In computing, a hyperlink, or a link, is a reference to data that the reader can directly follow either by clicking or tapping. A hyperlink points to a specific element within a document. Hypertext is text with hyperlinks; the text, linked from is called anchor text. A software system, used for viewing and creating hypertext is a hypertext system, to create a hyperlink is to hyperlink. A user following hyperlinks is said to browse the hypertext; the document containing a hyperlink is known as its source document. For example, in an online reference work such as Wikipedia, or Google, many words and terms in the text are hyperlinked to definitions of those terms. Hyperlinks are used to implement reference mechanisms such as tables of contents, bibliographies, indexes and glossaries. In some hypertext hyperlinks can be bidirectional: they can be followed in two directions, so both ends act as anchors and as targets. More complex arrangements exist, such as many-to-many links; the effect of following a hyperlink may vary with the hypertext system and may sometimes depend on the link itself.
Another possibility is transclusion, for which the link target is a document fragment that replaces the link anchor within the source document. Not only persons browsing the document follow hyperlinks; these hyperlinks may be followed automatically by programs. A program that traverses the hypertext, following each hyperlink and gathering all the retrieved documents is known as a Web spider or crawler. An inline link displays remote content without the need for embedding the content; the remote content may be accessed without the user selecting the link. An inline link may display a modified version of the content; the full content is usually available on demand, as is the case with print publishing software – e.g. with an external link. This allows for smaller file sizes and quicker response to changes when the full linked content is not needed, as is the case when rearranging a page layout. An anchor hyperlink is a link bound to a portion of a document—generally text, though not necessarily. For instance, it may be a hot area in an image, a designated irregular part of an image.
One way to define it is by a list of coordinates. For example, a political map of Africa may have each country hyperlinked to further information about that country. A separate invisible hot area interface allows for swapping skins or labels within the linked hot areas without repetitive embedding of links in the various skin elements. A fat link or a "multi-tailed link" is a hyperlink. Tim Berners-Lee saw the possibility of using hyperlinks to link any information to any other information over the Internet. Hyperlinks were therefore integral to the creation of the World Wide Web. Web pages are written in the hypertext mark-up language HTML; this is what a hyperlink to the home page of the W3C organization could look like in HTML code: This HTML code consists of several tags: The hyperlink starts with an anchor opening tag <a, includes a hyperlink reference href="http://www.w3.org" to the URL for the page. The URL is followed by >. The words that follow identify; these words are underlined and colored.
The anchor closing tag terminates the hyperlink code. Webgraph is a graph, formed from web pages as hyperlinks, as directed edges; the W3C Recommendation called XLink describes hyperlinks that offer a far greater degree of functionality than those offered in HTML. These extended links can be multidirectional, linking from and between XML documents, it can describe simple links, which are unidirectional and therefore offer no more functionality than hyperlinks in HTML. While wikis may use HTML-type hyperlinks, the use of wiki markup, a set of lightweight markup languages for wikis, provides simplified syntax for linking pages within wiki environments—in other words, for creating wikilinks; the syntax and appearance of wikilinks may vary. Ward Cunningham's original wiki software, the WikiWikiWeb used CamelCase for this purpose. CamelCase was used in the early version of Wikipedia and is still used in some wikis, such as TiddlyWiki, PmWiki. A common markup syntax is the use of double square brackets around the term to be wikilinked.
For example, the input "" is converted by wiki software using this markup syntax to a link to a zebras article. Hyperlinks used in wikis are classified as follows: Internal wikilinks or intrawiki links lead to pages within the same wiki website. Interwiki links are simplified markup hyperlinks that lead to pages of other wikis that are associated with the first. External links lead to other webpages. Wikilinks are visibly distinct from other text, if an internal wikilink leads to a page th
Caenorhabditis elegans is a free-living, transparent nematode, about 1 mm in length, that lives in temperate soil environments. It is the type species of its genus; the name is rhabditis and Latin elegans. In 1900, Maupas named it Rhabditides elegans, Osche placed it in the subgenus Caenorhabditis in 1952, in 1955, Dougherty raised Caenorhabditis to the status of genus. C. Elegans lacks respiratory or circulatory systems. Most of these nematodes are hermaphrodites and a few are males. Males have specialised tails for mating. In 1963, Sydney Brenner proposed research into C. elegans in the area of neuronal development. In 1974, he began research into the molecular and developmental biology of C. elegans, which has since been extensively used as a model organism. It was the first multicellular organism to have its whole genome sequenced, as of 2012, is the only organism to have its connectome completed. C. elegans is unsegmented and bilaterally symmetrical. It has a cuticle, four main epidermal cords, a fluid-filled pseudocoelom.
