Mahavira postulated the existence of microscopic creatures in the6th century BC.The mausoleum ofAkshamsaddin, who mentioned microorganisms in his book Maddat ul-Hayat in the 15th century, in Bolu, Turkiye.Antonie van Leeuwenhoek was the first to study microscopic organisms.Lazzaro Spallanzani showed that boiling a broth stopped it from decaying.
The possible existence of microscopic organisms was discussed for many centuries before their discovery in the 17th century. By the 6th century BC, theJains of present-day India postulated the existence of tiny organisms callednigodas.[3] 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.[4] According toMahavira, the 24th preacher of Jainism, the humans destroy these nigodas on a massive scale, when they eat, breathe, sit, and move.[3] Many modern Jains assert that Mahavira's teachings presage the existence of microorganisms as discovered by modern science.[5]
The earliest known idea to indicate the possibility of diseases spreading by yet unseen organisms was that of the Roman scholarMarcus Terentius Varro in a 1st-century BC book entitledOn 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]
Turkish scientistAkshamsaddin mentioned the microbe in his workMaddat ul-Hayat (The Material of Life) about two centuries prior to Leeuwenhoek's experimental discovery:
It is a mistake to assume that diseases appear in individuals one by one. Diseases are transmitted from person to person. This transmission takes place through small seeds that are invisible to the eye, but are still alive.[9][10]
In1546,Girolamo Fracastoro proposed thatepidemic diseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or even without contact over long distances.[11]
Louis Pasteur showed that Spallanzani's findings held even if air could enter through a filter that kept particles out.
Louis Pasteur (1822–1895) exposed boiled broths to the air, in vessels that contained a filter to prevent particles from passing through to thegrowth 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, asspores on dust, rather than spontaneously generated within the broth. Thus, Pasteur refuted the theory ofspontaneous generation and supported thegerm 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 withanthrax always had large numbers ofBacillus 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 asKoch'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 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 ofMartinus Beijerinck andSergei Winogradsky in the late 19th century that the true breadth of microbiology was revealed.[23] Beijerinck made two major contributions to microbiology: the discovery ofviruses and the development ofenrichment culture techniques.[24] While his work on thetobacco 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 ofchemolithotrophy and to thereby reveal the essential role played by microorganisms in geochemical processes.[25] He was responsible for the first isolation and description of bothnitrifying andnitrogen-fixing bacteria.[23] French-Canadian microbiologistFélix d'Hérelle co-discoveredbacteriophages and was one of the earliest applied microbiologists.[26]
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 areprokaryotic unicellular organisms, and form the first domain of life inCarl Woese'sthree-domain system. A prokaryote is defined as having nocell nucleus or othermembrane 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 bacterialcell membranes are made fromphosphoglycerides withester bonds, Achaean membranes are made ofether lipids.[43] Archaea were originally described asextremophiles living inextreme environments, such ashot springs, but have since been found in all types ofhabitats.[44] Only now are scientists beginning to realize how common archaea are in the environment, withThermoproteota (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 inammonia oxidation.[47]
The combined domains of archaea and bacteria make up the most diverse and abundant group oforganisms on Earth and inhabit practically all environments where the temperature is below +140 °C (284 °F). They are found inwater,soil,air, as themicrobiome of an organism,hot springs and even deep beneath the Earth's crust inrocks.[48] The number of prokaryotes is estimated to be around five nonillion, or 5 × 1030, accounting for at least half thebiomass 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 transferDNA from one cell to another through direct contact, particularly under stressful environmental conditions that causeDNA damage.[51][52]
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 asThiomargarita namibiensis.[53] Bacteria function and reproduce as individual cells, but they can often aggregate in multicellularcolonies.[54] Some species such asmyxobacteria can aggregate into complexswarming structures, operating as multicellular groups as part of theirlife cycle,[55] or form clusters inbacterial colonies such asE. coli.
