Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the outermost layer of the skin. There are three principal shapes of epithelial cell: squamous and cuboidal; these can be arranged in a single layer of cells as simple epithelium, either squamous, columnar, or cuboidal, or in layers of two or more cells deep as stratified, either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei; this sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include secretion, selective absorption, transcellular transport, sensing. Epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.
Cell junctions are well employed in epithelial tissues. In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells; the three principal shapes associated with epithelial cells are—squamous and columnar. Squamous epithelium has cells; this is found as the lining of the mouth, the blood vessels and in the alveoli of the lungs. Cuboidal epithelium has cells whose height and width are the same. Columnar epithelium has cells taller. By layer, epithelium is classed as either simple epithelium, only one cell thick or stratified epithelium having two or more cells in thickness or multi-layered – as stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium, both types of layering can be made up of any of the cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia; this kind of epithelium is therefore described as pseudostratified columnar epithelium.
Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where filtration occur; the thinness of the epithelial barrier facilitates these processes. In general, simple epithelial tissues are classified by the shape of their cells; the four major classes of simple epithelium are: simple squamous. Simple squamous. Simple cuboidal: these cells may have secretory, absorptive, or excretory functions. Examples include small collecting ducts of kidney and salivary gland. Simple columnar. Non-ciliated epithelium can possess microvilli; some tissues are referred to as simple glandular columnar epithelium. These secrete mucus and are found in stomach and rectum. Pseudostratified columnar epithelium; the ciliated type is called respiratory epithelium as it is exclusively confined to the larger respiratory airways of the nasal cavity and bronchi.
Stratified epithelium differs from simple epithelium. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar. Stratified epithelia can have the following specializations: The basic cell types are squamous and columnar classed by their shape. Cells of epithelial tissue are scutoid shaped packed and form a continuous sheet, they have no intercellular spaces. All epithelia is separated from underlying tissues by an extracellular fibrous basement membrane; the lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. The lining of the blood and lymphatic vessels are of a specialised form of epithelium called endothelium. Epithelium lines both the outside and the inside cavities and lumina of bodies; the outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells.
Tissues that line the inside of the mouth, the esophagus, the vagina, part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, make up the exocrine and endocrine glands; the outer surface of the cornea is covered with fast-growing regenerated epithelial cells. A specialised form of epithelium – endothelium forms the inner lining of blood vessels and the heart, is known as vascular endotheliu
An endospore is a dormant and non-reproductive structure produced by certain bacteria from the phylum Firmicutes. The name "endospore" is suggestive of a spore or seed-like form, it is a dormant form to which the bacterium can reduce itself. Endospore formation is triggered by a lack of nutrients, occurs in gram-positive bacteria. In endospore formation, the bacterium divides within its cell wall, one side engulfs the other. Endospores enable bacteria to lie dormant for extended periods centuries. There are many reports of spores remaining viable over 10,000 years, revival of spores millions of years old has been claimed. There is one report of viable spores of Bacillus marismortui in salt crystals 250 million years old; when the environment becomes more favorable, the endospore can reactivate itself to the vegetative state. Most types of bacteria cannot change to the endospore form. Examples of bacteria that can form endospores include Clostridium; the endospore consists of ribosomes and large amounts of dipicolinic acid.
Dipicolinic acid is a spore-specific chemical that appears to help in the ability for endospores to maintain dormancy. This chemical accounts for up to 10% of the spore's dry weight. Endospores can survive without nutrients, they are resistant to ultraviolet radiation, high temperature, extreme freezing and chemical disinfectants. Thermo-resistant endospores were first hypothesized by Ferdinand Cohn after studying Bacillus subtilis growth on cheese after boiling the cheese, his notion of spores being the reproductive mechanism for the growth was a large blow to the previous suggestions of spontaneous generation. Astrophysicist Steinn Sigurdsson said "There are viable bacterial spores that have been found that are 40 million years old on Earth – and we know they're hardened to radiation." Common anti-bacterial agents that work by destroying vegetative cell walls do not affect endospores. Endospores are found in soil and water, where they may survive for long periods of time. A variety of different microorganisms form "spores" or "cysts," but the endospores of low G+C gram-positive bacteria are by far the most resistant to harsh conditions.
