardial infarctions leading to heart attack.
- Cyclosporin A is a bioactive molecule used as an immunosuppressive agent in organ transplantation
- Statins produced by the yeast Monascus purpureus are commercialised as blood cholesterol lowering agents which act by competitively inhibiting the enzyme responsible for synthesis of cholesterol.
Use in science 
Microbes are also essential tools in biotechnology, biochemistry, genetics, and molecular biology. The yeasts (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are important model organisms 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. Microbes can be harnessed for uses such as creating steroids and treating skin diseases. Scientists are also considering using microbes for living fuel cells, and as a solution for pollution.
Use in warfare 
In the Middle Ages, diseased corpses were thrown into castles during sieges using catapults or other siege engines. Individuals near the corpses were exposed to the deadly pathogen and were likely to spread that pathogen to others.
Importance in human health 
Human digestion 
Microorganisms can form an endosymbiotic relationship with other, larger organisms. For example, the bacteria that live within the human digestive system contribute to gut immunity, synthesise vitamins such as folic acid and biotin, and ferment complex indigestible carbohydrates.
Diseases caused by microbes 
Microorganisms are the cause of many infectious diseases. The organisms involved include pathogenic bacteria, causing diseases such as plague, tuberculosis and anthrax; protozoa, causing diseases such as malaria, sleeping sickness and toxoplasmosis; and also fungi causing diseases such as ringworm, candidiasis or histoplasmosis. However, other diseases such as influenza, yellow fever or AIDS are caused by pathogenic viruses, which are not usually classified as living organisms and are not, therefore, microorganisms by the strict definition. As of 2007[update], 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.
Importance in ecology 
Microbes are critical to the processes of decomposition required to cycle nitrogen and other elements back to the natural world.
Hygiene is the avoidance of infection or food spoiling by eliminating microorganisms from the surroundings. As microorganisms, in particular bacteria, are found virtually everywhere, the levels of harmful microorganisms can be reduced to acceptable levels. However, in some cases, it is required that an object or substance be completely sterile, i.e. devoid of all living entities and viruses. A good example of this is a hypodermic needle.
In food preparation microorganisms are reduced by preservation methods (such as the addition of vinegar), clean utensils used in preparation, short storage periods, or by cool temperatures. If complete sterility is needed, the two most common methods are irradiation and the use of an autoclave, which resembles a pressure cooker.
There are several methods for investigating the level of hygiene in a sample of food, drinking water, equipment, etc. Water samples can be filtrated through an extremely fine filter. This filter is then placed in a nutrient medium. Microorganisms on the filter then grow to form a visible colony. Harmful microorganisms can be detected in food by placing a sample in a nutrient broth designed to enrich the organisms in question. Various methods, such as selective media or PCR, can then be used for detection. The hygiene of hard surfaces, such as cooking pots, can be tested by touching them with a solid piece of nutrient medium and then allowing the microorganisms to grow on it.
There are no conditions where all microorganisms would grow, and therefore often several different methods are needed. For example, a food sample might be analyzed on three different nutrient mediums designed to indicate the presence of "total" bacteria (conditions where many, but not all, bacteria grow), molds (conditions where the growth of bacteria is prevented by, e.g., antibiotics) and coliform bacteria (these indicate a sewage contamination).
See also 
- Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms (13th ed.). Pearson Education. p. 1096. ISBN 0-321-73551-X.
- Rybicki EP (1990). "The classification of organisms at the edge of life, or problems with virus systematics". S Aft J Sci 86: 182–6. ISSN 0038-2353.
- LWOFF A (1956). "The concept of virus". J. Gen. Microbiol. 17 (2): 239–53. PMID 13481308.
- Max Planck Society Research News Release Accessed 15 September 2012
- Christner BC, Morris CE, Foreman CM, Cai R, Sands DC (2008). "Ubiquity of biological ice nucleators in snowfall". Science 319 (5867): 1214. Bibcode:2008Sci...319.1214C. doi:10.1126/science.1149757. PMID 18309078.
- Choi, Charles Q. (17 March 2013). "Microbes Thrive in Deepest Spot on Earth". LiveScience. Retrieved 17 March 2013.
- Glud, Ronnie; Wenzhöfer, Frank; Middleboe, Mathias; Oguri, Kazumasa; Turnewitsch, Robert; Canfield, Donald E.; Kitazato, Hiroshi (17 March 2013). "High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth". Nature Geoscience 6 (4): 284. doi:10.1038/ngeo1773. Retrieved 17 March 2013.
- Oskin, Becky (14 March 2013). "Intraterrestrials: Life Thrives in Ocean Floor". LiveScience. Retrieved 17 March 2013.