Theirgenome is usually acircular bacterial chromosome – a single loop ofDNA, although they can also harbor small pieces of DNA calledplasmids. These plasmids can be transferred between cells throughbacterial conjugation. Bacteria have an enclosingcell wall, which provides strength and rigidity to their cells. They reproduce bybinary fission or sometimes bybudding, but do not undergomeioticsexual reproduction. However, many bacterial species can transfer DNA between individual cells by ahorizontal gene transfer process referred to as naturaltransformation.[56] Some species form extraordinarily resilientspores, 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]
Unicellular eukaryotes consist of a singlecell throughout their life cycle. This qualification is significant since mostmulticellular eukaryotes consist of a single cell called azygote only at the beginning of their life cycles. Microbial eukaryotes can be eitherhaploid ordiploid, and some organisms have multiplecell nuclei.
Unicellular eukaryotes usually reproduce asexually bymitosis under favorable conditions. However, under stressful conditions such as nutrient limitations and other conditions associated with DNA damage, they tend to reproduce sexually bymeiosis andsyngamy.[60]
Ofeukaryotic groups, theprotists are most commonlyunicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify.[61][62] Severalalgaespecies aremulticellular protists, andslime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.[63] 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.[64][65]
Thegreen algae are a large group of photosynthetic eukaryotes that include many microscopic organisms. Although some green algae are classified asprotists, others such ascharophyta are classified withembryophyte 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 colonialflagellates, usually but not always with twoflagella per cell, as well as various colonial,coccoid, and filamentous forms. In theCharales, 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.[67]
Microorganisms are found in almost everyhabitat present in nature, including hostile environments such as theNorth and South poles,deserts,geysers, androcks. They also include all themarine microorganisms of theoceans anddeep sea. Some types of microorganisms have adapted toextreme environments and sustained colonies; these organisms are known asextremophiles. Extremophiles have been isolated from rocks as much as 7 kilometres below the Earth's surface,[68] 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.[48] Extremophiles have been known to survive for a prolonged time in avacuum, and can be highly resistant toradiation, which may even allow them to survive in space.[69] Many types of microorganisms have intimatesymbiotic relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to thehost organism (parasitism). If microorganisms can causedisease in a host they are known aspathogens. Microorganisms play critical roles in Earth'sbiogeochemical cycles as they are responsible fordecomposition andnitrogen fixation.[70]
Bacteria useregulatory networks that allow them to adapt to almost every environmental niche on earth.[71][72] A network of interactions among diverse types of molecules including DNA, RNA, proteins and metabolites, is utilised by the bacteria to achieveregulation 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.[73] A complex organization of networks permits the microorganism to coordinate and integrate multiple environmental signals.[71]
These microorganisms in theroot 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 mutualisticsymbiosis between the two. However, these signals can be eavesdropped by other microorganisms, such as thesoil 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.[84]
Microorganisms are useful in producing foods, treating waste water, creating biofuels and a wide range of chemicals and enzymes. They are invaluable in research asmodel organisms. They have beenweaponised and sometimes used inwarfare andbioterrorism. They are vital to agriculture through their roles in maintainingsoil fertility and in decomposing organic matter. They also have applications in aquaculture, such as inbiofloc technology.
Growth of microorganisms contributes to ripening and flavor. The flavor and appearance of a particular cheese is due in large part to the microorganisms associated with it.Lactobacillus Bulgaricus is one of the microbes used in production ofdairy products
Alcoholic beverages
Yeast is used to convert sugar, grape juice, or malt-treated grain into alcohol. Other microorganisms may also be used; a mold converts starch into sugar to make the Japanese rice wine, sake.Acetobacter Aceti a kind of bacterium is used in production ofalcoholic beverages
Vinegar
Certain bacteria are used to convert alcohol into acetic acid, which gives vinegar its acid taste.Acetobacter Aceti is used on production of vinegar, which gives vinegar odor of alcohol and alcoholic taste
Citric acid
Certain fungi are used to make citric acid, a common ingredient of soft drinks and other foods.
Vitamins
Microorganisms are used to make vitamins, including C, B2 , B12.