It has some of the same organ systems as larger animals. About one in a thousand individuals is male and the rest are hermaphrodites; the basic anatomy of C. elegans includes a mouth, intestine and collagenous cuticle. Like all nematodes, they have neither a respiratory system; the four bands of muscles that run the length of the body are connected to a neural system that allows the muscles to move the animal's body only as dorsal bending or ventral bending, but not left or right, except for the head, where the four muscle quadrants are wired independently from one another. When a wave of dorsal/ventral muscle contractions proceeds from the back to the front of the animal, the animal is propelled backwards; when a wave of contractions is initiated at the front and proceeds posteriorly along the body, the animal is propelled forwards. Because of this dorsal/ventral bias in body bends, any normal living, moving individual tends to lie on either its left side or its right side when observed crossing a horizontal surface.
A set of ridges on the lateral sides of the body cuticle, the alae, is believed to give the animal added traction during these bending motions. In relation to lipid metabolism, C. elegans does not have any specialized adipose tissues, a pancreas, a liver, or blood to deliver nutrients compared to mammals. Neutral lipids are instead stored in the intestine and embryos; the epidermis corresponds to the mammalian adipocytes by being the main triglyceride depot. The pharynx is a muscular food pump in the head of C. elegans, triangular in cross-section. This transports it directly to the intestine. A set of "valve cells" connects the pharynx to the intestine, but how this valve operates is not understood. After digestion, the contents of the intestine are released via the rectum, as is the case with all other nematodes. No direct connection exists between the pharynx and the excretory canal, which functions in the release of liquid urine. Males have a single-lobed gonad, a vas deferens, a tail specialized for mating, which incorporates spicules.
Hermaphrodites have two ovaries and spermatheca, a single uterus. Numerous gut granules are present in the intestine of C. elegans, the functions of which are still not known, as are many other aspects of this nematode, despite the many years that it has been studied. These gut granules are found in all of the Rhabditida orders, they are similar to lysosomes in that they feature an acidic interior and the capacity for endocytosis, but they are larger, reinforcing the view of their being storage organelles. A remarkable feature of the granules is that when they are observed under ultraviolet light, they react by emitting an intense blue fluorescence. Another phenomenon seen is termed'death fluorescence'; as the worms die, a dramatic burst of blue fluorescence is emitted. This death fluorescence takes place in an anterior to posterior wave that moves along the intestine, is seen in both young and old worms, whether subjected to lethal injury or peacefully dying of old age. Many theories have been posited on the functions of the gut granules, with earlier ones being eliminated by findings.
They are thought to store zinc as one of their functions. Recent chemical analysis has identified the blue fluorescent material they contain as a glycosylated form of anthranilic acid; the need for the large amounts of AA the many gut granules contain is questioned. One possibility is. Another possibility is; this is seen a possible link to the melanin–containing melanosomes. The hermaphroditic worm is considered to be a specialized form of self-fertile female, as its soma is female; the hermaphroditic germline produces male gametes first, lays eggs through its uterus after internal fertilization. Hermaphrodites produce all their sperm in the L4 stage and produce only oocytes; the hermaphroditic gonad acts as an ovotestis with sperm cells being stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca. The male can inseminate the hermaphrodite; the sperm of
The house mouse is a small mammal of the order Rodentia, characteristically having a pointed snout, large rounded ears, a long and hairy tail. It is one of the most abundant species of the genus Mus. Although a wild animal, the house mouse has benefited from associating with human habitation to the point that wild populations are less common than the semi-tame populations near human activity; the house mouse has been domesticated as the pet or fancy mouse, as the laboratory mouse, one of the most important model organisms in biology and medicine. The complete mouse reference genome was sequenced in 2002. House mice have an adult body length of 7.5 -- a tail length of 5 -- 10 cm. The weight is 40–45 g. In the wild they vary in colour from light to dark agouti, but domesticated fancy mice and laboratory mice are produced in many colors ranging from white to champagne to black, they have short hair and some. The ears and tail have little hair; the hind feet are short compared to only 15 -- 19 mm long.