- 2002 WHO mortality data Accessed 20 January 2007
- Schopf J (2006). "Fossil evidence of Archaean life". Philos Trans R Soc Lond B Biol Sci 361 (1470): 869–85. doi:10.1098/rstb.2006.1834. PMC 1578735. PMID 16754604.
- Altermann W, Kazmierczak J (2003). "Archean microfossils: a reappraisal of early life on Earth". Res Microbiol 154 (9): 611–7. doi:10.1016/j.resmic.2003.08.006. PMID 14596897.
- Cavalier-Smith T (2006). "Cell evolution and Earth history: stasis and revolution". Philos Trans R Soc Lond B Biol Sci 361 (1470): 969–1006. doi:10.1098/rstb.2006.1842. PMC 1578732. PMID 16754610.
- Schopf J (1994). "Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic". Proc Natl Acad Sci USA 91 (15): 6735–42. Bibcode:1994PNAS...91.6735S. doi:10.1073/pnas.91.15.6735. PMC 44277. PMID 8041691.
- Stanley S (May 1973). "An Ecological Theory for the Sudden Origin of Multicellular Life in the Late Precambrian". Proc Natl Acad Sci USA 70 (5): 1486–9. Bibcode:1973PNAS...70.1486S. doi:10.1073/pnas.70.5.1486. PMC 433525. PMID 16592084.
- DeLong E, Pace N (2001). "Environmental diversity of bacteria and archaea". Syst Biol 50 (4): 470–8. doi:10.1080/106351501750435040. PMID 12116647.
- Schmidt A, Ragazzi E, Coppellotti O, Roghi G (2006). "A microworld in Triassic amber". Nature 444 (7121): 835. Bibcode:2006Natur.444..835S. doi:10.1038/444835a. PMID 17167469.
- Wolska K (2003). "Horizontal DNA transfer between bacteria in the environment". Acta Microbiol Pol 52 (3): 233–43. PMID 14743976.
- Enright M, Robinson D, Randle G, Feil E, Grundmann H, Spratt B (May 2002). "The evolutionary history of methicillin-resistant Staphylococcus aureus (MRSA)". Proc Natl Acad Sci USA 99 (11): 7687–92. Bibcode:2002PNAS...99.7687E. doi:10.1073/pnas.122108599. PMC 124322. PMID 12032344.
- Mahavira is dated 599 BCE - 527 BCE. See. Dundas, Paul; John Hinnels ed. (2002). The Jains. London: Routledge. ISBN 0-415-26606-8. p. 24
- Dundas, Paul (2002) p. 88
- *Jaini, Padmanabh (1998). The Jaina Path of Purification. New Delhi: Motilal Banarsidass. ISBN 81-208-1578-5. p. 109
- Varro On Agriculture 1,xii Loeb
- Tschanz, David W. "Arab Roots of European Medicine". Heart Views 4 (2).
- Colgan, Richard (2009). Advice to the Young Physician: On the Art of Medicine. Springer. p. 33. ISBN 978-1-4419-1033-2.
- Payne, A.S. The Cleere Observer: A Biography of Antoni Van Leeuwenhoek, p. 13, Macmillan, 1970
- Leeuwenhoek A (1753). "Part of a Letter from Mr Antony van Leeuwenhoek, concerning the Worms in Sheeps Livers, Gnats, and Animalcula in the Excrements of Frogs". Philosophical Transactions (1683–1775) 22 (260–276): 509–18. doi:10.1098/rstl.1700.0013. Retrieved 30 November 2006.
- Leeuwenhoek A (1753). "Part of a Letter from Mr Antony van Leeuwenhoek, F. R. S. concerning Green Weeds Growing in Water, and Some Animalcula Found about Them". Philosophical Transactions (1683–1775) 23 (277–288): 1304–11. doi:10.1098/rstl.1702.0042. Retrieved 30 November 2006.
- The Nobel Prize in Physiology or Medicine 1905 Nobelprize.org Accessed 22 November 2006.
- O'Brien S, Goedert J (1996). "HIV causes AIDS: Koch's postulates fulfilled". Curr Opin Immunol 8 (5): 613–18. doi:10.1016/S0952-7915(96)80075-6. PMID 8902385.
- Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P (2006). "Toward automatic reconstruction of a highly resolved tree of life". Science 311 (5765): 1283–7. Bibcode:2006Sci...311.1283C. doi:10.1126/science.1123061. PMID 16513982.
- Gold T (1992). "The deep, hot biosphere". Proc. Natl. Acad. Sci. U.S.A. 89 (13): 6045–9. Bibcode:1992PNAS...89.6045G. doi:10.1073/pnas.89.13.6045. PMC 49434. PMID 1631089.
- Whitman W, Coleman D, Wiebe W (1998). "Prokaryotes: The unseen majority". Proc Natl Acad Sci USA 95 (12): 6578–83. Bibcode:1998PNAS...95.6578W. doi:10.1073/pnas.95.12.6578. PMC 33863. PMID 9618454.