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 aslow sand filter.[89]Anaerobic digestion bymethanogens generate usefulmethane gas as a by-product.[90]
In theMiddle Ages, as an early example ofbiological warfare, diseased corpses were thrown into castles duringsieges using catapults or othersiege engines. Individuals near the corpses were exposed to the pathogen and were likely to spread that pathogen to others.[104]
Microbes can makenutrients and minerals in the soil available to plants, producehormones that spur growth, stimulate the plantimmune system and trigger or dampen stress responses. In general a more diverse set ofsoil microbes results in fewer plant diseases and higher yield.[107]
Hygiene is a set of practices to avoidinfection orfood spoilage by eliminating microorganisms from the surroundings. As microorganisms, in particular bacteria, are found virtually everywhere,harmful microorganisms may be reduced to acceptable levels rather than actually eliminated. In food preparation, microorganisms are reduced bypreservation methods such as cooking, cleanliness of utensils, short storage periods, or by low temperatures. If complete sterility is needed, as with surgical equipment, anautoclave is used to kill microorganisms with heat and pressure.[113][114]
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.
^The wordmicroorganism (/ˌmaɪkroʊˈɔːrɡənɪzəm/) usescombining forms ofmicro- (from theGreek:μικρός,mikros, 'small') andorganism fromὀργανισμός,organismós, 'organism'). It is usually written as a single word but is sometimeshyphenated (micro-organism), especially in older texts. The informal synonymmicrobe (/ˈmaɪkroʊb/) comes frommikros andβίος,bíos, 'life'.
^Thepiezophilic bacteriaHalomonas 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.[79]
^Osman Şevki Uludağ:Beş Buçuk Asırlık Türk Tabâbet Tarihi (Five and a Half Centuries of Turkish Medical History). Istanbul, 1969, pp. 35–36
^Nutton, Vivian (1990). "The Reception of Fracastoro's Theory of Contagion: The Seed That Fell among Thorns?".Osiris. 2nd Series, Vol. 6, Renaissance Medical Learning: Evolution of a Tradition:196–234.doi:10.1086/368701.JSTOR301787.PMID11612689.S2CID37260514.
^abMadigan, M.; Martinko, J., eds. (2006).Brock Biology of Microorganisms (13th ed.). Pearson Education. p. 1096.ISBN978-0-321-73551-5.
^Johnson, J. (2001) [1998]."Martinus Willem Beijerinck".APSnet. American Phytopathological Society. Archived fromthe original on 20 June 2010. Retrieved2 May 2010. Retrieved from Internet Archive 12 January 2014.
^Yamaguchi, Masashi; et al. (1 December 2012). "Prokaryote or eukaryote? A unique microorganism from the deep sea".Journal of Electron Microscopy.61 (6):423–431.doi:10.1093/jmicro/dfs062.PMID23024290.
^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 PublishingISBN978-3-319-65535-2 DOI 10.1007/978-3-319-65536-9
^Bernstein, H.; Bernstein, C.; Michod, R. E. (2012)."Chapter 1". In Kimura, Sakura; Shimizu, Sora (eds.).DNA repair as the primary adaptive function of sex in bacteria and eukaryotes. DNA Repair: New Research. Nova Sci. Publ. pp. 1–49.ISBN978-1-62100-808-8. Archived fromthe original on 22 July 2018.
^Anderson, A. W.; Nordan, H. C.; Cain, R. F.; Parrish, G.; Duggan, D. (1956). "Studies on a radio-resistant micrococcus. I. Isolation, morphology, cultural characteristics, and resistance to gamma radiation".Food Technol.10 (1):575–577.
^Soni, S. K. (2007).Microbes: A Source of Energy for 21st Century. New India Publishing.ISBN978-81-89422-14-1.
^Moses, Vivian; et al. (1999).Biotechnology: The Science and the Business. CRC Press. p. 563.ISBN978-90-5702-407-8.
^Langford, Roland E. (2004).Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological. Wiley-IEEE. p. 140.ISBN978-0-471-46560-7.
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