Some classes of bacteria can turn into exospores known as microbial cysts, instead of endospores. Exospores and endospores are two kinds of "hibernating" or dormant stages seen in some classes of microorganisms. Bacteria produce a single endospore internally; the spore is sometimes surrounded by a thin covering known as the exosporium, which overlies the spore coat. The spore coat, which acts like a sieve that excludes large toxic molecules like lysozyme, is resistant to many toxic molecules and may contain enzymes that are involved in germination, it is composed of keratin and other core specific proteins, which makes the endospore hardy. The cortex consists of peptidoglycan; the core wall surrounds the protoplast or core of the endospore. The core contains the spore chromosomal DNA, encased in chromatin-like proteins known as SASPs, that protect the spore DNA from UV radiation and heat; the core contains normal cell structures, such as ribosomes and other enzymes, but is not metabolically active.
Up to 20% of the dry weight of the endospore consists of calcium dipicolinate within the core, thought to stabilize the DNA. Dipicolinic acid could be responsible for the heat resistance of the spore, calcium may aid in resistance to heat and oxidizing agents. However, mutants resistant to heat but lacking dipicolinic acid have been isolated, suggesting other mechanisms contributing to heat resistance are at work. Small acid-soluble proteins are found in endospores; these proteins bind and condense the DNA, are in part responsible for resistance to UV light and DNA-damaging chemicals. Visualising endospores under light microscopy can be difficult due to the impermeability of the endospore wall to dyes and stains. While the rest of a bacterial cell may stain, the endospore is left colourless. To combat this, a special stain technique called; that allows the endospore to show up as red. Another staining technique for endospores is the Schaeffer-Fulton stain, which stains endospores green and bacterial bodies red.
The arrangement of spore layers is as follows: Exosporium Spore coat Spore cortex Core wall The position of the endospore differs among bacterial species and is useful in identification. The main types within the cell are terminal and centrally placed endospores. Terminal endospores are seen at the poles of cells, whereas central endospores are more or less in the middle. Subterminal endospores are those between these two extremes seen far enough towards the poles but close enough to the center so as not to be considered either terminal or central. Lateral endospores are seen occasionally. Examples of bacteria having terminal endospores include Clostridium tetani, the pathogen that causes the disease tetanus. Bacteria having a centrally placed endospore include Bacillus cereus. Sometimes the endospore can be so large; this is typical of Clostridium tetani. Under conditions of starvation the lack of carbon and nitrogen sources, a single endospore forms within some of the bacteria; the process is called sporulation.
When a bacterium detects environmental conditions are becoming unfavourable it may start the process of endosporulation, which takes about eight hours. The DNA is replicated and a membra
Lysis refers to the breaking down of the membrane of a cell by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a lysate. In molecular biology and cell biology laboratories, cell cultures may be subjected to lysis in the process of purifying their components, as in protein purification, DNA extraction, RNA extraction, or in purifying organelles. Many species of bacteria are subject to lysis by the enzyme lysozyme, found in animal saliva, egg white, other secretions. Phage lytic enzymes produced during bacteriophage infection are responsible for the ability of these viruses to lyse bacterial cells. Penicillin and related β-lactam antibiotics cause the death of bacteria through enzyme-mediated lysis that occurs after the drug causes the bacterium to form a defective cell wall. If the cell wall is lost, the bacterium is referred as a protoplast, if penicillin was used on gram-positive bacteria, spheroplast, if penicillin was used on gram-negative bacteria.
Cytolysis occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell. Cytolysis can be prevented by several different mechanisms, including the contractile vacuole that exists in some paramecia, which pump water out of the cell. Cytolysis does not occur under normal conditions in plant cells because plant cells have a strong cell wall that contains the osmotic pressure, or turgor pressure, that would otherwise cause cytolysis to occur. Oncolysis refers of a tumour, it is used to refer to the reduction of any swelling. Plasmolysis is the contraction of cells within plants due to the loss of water through osmosis. In a hypertonic environment, the cell membrane peels off of the cell wall and the vacuole collapses; these cells will wilt and die unless the flow of water caused by osmosis can stop the contraction of the cell membrane. Erythrocytes' hemoglobin release free radicals in response to pathogens; this can damage the pathogens. Cell lysis is used in laboratories to purify or further study their contents.