- Schulz H, Jorgensen B (2001). "Big bacteria". Annu Rev Microbiol 55: 105–37. doi:10.1146/annurev.micro.55.1.105. PMID 11544351.
- Shapiro JA (1998). "Thinking about bacterial populations as multicellular organisms". Annu. Rev. Microbiol. 52: 81–104. doi:10.1146/annurev.micro.52.1.81. PMID 9891794.
- Eagon R (1962). "PSEUDOMONAS NATRIEGENS, A MARINE BACTERIUM WITH A GENERATION TIME OF LESS THAN 10 MINUTES". J Bacteriol 83 (4): 736–7. PMC 279347. PMID 13888946.
- Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proc Natl Acad Sci USA 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
- De Rosa M, Gambacorta A, Gliozzi A (1 March 1986). "Structure, biosynthesis, and physicochemical properties of archaebacterial lipids". Microbiol. Rev. 50 (1): 70–80. PMC 373054. PMID 3083222.
- Robertson C, Harris J, Spear J, Pace N (2005). "Phylogenetic diversity and ecology of environmental Archaea". Curr Opin Microbiol 8 (6): 638–42. doi:10.1016/j.mib.2005.10.003. PMID 16236543.
- Karner MB, DeLong EF, Karl DM (2001). "Archaeal dominance in the mesopelagic zone of the Pacific Ocean". Nature 409 (6819): 507–10. doi:10.1038/35054051. PMID 11206545.
- Sinninghe Damsté JS, Rijpstra WI, Hopmans EC, Prahl FG, Wakeham SG, Schouten S (June 2002). "Distribution of Membrane Lipids of Planktonic Crenarchaeota in the Arabian Sea". Appl. Environ. Microbiol. 68 (6): 2997–3002. doi:10.1128/AEM.68.6.2997-3002.2002. PMC 123986. PMID 12039760.
- Leininger S, Urich T, Schloter M, et al. (2006). "Archaea predominate among ammonia-oxidizing prokaryotes in soils". Nature 442 (7104): 806–9. Bibcode:2006Natur.442..806L. doi:10.1038/nature04983. PMID 16915287.
- Eukaryota: More on Morphology. (Retrieved 10 October 2006)
- Dyall S, Brown M, Johnson P (2004). "Ancient invasions: from endosymbionts to organelles". Science 304 (5668): 253–7. Bibcode:2004Sci...304..253D. doi:10.1126/science.1094884. PMID 15073369.
- Cavalier-Smith T (1 December 1993). "Kingdom protozoa and its 18 phyla". Microbiol. Rev. 57 (4): 953–94. PMC 372943. PMID 8302218.
- Corliss JO (1992). "Should there be a separate code of nomenclature for the protists?". BioSystems 28 (1–3): 1–14. doi:10.1016/0303-2647(92)90003-H. PMID 1292654.
- Devreotes P (1989). "Dictyostelium discoideum: a model system for cell-cell interactions in development". Science 245 (4922): 1054–8. Bibcode:1989Sci...245.1054D. doi:10.1126/science.2672337. PMID 2672337.
- Slapeta J, Moreira D, López-García P (2005). "The extent of protist diversity: insights from molecular ecology of freshwater eukaryotes". Proc. Biol. Sci. 272 (1576): 2073–81. doi:10.1098/rspb.2005.3195. PMC 1559898. PMID 16191619.
- Moreira D, López-García P (2002). "The molecular ecology of microbial eukaryotes unveils a hidden world". Trends Microbiol. 10 (1): 31–8. doi:10.1016/S0966-842X(01)02257-0. PMID 11755083.
- At least one animal group is unicellular in its adult form: see Myxozoa.
- Lapinski J, Tunnacliffe A (2003). "Anhydrobiosis without trehalose in bdelloid rotifers". FEBS Lett. 553 (3): 387–90. doi:10.1016/S0014-5793(03)01062-7. PMID 14572656.
- Kumamoto CA, Vinces MD (2005). "Contributions of hyphae and hypha-co-regulated genes to Candida albicans virulence". Cell. Microbiol. 7 (11): 1546–54. doi:10.1111/j.1462-5822.2005.00616.x. PMID 16207242.
- Thomas, David C. (2002). Seaweeds. London: Natural History Museum. ISBN 0-565-09175-1.
- Szewzyk U, Szewzyk R, Stenström T (1994). "Thermophilic, anaerobic bacteria isolated from a deep borehole in granite in Sweden". Proc Natl Acad Sci USA 91 (5): 1810–3. Bibcode:1994PNAS...91.1810S. doi:10.1073/pnas.91.5.1810. PMC 43253. PMID 11607462.