The voice is a high-pitched squeak. House mice thrive under a variety of conditions. Newborn males and females can be distinguished on close examination as the anogenital distance in males is about double that of the female. From the age of about 10 days, females have five pairs of mammary nipples; when sexually mature, the most striking and obvious difference is the presence of testicles on the males. These can be retracted into the body; the tail, used for balance, has only a thin covering of hair as it is the main peripheral organ of heat loss in thermoregulation along with—to a lesser extent—the hairless parts of the paws and ears. Blood flow to the tail can be controlled in response to changes in ambient temperature using a system of arteriovenous anastomoses to increase the temperature of the skin on the tail by as much as 10 °C to lose body heat. Tail length varies according to the environmental temperature of the mouse during postnatal development, so mice living in colder regions tend to have shorter tails.
The tail is used for balance when the mouse is climbing or running, or as a base when the animal stands on its hind legs, to convey information about the dominance status of an individual in encounters with other mice. In addition to the regular pea-sized thymus organ in the chest, house mice have a second functional pinhead-sized thymus organ in the neck next to the trachea. Mice are mammals of the Glires clade, which means they are amongst the closest relatives of humans other than lagomorphs, flying lemurs and other primates; the three accepted subspecies are treated as distinct species: southeastern Asian house mouse western European house mouse. Some populations are hybrids including the Japanese house mouse; the standard species karyotype is composed of 40 chromosomes. Within Western Europe there are numerous populations - chromosomal races - with a reduced chromosome count arising from Robertsonian fusion. House mice run, walk, or stand on all fours, but when eating, fighting, or orienting themselves, they rear up on their hind legs with additional support from the tail - a behavior known as "tripoding".
Mice are good jumpers and swimmers, are considered to be thigmotactic, i.e. attempt to maintain contact with vertical surfaces. Mice are crepuscular or nocturnal; the average sleep time of a captive house mouse is reported to be 12.5 hours per day. They live in a wide variety of hidden places near food sources, construct nests from various soft materials. Mice are territorial, one dominant male lives together with several females and young. Dominant males respect each other's territories and enter another's territory only if it is vacant. If two or more males are housed together in a cage, they become aggressive unless they have been raised together from birth. House mice feed on plant matter, but are omnivorous, they eat their own faeces to acquire nutrients produced by bacteria in their intestines. House mice, like most other rodents, do not vomit. Mice are afraid of rats which kill and eat them, a behavior known as muricide. Despite this, free-living populations of rats and mice do exist together in forest areas in New Zealand, North America, elsewhere.
House mice are poor competitors and in most areas cannot survive away from human settlements in areas where other small mammals, such as wood mice, are present. However, in some areas, mice are able to coexist with other small rodent species; the social behavior of the house mouse is not r
In taxonomy, Homo sapiens is the only extant human species. The name was introduced in 1758 by Carl Linnaeus. Extinct species of the genus Homo include Homo erectus, extant during 1.9 to 0.4 million years ago, a number of other species. The age of speciation of H. sapiens out of ancestral H. erectus is estimated to have been 350,000 years ago. Sustained archaic admixture is known to have taken place both in Africa and in Eurasia, between about 100,000 and 30,000 years ago; the term anatomically modern humans is used to distinguish H. sapiens having an anatomy consistent with the range of phenotypes seen in contemporary humans from varieties of extinct archaic humans. This is useful for times and regions where anatomically modern and archaic humans co-existed, for example, in Paleolithic Europe; the binomial name Homo sapiens was coined by Linnaeus, 1758. The Latin noun homō means "human being", while the participle sapiēns means "discerning, sensible"; the species was thought to have emerged from a predecessor within the genus Homo around 300,000 to 200,000 years ago.