Lysis in the laboratory may be affected by other chaotropic agents. Mechanical disruption of cell membranes, as by repeated freezing and thawing, pressure, or filtration may be referred to as lysis. Many laboratory experiments are sensitive to the choice of lysis mechanism; the unprocessed solution after lysis but before any further extraction steps is referred to as a crude lysate. For example, lysis is used in western and Southern blotting to analyze the composition of specific proteins and nucleic acids individually or as complexes. Depending on the detergent used, either all or some membranes are lysed. For example, if only the cell membrane is lysed gradient centrifugation can be used to collect certain organelles. Lysis is used for protein purification, DNA extraction, RNA extraction. Cell disruption Cell unroofing Crenation Hemolysis Lysogenic Pitted keratolysis
Kiyoshi Shiga was a Japanese physician and bacteriologist. Shiga was born in Sendai, Miyagi Prefecture, though his original family name was Satō, he graduated from the Medical School of Tokyo Imperial University in 1896 and went to work at the Institute for the Study of Infectious Diseases under Dr. Kitasato Shibasaburō. Shiga became famous for the discovery of Shigella dysenteriae, the organism that causes dysentery, in 1897, during a severe epidemic in which more than 90,000 cases were reported, with a mortality rate approaching 30%; the bacterium Shigella was thus named after him, as well as the Shiga toxin, produced by the bacterium. After the discovery of Shigella, Shiga worked with Paul Ehrlich in Germany from 1901 to 1905. After returning to Japan, he resumed the study of infectious diseases with Dr. Kitasato, he became a professor at Keio University in 1920. From 1929 to 1931, Shiga was the president of Keijō Imperial University in Keijo and was senior medical advisor to the Japanese Governor-General of Korea.
Shiga was a recipient of the Order of Culture in 1944. He was awarded the Order of the Sacred Treasure, 1st class, on his death in 1957. Csuros, Maria. Microbiological Examination of Water and Wastewater. CRC Press. ISBN 1-56670-179-1 Kleinman. Pediatric Gastrointestinal Disease. ISBN 1-55009-364-9
Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. A few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, are present in most of its habitats. Bacteria inhabit soil, acidic hot springs, radioactive waste, the deep portions of Earth's crust. Bacteria live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, only about half of the bacterial phyla have species that can be grown in the laboratory; the study of bacteria is known as a branch of microbiology. There are 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. There are 5×1030 bacteria on Earth, forming a biomass which exceeds that of all plants and animals. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere.
The nutrient cycle includes the decomposition of dead bodies. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Data reported by researchers in October 2012 and published in March 2013 suggested that bacteria thrive in the Mariana Trench, with a depth of up to 11 kilometres, is the deepest known part of the oceans. Other researchers reported related studies that microbes thrive inside rocks up to 580 metres below the sea floor under 2.6 kilometres of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere—they're adaptable to conditions, survive wherever they are."The famous notion that bacterial cells in the human body outnumber human cells by a factor of 10:1 has been debunked. There are 39 trillion bacterial cells in the human microbiota as personified by a "reference" 70 kg male 170 cm tall, whereas there are 30 trillion human cells in the body.
This means that although they do have the upper hand in actual numbers, it is only by 30%, not 900%. The largest number exist in the gut flora, a large number on the skin; the vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, though many are beneficial in the gut flora. However several species of bacteria are pathogenic and cause infectious diseases, including cholera, anthrax and bubonic plague; the most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year in sub-Saharan Africa. In developed countries, antibiotics are used to treat bacterial infections and are used in farming, making antibiotic resistance a growing problem. In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium and other metals in the mining sector, as well as in biotechnology, the manufacture of antibiotics and other chemicals.
Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two different groups of organisms that evolved from an ancient common ancestor; these evolutionary domains are called Archaea. The word bacteria is the plural of the New Latin bacterium, the latinisation of the Greek βακτήριον, the diminutive of βακτηρία, meaning "staff, cane", because the first ones to be discovered were rod-shaped; the ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, bacteria and archaea were the dominant forms of life. Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species.
