The organisms that constitute the microbial world are characterized as either prokaryotes or eukaryotes;Eukaryotic microorganisms possess membrane-boundorganelles and includefungi andprotists, whereasprokaryotic organisms are conventionally classified as lacking membrane-bound organelles and includeBacteria andArchaea.[3][4]Microbiologists traditionally relied on culture, staining, andmicroscopy for the isolation and identification of microorganisms. However, less than 1% of the microorganisms present in common environments can be cultured in isolation using current means.[5] With the emergence ofbiotechnology, Microbiologists currently rely onmolecular biology tools such asDNA sequence-based identification, for example, the16S rRNA gene sequence used for bacterial identification.
Viruses have been variably classified as organisms[6] because they have been considered either very simple microorganisms or very complex molecules.Prions, never considered microorganisms, have been investigated by virologists; however, as the clinical effects traced to them were originally presumed due to chronic viral infections, virologists took a search—discovering "infectious proteins".
The existence of microorganisms was predicted many centuries before they were first observed, for example by theJains in India and byMarcus Terentius Varro in ancient Rome. The first recorded microscope observation was of the fruiting bodies of moulds, byRobert Hooke in 1666, but the Jesuit priestAthanasius Kircher was likely the first to see microbes, which he mentioned observing in milk and putrid material in 1658.Antonie van Leeuwenhoek is considered afather of microbiology as he observed and experimented withmicroscopic organisms in the 1670s, using simplemicroscopes of his design. Scientific microbiology developed in the 19th century through the work ofLouis Pasteur and in medical microbiologyRobert Koch.
Avicenna postulated the existence of microorganisms.
The existence of microorganisms was hypothesized for many centuries before their actual discovery. The existence of unseen microbiological life was postulated byJainism which is based onMahavira's teachings as early as 6th century BCE (599 BC - 527 BC).[7]: 24 Paul Dundas notes that Mahavira asserted the existence of unseen microbiological creatures living in earth, water, air and fire.[7]: 88 Jain scriptures describenigodas which are sub-microscopic creatures living in large clusters and having a very short life, said to pervade every part of the universe, even in tissues of plants and flesh of animals.[8] TheRomanMarcus Terentius Varro made references to microbes when he warned against locating a homestead in the vicinity of swamps "because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and thereby cause serious diseases."[9]
Persian scientists hypothesized the existence of microorganisms, such asAvicenna in his bookThe Canon of Medicine,Ibn Zuhr (also known as Avenzoar) who discoveredscabies mites, andAl-Razi who gave the earliest known description ofsmallpox in his bookThe Virtuous Life (al-Hawi).[10] The tenth-centuryTaoistBaoshengjing describes "countless micro organic worms" which resemble vegetableseeds, which prompted Dutch sinologistKristofer Schipper to claim that "the existence of harmful bacteria was known to the Chinese of the time."[11]
In 1546,Girolamo Fracastoro proposed thatepidemicdiseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or vehicle transmission.[12]
Kircher was among the first to design magic lanterns for projection purposes, and so he was well acquainted with the properties of lenses.[16] He wrote "Concerning the wonderful structure of things in nature, investigated by Microscope" in 1646, stating "who would believe that vinegar and milk abound with an innumerable multitude of worms." He also noted that putrid material is full of innumerable creeping animalcules. He published hisScrutinium Pestis (Examination of the Plague) in 1658, stating correctly that the disease was caused by microbes, though what he saw was most likely red or white blood cells rather than the plague agent itself.[16]
Innovativelaboratory glassware and experimental methods developed byLouis Pasteur and other biologists contributed to the young field of bacteriology in the late 19th century.
While Pasteur and Koch are often considered the founders of microbiology, their work 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 late 19th century and the work ofMartinus Beijerinck andSergei Winogradsky that the true breadth of microbiology was revealed.[2] Beijerinck made two major contributions to microbiology: the discovery ofviruses and the development ofenrichment culture techniques.[21] 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.[22] He was responsible for the first isolation and description of bothnitrifying andnitrogen-fixing bacteria.[2] French-Canadian microbiologistFelix d'Herelle co-discoveredbacteriophages in 1917 and was one of the earliest applied microbiologists.[23]
Thebranches of microbiology can be classified into applied sciences, or divided according to taxonomy, as is the case withbacteriology,mycology,protozoology,virology,phycology, andmicrobial ecology. There is considerable overlap between the specific branches of microbiology with each other and with other disciplines, and certain aspects of these branches can extend beyond the traditional scope of microbiology.[25][26] A pure research branch of microbiology is termedcellular microbiology.
Bacteria can be used for the industrial production ofamino acids.organic acids,vitamin,proteins,antibiotics and other commercially used metabolites which are produced by microorganisms.Corynebacterium glutamicum is one of the most important bacterial species with an annual production of more than two million tons of amino acids, mainly L-glutamate and L-lysine.[29] Since some bacteria have the ability to synthesize antibiotics, they are used for medicinal purposes, such asStreptomyces to makeaminoglycoside antibiotics.[30]
Microorganisms are beneficial formicrobial biodegradation orbioremediation of domestic, agricultural and industrial wastes and subsurfacepollution in soils, sediments and marine environments. The ability of each microorganism to degradetoxic waste depends on the nature of eachcontaminant. Since sites typically have multiple pollutant types, the most effective approach tomicrobial biodegradation is to use a mixture of bacterial and fungal species and strains, each specific to thebiodegradation of one or more types of contaminants.[32]
Symbiotic microbial communities confer benefits to their human and animal hosts health including aiding digestion, producing beneficial vitamins and amino acids, and suppressing pathogenic microbes. Some benefit may be conferred by eating fermented foods,probiotics (bacteria potentially beneficial to the digestive system) orprebiotics (substances consumed to promote the growth of probiotic microorganisms).[33][34] The ways the microbiome influences human and animal health, as well as methods to influence the microbiome are active areas of research.[35]
Research has suggested that microorganisms could be useful in the treatment ofcancer. Various strains of non-pathogenicclostridia can infiltrate and replicate within solidtumors. Clostridial vectors can be safely administered and their potential to deliver therapeutic proteins has been demonstrated in a variety of preclinical models.[36]
Some bacteria are used to study fundamental mechanisms. An example of model bacteria used to studymotility[37] or the production of polysaccharides and development isMyxococcus xanthus.[38]
^Fracastoro G (1930) [1546].De Contagione et Contagiosis Morbis [On Contagion and Contagious Diseases] (in Latin). Translated byWright WC. New York: G.P. Putnam.
^Gest H (2005). "The remarkable vision of Robert Hooke (1635-1703): first observer of the microbial world".Perspectives in Biology and Medicine.48 (2):266–272.doi:10.1353/pbm.2005.0053.PMID15834198.S2CID23998841.
^Johnson J (2001) [1998]."Martinus Willem Beijerinck".APSnet. American Phytopathological Society. Archived fromthe original on 2010-06-20. RetrievedMay 2, 2010. Retrieved from Internet Archive January 12, 2014.
^Mengesha A, Dubois L, Paesmans K, Wouters B, Lambin P, Theys J (2009). "Clostridia in Anti-tumor Therapy". In Brüggemann H, Gottschalk G (eds.).Clostridia: Molecular Biology in the Post-genomic Era. Caister Academic Press.ISBN978-1-904455-38-7.
^Zusman DR, Scott AE, Yang Z, Kirby JR (November 2007). "Chemosensory pathways, motility and development in Myxococcus xanthus".Nature Reviews. Microbiology.5 (11):862–872.doi:10.1038/nrmicro1770.PMID17922045.S2CID2340386.
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