A problem with the morphological classification of "anatomically modern" was that it would not have included certain extant populations. For this reason, a lineage-based definition of H. sapiens has been suggested, in which H. sapiens would by definition refer to the modern human lineage following the split from the Neanderthal lineage. Such a cladistic definition would extend the age of H. sapiens to over 500,000 years. Extant human populations have been divided into subspecies, but since around the 1980s all extant groups have tended to be subsumed into a single species, H. sapiens, avoiding division into subspecies altogether. Some sources show Neanderthals as a subspecies; the discovered specimens of the H. rhodesiensis species have been classified by some as a subspecies, although it remains more common to treat these last two as separate species within the genus Homo rather than as subspecies within H. sapiens. The subspecies name H. sapiens sapiens is sometimes used informally instead of "modern humans" or "anatomically modern humans".
It has no formal authority associated with it. By the early 2000s, it had become common to use H. s. sapiens for the ancestral population of all contemporary humans, as such it is equivalent to the binomial H. sapiens in the more restrictive sense. The speciation of H. sapiens out of archaic human varieties derived from H. erectus is estimated as having taken place over 350,000 years ago, as the Khoisan split from other populations is dated between 260,000 and 350,000 years ago. An alternative suggestion defines H. sapiens cladistically as including the lineage of modern humans since the split from the lineage of Neanderthals 500,000 to 800,000 years ago. The time of divergence between archaic H. sapiens and ancestors of Neanderthals and Denisovans caused by a genetic bottleneck of the latter was dated at 744,000 years ago, combined with repeated early admixture events and Denisovans diverging from Neanderthals 300 generations after their split from H. sapiens, as calculated by Rogers et al..
The derivation of a comparatively homogeneous single species of H. sapiens from more diverse varieties of archaic humans was debated in terms of two competing models during the 1980s: "recent African origin" postulated the emergence of H. sapiens from a single source population in Africa, which expanded and led to the extinction of all other human varieties, while the "multiregional evolution" model postulated the survival of regional forms of archaic humans converging into the modern human varieties by the mechanism of clinal variation, via genetic drift, gene flow and selection throughout the Pleistocene. Since the 2000s, the availability of data from archaeogenetics and population genetics has led to the emergence of a much more detailed picture, intermediate between the two competing scenarios outlined above: The recent Out-of-Africa expansion accounts for the predominant part of modern human ancestry, while there were significant admixture events with regional archaic humans. Since the 1970s, the Omo remains, dated to some 195,000 years ago, have been taken as the conventional cut-off point for the emergence of "anatomically modern humans".
Since the 2000s, the discovery of older remains with comparable characteristics, the discovery of ongoing hybridization between "modern" and "archaic" populations after the time of the Omo remains, have opened up a renewed debate on the "age of H. sapiens", in journalistic publications cast into terms of "H. sapiens may be older than thought". H. s. idaltu, dated to 160,000 years ago, has been postulated as an extinct subspecies of H. sapiens in 2003. H. Neanderthalensis, which became extinct about 40,000 years ago, has been classified as a subspecies, H. s. neanderthalensis. H. heidelbergensis, dated 600,000 to 300,000 years ago, has long been thought to be a candidate for the last common ancestor of the Neanderthal and modern human lineages. However, genetic evidence from the Sima de los Huesos fossils published in 2016 seems to suggest that H. heidelbergensis in its entirety should be included in the Neanderthal lineage, as "pre-Neanderthal" or "early Neanderthal", while the divergence time between the Neanderthal and
Saccharomyces cerevisiae is a species of yeast. It has been instrumental to winemaking and brewing since ancient times, it is believed to have been isolated from the skin of grapes. It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium, it is the microorganism behind the most common type of fermentation. S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. It reproduces by a division process known as budding. Many proteins important in human biology were first discovered by studying their homologs in yeast. S. cerevisiae is the only yeast cell known to have Berkeley bodies present, which are involved in particular secretory pathways. Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis. S. cerevisiae, a yeast, have been found to contribute to the smell of bread by Schieberle. "Saccharomyces" derives from Latinized Greek and means "sugar-mold" or "sugar-fungus", saccharon being the combining form "sugar" and myces being "fungus".