However, gene sequences can be used to reconstruct the bacterial phylogeny, these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage. The most recent common ancestor of bacteria and archaea was a hyperthermophile that lived about 2.5 billion–3.2 billion years ago. Bacteria were involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves related to the Archaea; this involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Some eukaryotes that contained mitochondria engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants; this is known as primary endosymbiosis. Bacteria display a wide diversity of sizes, called morphologies.
Bacterial cells are about one-tenth the size of eukaryotic cells
Type three secretion system
Type three secretion system is a protein appendage found in several Gram-negative bacteria. In pathogenic bacteria, the needle-like structure is used as a sensory probe to detect the presence of eukaryotic organisms and secrete proteins that help the bacteria infect them; the secreted effector proteins are secreted directly from the bacterial cell into the eukaryotic cell, where they exert a number of effects that help the pathogen to survive and to escape an immune response. The term Type III secretion system was coined in 1993; this secretion system is distinguished from at least five other secretion systems found in Gram-negative bacteria. Many animal and plant associated bacteria possess similar T3SSs; these T3SSs are similar as a result of divergent evolution and phylogenetic analysis supports a model in which gram-negative bacteria can transfer the T3SS gene cassette horizontally to other species. The most researched T3SSs are from species of Shigella, Escherichia coli, Burkholderia, Chlamydia and the plant pathogens Erwinia and Xanthomonas, the plant symbiont Rhizobium.
The T3SS is composed of 30 different proteins, making it one of the most complex secretion systems. Its structure shows many similarities with bacterial flagella; some of the proteins participating in T3SS share amino-acid sequence homology to flagellar proteins. Some of the bacteria possessing a T3SS have flagella as well and are motile, some do not. Technically speaking, type III secretion is used both for secreting infection-related proteins and flagellar components. However, the term "type III secretion" is used in relation to the infection apparatus; the bacterial flagellum shares a common ancestor with the type III secretion system. T3SSs are essential for the pathogenicity of many pathogenic bacteria. Defects in the T3SS may render a bacterium non-pathogenic, it has been suggested that some non-invasive strains of gram-negative bacteria have lost the T3SS because the energetically costly system is no longer of use. Although traditional antibiotics were effective against these bacteria in the past, antibiotic-resistant strains emerge.
Understanding the way the T3SS works and developing drugs targeting it have become an important goal of many research groups around the world since the late 1990s. The hallmark of T3SS is the needle. Bacterial proteins that need to be secreted pass from the bacterial cytoplasm through the needle directly into the host cytoplasm. Three membranes separate the two cytoplasms: the double membrane of the Gram-negative bacterium and the eukaryotic membrane; the needle provides a smooth passage through those selective and impermeable membranes. A single bacterium can have several hundred needle complexes spread across its membrane, it has been proposed that the needle complex is a universal feature of all T3SSs of pathogenic bacteria. The needle complex starts at the cytoplasm of the bacterium, crosses the two membranes and protrudes from the cell; the part anchored in the membrane is the base of the T3SS. The extracellular part is the needle. A so-called inner rod connects the needle to the base; the needle itself, although the biggest and most prominent part of the T3SS, is made out of many units of a single protein.
The majority of the different T3SS proteins are therefore those that build the base and those that are secreted into the host. As mentioned above, the needle complex shares similarities with bacterial flagella. More the base of the needle complex is structurally similar to the flagellar base; the base is composed of several circular rings and is the first structure, built in a new needle complex. Once the base is completed, it serves as a secretion machine for the outer proteins. Once the whole complex is completed the system switches to secreting proteins that are intended to be delivered into host cells; the needle is presumed to be built from bottom to top. The needle subunit is one of the smallest T3SS proteins, measuring at around 9 kDa. 100−150 subunits comprise each needle. The T3SS needle measures 8 nm in external width, it needs to have a minimal length so that other extracellular bacterial structures do not interfere with secretion. The hole of the needle has a 3 nm diameter. Most folded effector proteins are too large to pass through the needle opening, so most secreted proteins must pass through the needle unfolded, a task carried out by the ATPase at the base of the structure.
The T3SS proteins can be grouped into three categories: Structural proteins: build the base, the inner rod and the needle. Effector proteins: get secreted into the host cell and promote infection / suppress host cell defences. Chaperones: bind effectors in the bacterial cytoplasm, protect them from aggregation and degradation and direct them towards the needle complex. Most T3SS genes are laid out in ope