Listeria
Listeria is a genus of bacteria that, until 1992, contained 10 known species, each containing two subspecies. As of 2019, 20 species were identified. Named after the British pioneer of sterile surgery Joseph Lister, the genus received its current name in 1940. Listeria species are Gram-positive, rod-shaped, facultatively anaerobic, do not produce endospores; the major human pathogen in the genus Listeria is L. monocytogenes. It is the causative agent of the rare bacterial disease listeriosis, an infection caused by eating food contaminated with the bacteria. Listeriosis can cause serious illness in pregnant women, adults with weakened immune systems and the elderly, may cause gastroenteritis in others who have been infected. Listeriosis is a serious disease for humans; the two main clinical manifestations are sepsis and meningitis. Meningitis is complicated by encephalitis, when it is known as meningoencephalitis, a pathology, unusual for bacterial infections. L. ivanovii is a pathogen of mammals ruminants, has caused listeriosis in humans.
The incubation period can vary between 70 days. The first documented case of listeriosis was in 1924. In the late 1920s, two researchers independently identified L. monocytogenes from animal outbreaks. They proposed the genus Listerella in honor of surgeon and early antiseptic advocate Joseph Lister, but that name was in use for a slime mold and a protozoan; the genus Listeria was proposed and accepted. All species within the genus Listeria are Gram-positive, catalase-positive rods and do not produce endospores; the genus Listeria was classified in the family Corynebacteriaceae through the seventh edition of Bergey's Manual of Systematic Bacteriology. The 16S rRNA cataloging studies of Stackebrandt, et al. demonstrated that L. monocytogenes is a distinct taxon within the Lactobacillus-Bacillus branch of the bacterial phylogeny constructed by Woese. In 2004, the genus was placed in the newly created family Listeriaceae; the only other genus in the family is Brochothrix. The genus Listeria contains 20 species: L. aquatica, L. booriae, L. cornellensis, L. costaricensis, L. goaensis, L. fleischmannii, L. floridensis, L. grandensis, L. grayi, L. innocua, L. ivanovii, L. marthii, L. monocytogenes, L. newyorkensis, L. riparia, L. rocourtiae, L. seeligeri, L. thailandensis, L. weihenstephanensis, L. welshimeri.
Listeria dinitrificans thought to be part of the genus Listeria, was reclassified into the new genus Jonesia. Under the microscope, Listeria species appear as small rods, which are sometimes arranged in short chains. In direct smears, they may be coccoid, so they can be mistaken for streptococci. Longer cells may resemble corynebacteria. Flagella are produced at room temperature, but not at 37 °C. Hemolytic activity on blood agar has been used as a marker to distinguish L. monocytogenes from other Listeria species, but it is not an definitive criterion. Further biochemical characterization may be necessary to distinguish between the different species of Listeria. Listeria can be found in soil. Animals can be carriers. Listeria has been found in uncooked meats, uncooked vegetables, fruit such as rockmelon and apples, pasteurized or unpasteurized milk, foods made from milk, processed foods. Pasteurization and sufficient cooking kill Listeria. For example, meat-processing plants producing ready-to-eat foods, such as hot dogs and deli meats, must follow extensive sanitation policies and procedures to prevent Listeria contamination.
Listeria monocytogenes is found in soil, stream water, sewage and food. Listeria is responsible for listeriosis, a rare but lethal foodborne illness; the case fatality rate for those with a severe form of infection may approach 25%. Although L. monocytogenes has low infectivity, it is hardy and can grow in temperatures from 4 °C to 37 °C. Listeriosis is a serious illness, the disease may manifest as meningitis, or affect newborns due to its ability to penetrate the endothelial layer of the placenta. Listeria uses the cellular machinery to move around inside the host cell, it induces directed polymerization of actin by the ActA transmembrane protein, thus pushing the bacterial cell around. L. Monocytogenes, for example, encodes virulence genes; the expression of virulence factor is optimal at 39 °C, is controlled by a transcriptional activator, PrfA, whose expression is thermoregulated by the PrfA thermoregulator UTR element. At low temperatures, the PrfA transcript is not translated due to structural elements near the ribosome binding site.