Cerevisiae comes from Latin and means "of beer". Other names for the organism are: Brewer's yeast, though other species are used in brewing Ale yeast Top-fermenting yeast Baker's yeast Ragi yeast, in connection to making tapai Budding yeastThis species is the main source of nutritional yeast and yeast extract. In the 19th century, bread bakers obtained their yeast from beer brewers, this led to sweet-fermented breads such as the Imperial "Kaisersemmel" roll, which in general lacked the sourness created by the acidification typical of Lactobacillus. However, beer brewers switched from top-fermenting to bottom-fermenting yeast and this created a shortage of yeast for making bread, so the Vienna Process was developed in 1846. While the innovation is popularly credited for using steam in baking ovens, leading to a different crust characteristic, it is notable for including procedures for high milling of grains, cracking them incrementally instead of mashing them with one pass. Refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains.
In 1879, Great Britain introduced specialized growing vats for the production of S. cerevisiae, in the United States around the turn of the century centrifuges were used for concentrating the yeast, making modern commercial yeast possible, turning yeast production into a major industrial endeavor. The slurry yeast made by small bakers and grocery shops became cream yeast, a suspension of live yeast cells in growth medium, compressed yeast, the fresh cake yeast that became the standard leaven for bread bakers in much of the Westernized world during the early 20th century. During World War II, Fleischmann's developed a granulated active dry yeast for the United States armed forces, which did not require refrigeration and had a longer shelf-life and better temperature tolerance than fresh yeast; the company created yeast that would rise twice as fast. Lesaffre would create instant yeast in the 1970s, which has gained considerable use and market share at the expense of both fresh and dry yeast in their various applications.
In nature, yeast cells are found on ripe fruits such as grapes. Since S. cerevisiae is not airborne, it requires a vector to move. Queens of social wasps overwintering as adults can harbor yeast cells from autumn to spring and transmit them to their progeny; the intestine of Polistes dominula, a social wasp, hosts S. cerevisiae strains as well as S. cerevisiae × S. paradoxus hybrids. Stefanini et al. showed that the intestine of Polistes dominula favors the mating of S. cerevisiae strains, both among themselves and with S. paradoxus cells by providing environmental conditions prompting cell sporulation and spores germination. The optimum temperature for growth of S. cerevisiae is 30–35 °C. Two forms of yeast cells can survive and grow: diploid; the haploid cells undergo a simple lifecycle of mitosis and growth, under conditions of high stress will, in general, die. This is the asexual form of the fungus; the diploid cells undergo a simple lifecycle of mitosis and growth. The rate at which the mitotic cell cycle progresses differs between haploid and diploid cells.
Under conditions of stress, diploid cells can undergo sporulation, entering meiosis and producing four haploid spores, which can subsequently mate. This is the sexual form of the fungus. Under optimal conditions, yeast cells can double their population every 100 minutes. However, growth rates vary enormously both between environments. Mean replicative lifespan is about 26 cell divisions. In the wild, recessive deleterious mutations accumulate during long periods of asexual reproduction of diploids, are purged during selfing: this purging has been termed "genome renewal". All strains of S. cerevisiae can grow aerobically on glucose and treh
Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, dictated by the nucleotide sequence of their genes, which results in protein folding into a specific three-dimensional structure that determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are considered to be proteins and are called peptides, or sometimes oligopeptides; the individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, encoded in the genetic code.
In general, the genetic code specifies 20 standard amino acids. Shortly after or during synthesis, the residues in a protein are chemically modified by post-translational modification, which alters the physical and chemical properties, stability and the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can work together to achieve a particular function, they associate to form stable protein complexes. Once formed, proteins only exist for a certain period and are degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan covers a wide range, they can exist for years with an average lifespan of 1 -- 2 days in mammalian cells. Abnormal or misfolded proteins are degraded more either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells.
Many proteins are enzymes that are vital to metabolism. Proteins have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for use in the metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation and chromatography. Methods used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, a variable side chain are bonded.
Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; the amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, the linked series of carbon and oxygen atoms are known as the main chain or protein backbone; the peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone; the end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus.
The words protein and peptide are a little ambiguous and can overlap in meaning. Protein is used to refer to the complete biological molecule in a stable conformation, whereas peptide is reserved for a short amino acid oligomers lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids regardless of length, but implies an absence of a defined conformation. Proteins can interact with many types of molecules, including with other proteins, with lipids, with carboyhydrates, with DNA, it has been estimated. Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on the order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more pro
Escherichia coli known as E. coli, is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia, found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in their hosts, are responsible for product recalls due to food contamination. The harmless strains are part of the normal microbiota of the gut, can benefit their hosts by producing vitamin K2, preventing colonization of the intestine with pathogenic bacteria, having a symbiotic relationship. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh fecal matter under aerobic conditions for 3 days, but its numbers decline afterwards. E. Coli and other facultative anaerobes constitute about 0.1% of gut microbiota, fecal–oral transmission is the major route through which pathogenic strains of the bacterium cause disease. Cells are able to survive outside the body for a limited amount of time, which makes them potential indicator organisms to test environmental samples for fecal contamination.