- Horneck G (1981). "Survival of microorganisms in space: a review". Adv Space Res 1 (14): 39–48. doi:10.1016/0273-1177(81)90241-6. PMID 11541716.
- Strain 121, a hyperthermophilic archaea, has been shown to reproduce at 121 °C (250 °F), and survive at 130 °C (266 °F).
- Some Psychrophilic bacteria can grow at −17 °C (1 °F), and can survive near absolute zero.
- Picrophilus can grow at pH -0.06.
- The alkaliphilic bacteria Bacillus alcalophilus can grow at up to pH 11.5.
- Dyall-Smith, Mike, HALOARCHAEA, University of Melbourne. See also Haloarchaea.
- The piezophilic bacteria Halomonas salaria requires a pressure of 1,000 atm; nanobes, a speculative organism, have been reportedly found in the earth's crust at 2,000 atm.
- See Deinococcus radiodurans
- Cavicchioli R (2002). "Extremophiles and the search for extraterrestrial life". Astrobiology 2 (3): 281–92. Bibcode:2002AsBio...2..281C. doi:10.1089/153110702762027862. PMID 12530238.
- Barea J, Pozo M, Azcón R, Azcón-Aguilar C (2005). "Microbial co-operation in the rhizosphere". J Exp Bot 56 (417): 1761–78. doi:10.1093/jxb/eri197. PMID 15911555.
- Gillen, Alan L. (2007). The Genesis of Germs: The Origin of Diseases and the Coming Plagues. New Leaf Publishing Group. p. 10. ISBN 0-89051-493-3.
- "Dairy Microbiology". University of Guelph. Retrieved 9 October 2006.
- Gray, N.F. (2004). Biology of Wastewater Treatment. Imperial College Press. p. 1164. ISBN 1-86094-332-2.
- Kitani, Osumu and Carl W. Hall (1989). Biomass Handbook. Taylor & Francis US. p. 256. ISBN 2-88124-269-3.
- Pimental, David (2007). Food, Energy, and Society. CRC Press. p. 289. ISBN 1-4200-4667-5.
- Tickell, Joshua et al. (2000). From the Fryer to the Fuel Tank: The Complete Guide to Using Vegetable Oil as an Alternative Fuel. Biodiesel America. p. 53. ISBN 0-9707227-0-2.
- Inslee, Jay et al. (2008). Apollo's Fire: Igniting America's Clean Energy Economy. Island Press. p. 157. ISBN 1-59726-175-0.
- Biology textbook for class XII. National council of educational research and training. p. 183. ISBN 81-7450-639-X.
- Castrillo JI, Oliver SG (2004). "Yeast as a touchstone in post-genomic research: strategies for integrative analysis in functional genomics". J. Biochem. Mol. Biol. 37 (1): 93–106. doi:10.5483/BMBRep.2004.37.1.093. PMID 14761307.
- Suter B, Auerbach D, Stagljar I (2006). "Yeast-based functional genomics and proteomics technologies: the first 15 years and beyond". BioTechniques 40 (5): 625–44. doi:10.2144/000112151. PMID 16708762.
- Sunnerhagen P (2002). "Prospects for functional genomics in Schizosaccharomyces pombe". Curr. Genet. 42 (2): 73–84. doi:10.1007/s00294-002-0335-6. PMID 12478386.
- Soni, S.K. (2007). Microbes: A Source of Energy for 21st Century. New India Publishing. ISBN 81-89422-14-6.
- Moses, Vivian et al. (1999). Biotechnology: The Science and the Business. CRC Press. p. 563. ISBN 90-5702-407-1.
- Langford, Roland E. (2004). Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological. Wiley-IEEE. p. 140. ISBN 0-471-46560-7.
- O'Hara A, Shanahan F (2006). "The gut flora as a forgotten organ". EMBO Rep 7 (7): 688–93. doi:10.1038/sj.embor.7400731. PMC 1500832. PMID 16819463.
- Eckburg P, Lepp P, Relman D (2003). "Archaea and Their Potential Role in Human Disease". Infect Immun 71 (2): 591–6. doi:10.1128/IAI.71.2.591-596.2003. PMC 145348. PMID 12540534.
- Lepp P, Brinig M, Ouverney C, Palm K, Armitage G, Relman D (2004). "Methanogenic Archaea and human periodontal disease". Proc Natl Acad Sci USA 101 (16): 6176–81. Bibcode:2004PNAS..101.6176L. doi:10.1073/pnas.0308766101. PMC 395942. PMID 15067114.
- Our Microbial Planet A free poster from the National Academy of Sciences about the positive roles of microbes.
- "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
- Medical Microbiology 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
- MicrobeID.com Online Bacteria Identification Key and Probabilistic Identification Databases