As the bacteria infect the host, the temperature of the host melts the structure and allows translation initiation for the virulent genes. The majority of Listeria bacteria are targeted by the immune system before they are able to cause infection; those that escape the immune system's initial response, spread through intracellular mechanisms and are, guarded against circulating immune factors. To invade, Listeria induces macrophage phagocytic uptake by displaying D-galactose in their teichoic acids that are bound by the macrophage's polysaccharide s. Other important adhesins are the internalins. Listeria uses internalin B to bind to cellular receptors. Internalin A binds to E-cadherin. If both of these receptors have a high enough aff
Microbiology
Microbiology is the study of microorganisms, those being unicellular, multicellular, or acellular. Microbiology encompasses numerous sub-disciplines including virology, parasitology and bacteriology. Eukaryotic microorganisms possess membrane-bound cell organelles and include fungi and protists, whereas prokaryotic organisms—all of which are microorganisms—are conventionally classified as lacking membrane-bound organelles and include Bacteria and Archaea. Microbiologists traditionally relied on culture and microscopy. However, less than 1% of the microorganisms present in common environments can be cultured in isolation using current means. Microbiologists rely on molecular biology tools such as DNA sequence based identification, for example 16s rRNA gene sequence used for bacteria identification. Viruses have been variably classified as organisms, as they have been considered either as simple microorganisms or complex molecules. Prions, never considered as microorganisms, have been investigated by virologists, however, as the clinical effects traced to them were presumed due to chronic viral infections, virologists took search—discovering "infectious proteins".
The existence of microorganisms was predicted many centuries before they were first observed, for example by the Jains in India and by Marcus Terentius Varro in ancient Rome. The first recorded microscope observation was of the fruiting bodies of moulds, by Robert Hooke in 1666, but the Jesuit priest Athanasius Kircher was the first to see microbes, which he mentioned observing in milk and putrid material in 1658. Antonie van Leeuwenhoek is considered a father of microbiology as he observed and experimented with microscopic organisms in 1676, using simple microscopes of his own design. Scientific microbiology developed in the 19th century through the work of Louis Pasteur and in medical microbiology Robert Koch; the existence of microorganisms was hypothesized for many centuries before their actual discovery. The existence of unseen microbiological life was postulated by Jainism, based on Mahavira’s teachings as early as 6th century BCE. Paul Dundas notes that Mahavira asserted the existence of unseen microbiological creatures living in earth, water and fire.
Jain scriptures describe nigodas which are sub-microscopic creatures living in large clusters and having a short life, said to pervade every part of the universe in tissues of plants and flesh of animals. The Roman Marcus 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."In the golden age of Islamic civilization, Iranian scientists hypothesized the existence of microorganisms, such as Avicenna in his book The Canon of Medicine, Ibn Zuhr who discovered scabies mites, Al-Razi who gave the earliest known description of smallpox in his book The Virtuous Life. In 1546, Girolamo Fracastoro proposed that epidemic diseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or vehicle transmission.
In 1676, Antonie van Leeuwenhoek, who lived most of his life in Delft, observed bacteria and other microorganisms using a single-lens microscope of his own design. He is considered a father of microbiology as he pioneered the use of simple single-lensed microscopes of his own design. While Van Leeuwenhoek is cited as the first to observe microbes, Robert Hooke made his first recorded microscopic observation, of the fruiting bodies of moulds, in 1665, it has, been suggested that a Jesuit priest called Athanasius Kircher was the first to observe microorganisms. Kircher was among the first to design magic lanterns for projection purposes, so he must have been well acquainted with the properties of lenses, 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 noted that putrid material is full of innumerable creeping animalcules. He published his Scrutinium Pestis in 1658, stating that the disease was caused by microbes, though what he saw was most red or white blood cells rather than the plague agent itself.