A growing body of research, has examined environmentally persistent E. coli which can survive for extended periods outside a host. The bacterium can be grown and cultured and inexpensively in a laboratory setting, has been intensively investigated for over 60 years. E. coli is a chemoheterotroph whose chemically defined medium must include a source of carbon and energy. E. coli is the most studied prokaryotic model organism, an important species in the fields of biotechnology and microbiology, where it has served as the host organism for the majority of work with recombinant DNA. Under favorable conditions, it takes up to 20 minutes to reproduce. E. coli is a facultative anaerobic and nonsporulating bacterium. Cells are rod-shaped, are about 2.0 μm long and 0.25–1.0 μm in diameter, with a cell volume of 0.6–0.7 μm3. E. Coli stains Gram-negative because its cell wall is composed of a thin peptidoglycan layer and an outer membrane. During the staining process, E. coli picks up the color of the counterstain safranin and stains pink.
The outer membrane surrounding the cell wall provides a barrier to certain antibiotics such that E. coli is not damaged by penicillin. Strains that possess flagella are motile; the flagella have a peritrichous arrangement. It attaches and effaces to the microvilli of the intestines via an adhesion molecule known as intimin. E. coli can live on a wide variety of substrates and uses mixed-acid fermentation in anaerobic conditions, producing lactate, ethanol and carbon dioxide. Since many pathways in mixed-acid fermentation produce hydrogen gas, these pathways require the levels of hydrogen to be low, as is the case when E. coli lives together with hydrogen-consuming organisms, such as methanogens or sulphate-reducing bacteria. Optimum growth of E. coli occurs at 37 °C, but some laboratory strains can multiply at temperatures up to 49 °C. E. coli grows in a variety of defined laboratory media, such as lysogeny broth, or any medium that contains glucose, ammonium phosphate monobasic, sodium chloride, magnesium sulfate, potassium phosphate dibasic, water.
Growth can be driven by aerobic or anaerobic respiration, using a large variety of redox pairs, including the oxidation of pyruvic acid, formic acid and amino acids, the reduction of substrates such as oxygen, fumarate, dimethyl sulfoxide, trimethylamine N-oxide. E. coli is classified as a facultative anaerobe. It uses oxygen when it is available, it can, continue to grow in the absence of oxygen using fermentation or anaerobic respiration. The ability to continue growing in the absence of oxygen is an advantage to bacteria because their survival is increased in environments where water predominates; the bacterial cell cycle is divided into three stages. The B period occurs between the beginning of DNA replication; the C period encompasses the time it takes to replicate the chromosomal DNA. The D period refers to the stage between the conclusion of DNA replication and the end of cell division; the doubling rate of E. coli is higher. However, the length of the C and D periods do not change when the doubling time becomes less than the sum of the C and D periods.
At the fastest growth rates, replication begins before the previous round of replication has completed, resulting in multiple replication forks along the DNA and overlapping cell cycles. E. coli and related bacteria possess the ability to transfer DNA via bacterial conjugation or transduction, which allows genetic material to spread horizontally through an existing population. The process of transduction, which uses the bacterial virus called a bacteriophage, is where the spread of the gene encoding for the Shiga toxin from the Shigella bacteria to E. coli helped produce E. coli O157:H7, the Shiga toxin-producing strain of E. coli. E. coli encompasses an enormous population of bacteria that exhibit a high degree of both genetic and phenotypic diversity. Genome sequencing of a large number of isolates of E. coli and related bacteria shows that a taxonomic reclassification would be desirable. However, this has not been done due to its medical importance, E. coli remains one of the most diverse bacterial species: only 20% of the genes in a typical E. coli genome is shared among all strains.
In fact, from the evolutionary point of view, the members of genus Shigella (S. dysenteriae, S. fle