The field of bacteriology was founded in the 19th century by Ferdinand Cohn, a botanist whose studies on algae and photosynthetic bacteria led him to describe several bacteria including Bacillus and Beggiatoa. Cohn was the first to formulate a scheme for the taxonomic classification of bacteria, to discover endospores. Louis Pasteur and Robert Koch were contemporaries of Cohn, are considered to be the father of microbiology and medical microbiology, respectively. Pasteur is most famous for his series of experiments designed to disprove the widely held theory of spontaneous generation, thereby solidifying microbiology's identity as a biological science. One of his students, Adrien Certes, is considered the founder of marine microbiology. Pasteur designed methods for food preservation and vaccines against several diseases such as anthrax, fowl cholera and rabies. Koch is best known for his contributions to the germ theory of disease, proving that specific diseases were caused by specific pathogenic microorganisms.
He developed a series of criteria. Koch was one of the first scientists to focus on the i
Invertebrate
Invertebrates are animals that neither possess nor develop a vertebral column, derived from the notochord. This includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include arthropods, mollusks and cnidarians; the majority of animal species are invertebrates. Many invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata; some of the so-called invertebrates, such as the Tunicata and Cephalochordata are more related to the vertebrates than to other invertebrates. This makes the invertebrates paraphyletic, so the term has little meaning in taxonomy; the word "invertebrate" comes from the Latin word vertebra, which means a joint in general, sometimes a joint from the spinal column of a vertebrate. The jointed aspect of vertebra is derived from the concept of turning, expressed in the root verto or vorto, to turn; the prefix in- means "not" or "without". The term invertebrates is not always precise among non-biologists since it does not describe a taxon in the same way that Arthropoda, Vertebrata or Manidae do.
Each of these terms describes a valid taxon, subphylum or family. "Invertebrata" is a term of convenience, not a taxon. The Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of "Vertebrata" and "Invertebrata" has limited practicality. In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord; that would at least circumscribe the Chordata. However the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or bauplan. Despite this, the concept of invertebrates as a taxon of animals has persisted for over a century among the laity, within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata; the following text reflects earlier scientific understanding of the term and of those animals which have constituted it.
According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely varied body plans. Many have like jellyfish or worms. Others have outer shells like those of insects and crustaceans; the most familiar invertebrates include the Protozoa, Coelenterata, Nematoda, Echinodermata and Arthropoda. Arthropoda include insects and arachnids. By far the largest number of described invertebrate species are insects; the following table lists the number of described extant species for major invertebrate groups as estimated in the IUCN Red List of Threatened Species, 2014.3. The IUCN estimates that 66,178 extant vertebrate species have been described, which means that over 95% of the described animal species in the world are invertebrates; the trait, common to all invertebrates is the absence of a vertebral column: this creates a distinction between invertebrates and vertebrates. The distinction is one of convenience only. Being animals, invertebrates are heterotrophs, require sustenance in the form of the consumption of other organisms.
With a few exceptions, such as the Porifera, invertebrates have bodies composed of differentiated tissues. There is typically a digestive chamber with one or two openings to the exterior; the body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. A minority, exhibit no symmetry. One example of asymmetric invertebrates includes all gastropod species; this is seen in snails and sea snails, which have helical shells. Slugs appear externally symmetrical. Other gastropods develop external asymmetry, such as Glaucus atlanticus that develops asymmetrical cerata as they mature; the origin of gastropod asymmetry is a subject of scientific debate. Other examples of asymmetry are found in hermit crabs, they have one claw much larger than the other. If a male fiddler loses its large claw, it will grow another on the opposite side after moulting. Sessile animals such as sponges are asymmetrical alongside coral colonies. Neurons differ in invertebrates from mammalian cells.
Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. The first invertebrate in which a neuron cell was identified was the medicinal leech, Hirudo medicinalis. Learning and memory using nociceptors in the sea hare, Aplysia has been described. Mollusk neurons are able to detect tissue trauma. Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs and arthropods. One type of invertebrate respi
Uterine microbiome
The uterine microbiome is the commensal, bacteria, yeasts/fungi present in a healthy uterus, amniotic fluid and endometrium and the specific environment which they inhabit. It has been only confirmed that the uterus and its tissues are not sterile. Due to improved 16S rRNA gene sequencing techniques, detection of bacteria that are present in low numbers is possible. Using this procedure that allows the detection of bacteria that cannot be cultured outside the body, studies of microbiota present in the uterus are expected to increase. Bacteria and one genus of yeasts are a normal part of the uterus before and during pregnancy; the uterus has been found to possess its own characteristic microbiome that differs from the vaginal microbiome. Despite its close spatial connection with the vagina, the microbiome of the uterus more resembles the commensal bacteria found in the oral cavity. In addition, the immune system is able to differentiate between those bacteria found in the uterus and those that are pathogenic.
Hormonal changes have an effect on the microbiota of the uterus. The organisms listed below have been identified as commensals in the healthy uterus; some have the potential for growing to the point of causing disease: Other taxa can be present, without causing disease or an immune response. Their presence is associated with negative birth outcomes. Prophylactic antibiotics have been injected into the uterus to treat infertility; this has been done before the transfer of embryos with the intent to improve implantation rates. No association exists between antibiotic treatment. Infertility treatments progress to the point where a microbiological analysis of the uterine microbiota is performed. Preterm birth is associated with certain species of bacteria that are not part of the healthy uterine microbiome; the immune response becomes more pronounced. Investigations into reproductive-associated microbiomes began around 1885 by Theodor Escherich, he wrote. There was a general consensus at the time and recently that the uterus was sterile and this was referred to as the sterile womb paradigm.
Other investigations used sterile diapers for meconium collection. No bacteria were able to be cultured from the samples. Other studies showed that bacteria were detected and were directly proportional to the time between birth and the passage of meconium. Investigations into the role of the uterine microbiome in the development of the infant microbiome are ongoing. Human microbiome Human microbiome project Human virome List of antimicrobial peptides in the female reproductive tract List of bacterial vaginosis microbiota Placental microbiome Vaginal epithelium Vaginal microbiota in pregnancy
Skin flora
The term skin flora refers to the microorganisms which reside on the skin human skin. Many of them are bacteria of which there are around 1000 species upon human skin from nineteen phyla. Most are found in the upper parts of hair follicles. Skin flora is non-pathogenic, either commensal or mutualistic; the benefits bacteria can offer include preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin's immune system. However, resident microbes can cause skin diseases and enter the blood system, creating life-threatening diseases in immunosuppressed people. A major non-human skin flora is Batrachochytrium dendrobatidis, a chytrid and non-hyphal zoosporic fungus that causes chytridiomycosis, an infectious disease thought to be responsible for the decline in amphibian populations; the estimate of the number of species present on skin bacteria has been radically changed by the use of 16S ribosomal RNA to identify bacterial species present on skin samples direct from their genetic material.
Such identification had depended upon microbiological culture upon which many varieties of bacteria did not grow and so were hidden to science. Staphylococcus epidermidis and Staphylococcus aureus were thought from cultural based research to be dominant; however 16S ribosomal RNA research finds that while common, these species make up only 5% of skin bacteria. However, skin variety provides a diverse habitat for bacteria. Most come from four phyla: Actinobacteria, Firmicutes and Bacteroidetes. There are three main ecological areas: sebaceous and dry. Propionibacteria and Staphylococci species were the main species in sebaceous areas. In moist places on the body Corynebacteria together with Staphylococci dominate. In dry areas, there is a mixture of species but b-Proteobacteria and Flavobacteriales are dominant. Ecologically, sebaceous areas had greater species richness than dry one; the areas with least similarity between people in species were the spaces between fingers, the spaces between toes and umbilical cord stump.
Most were beside the nostril, on the back. A study of the area between toes in 100 young adults found 14 different genera of fungi; these include yeasts such as Candida albicans, Rhodotorula rubra and Trichosporon cutaneum, dermatophytes such as Microsporum gypseum, Trichophyton rubrum and nondermatophyte fungi such as Rhizopus stolonifer, Trichosporon cutaneum, Scopulariopsis brevicaulis, Alternaria alternata, Aspergillus flavus and Penicillium species. A study by the National Human Genome Research Institute in Bethesda, researched the DNA of human skin fungi at 14 different locations on the body; these were the ear canal, between the eyebrows, the back of the head, behind the ear, the heel, between the toes, back, nostrils, chest and the crook of the elbow. The study showed a large fungal diversity across the body, the richest habitat being the heel, which hosts about 80 species of fungi. By way of contrast, there are 40 between the toes. Other rich areas are the palm and inside the elbow, with from 18 to 32 species.
The head and the trunk hosted between 10 each. The umbilicus, or navel, is an area of the body, exposed to UV light, soaps, or bodily secretions and because it is an undisturbed community of bacteria it is an excellent part of the skin microbiome to study; the navel, or umbilicus is a moist microbiome of the body, that contains a large amount of bacteria bacteria that favors moist conditions such as Corynebacterium and Staphylococcus. The Belly Button Biodiversity Project began at North Carolina State University in early 2011 with two initial groups of 35 and 25 volunteers. Volunteers were given sterile cotton swabs and were asked to insert the cotton swabs into their navels, to turn the cotton swab around three times and return the cotton swab to the researchers in a vial that contained a 0.5 ml 10% phosphate saline buffer. Researchers at North Carolina State University, led by Jiri Hulcr grew the samples in a culture until the bacterial colonies were large enough to be photographed and these pictures were posted on the Belly Button Biodiversity Project's website.
These samples were analyzed using 16S rDNA libraries so that strains that did not grow well in cultures could be identified. The researchers at North Carolina State University discovered that while it was difficult to predict every strain of bacteria in the microbiome of the navel that they could predict which strains would be prevalent and which strains of bacteria would be quite rare in the microbiome, it was found that the navel microbiomes only contained a few prevalent types of bacteria and many different types of rare bacteria. Other types of rare organisms were discovered inside the navels of the volunteers including three types of Archaea and two of the three types of Archaea were found in one volunteer who claimed not to have bathed or showered for many years. Staphylococcus and Corynebacterium wer
Polychaete
The Polychaeta known as the bristle worms or polychaetes, are a paraphyletic class of annelid worms marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin; as such, polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common representatives include the sandworm or clam worm Alitta. Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to a 2- to 3-cm specimen observed by the robot ocean probe Nereus at the bottom of the Challenger Deep, the deepest known spot in the Earth's oceans. Only 168 species are known from fresh waters. Polychaetes are segmented worms less than 10 cm in length, although ranging at the extremes from 1 mm to 3 m, in Eunice aphroditois.
They can sometimes be brightly coloured, may be iridescent or luminescent. Each segment bears a pair of paddle-like and vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called setae, project from the parapodia. However, polychaetes vary from this generalised pattern, can display a range of different body forms; the most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different ecological niches, including burrowing, pelagic life, tube-dwelling or boring and parasitism, requiring various modifications to their body structures. The head, or prostomium, is well developed, compared with other annelids, it projects forward over the mouth. The head includes two to four pair of eyes, although some species are blind; these are fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision.
The head includes a pair of antennae, tentacle-like palps, a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, help the worm to seek out food; the outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, a peritoneum surrounding the body cavity. Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it's more continuous; the mouth of polychaetes is located on the peristomium, the segment behind the prostomium, varies in form depending on their diets, since the group includes predators, filter feeders and parasites. In general, they possess a pair of jaws and a pharynx that can be everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy proboscis.
The digestive tract is a simple tube with a stomach part way along. The smallest species, those adapted to burrowing, lack gills, breathing only through their body surfaces. Most other species have external gills associated with the parapodia. A simple but well-developed circulatory system is present; the two main blood vessels furnish smaller vessels to supply the gut. Blood flows forward in the dorsal vessel, above the gut, returns down the body in the ventral vessel, beneath the gut; the blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in the coelomic fluid that fills their body cavities; the blood may have any of three different respiratory pigments. The most common of these is haemoglobin, but some groups have haemerythrin or the green-coloured chlorocruorin, instead.
The nervous system consists of a single or double ventral nerve cord running the length of the body, with ganglia and a series of small nerves in each segment. The brain is large, compared with that of other annelids, lies in the upper part of the head. An endocrine gland is attached to the ventral posterior surface of the brain, appears to be involved in reproductive activity. In addition to the sensory organs on the head, photosensitive eye spots and numerous additional sensory nerve endings, most in involved with the sense of touch occur on the body. Polychaetes have a varying number of protonephridia or metanephridia for excreting waste, which in some cases can be complex in structure; the body contains greenish "chloragogen" tissue, similar to that found in oligochaetes, which appears to function in metabolism, in a similar fashion to that of the vertebrate liver. The cuticle may be 200 nm to 13 mm thick, their jaws are formed from sclerotised collagen, their setae from sclerotised chitin.
Polychaetes are variable in both form and lifestyle, include a few taxa that swim among the plankton or above the abyssal plain. Most burrow or build tubes in the sediment, some live as commensals. A few are pa
Eukaryote
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. Eukaryotes belong to Eukarya, their name comes from the Greek εὖ and κάρυον. Eukaryotic cells contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells; these act as sex cells. Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis.
The domain Eukaryota appears to be monophyletic, makes up one of the domains of life in the three-domain system. The two other domains and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things. However, due to their much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved 1.6–2.1 billion years ago, during the Proterozoic eon. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton; the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1937 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes; however he mentioned this in only one paragraph, the idea was ignored until Chatton's statement was rediscovered by Stanier and van Niel.
In 1905 and 1910, the Russian biologist Konstantin Mereschkowski argued that plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, itself formed by symbiosis between an amoeba-like host and a bacterium-like cell that formed the nucleus. Plants had thus inherited photosynthesis from cyanobacteria. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, On the origin of mitosing cells. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA; this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles and chloroplasts. In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria. In 1979, G. W. Gould and G. J. Dring suggested that the eukaryotic cell's nucleus came from the ability of Gram-positive bacteria to form endospores. In 1987 and papers, Thomas Cavalier-Smith proposed instead that the membranes of the nucleus and endoplasmic reticulum first formed by infolding a prokaryote's plasma membrane.
In the 1990s, several other biologists proposed endosymbiotic origins for the nucleus reviving Mereschkowski's theory. Eukaryotic cells are much larger than those of prokaryotes having a volume of around 10,000 times greater than the prokaryotic cell, they have a variety of internal membrane-bound structures, called organelles, a cytoskeleton composed of microtubules and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division. Eukaryote cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and pinches off to form a vesicle, it is probable that most other membrane-bound organelles are derived from such vesicles.
Alternatively some products produced by the cell can leave in a vesicle through exocytosis. The nucleus is surrounded with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, involved in protein transport and maturation, it includes the rough endoplasmic reticulum where ribosomes are attached to synthesize proteins, which enter the interior space or lumen. Subsequently, they enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, the Golgi apparatus. Vesicles may be specialized for various purposes. For instance, lysosomes contain digestive enzymes that break down most biomolecules in the cytoplasm. Peroxisomes are used to break down peroxide, otherwise toxic. Many protozoans have contractile vacuoles, which collect and expel excess water, extrusomes, which expel material used to deflect predators or capture prey.
In higher plants, most of a cell's volume is taken up by a central vacuole, whi
