Rodents are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents, they are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments. Species can be fossorial, or semiaquatic. Well-known rodents include mice, squirrels, prairie dogs, chinchillas, beavers, guinea pigs, hamsters and capybaras. Other animals such as rabbits and pikas, whose incisors grow continually, were once included with them, but are now considered to be in a separate order, the Lagomorpha. Nonetheless and Lagomorpha are sister groups, sharing a most recent common ancestor and forming the clade of Glires. Most rodents are small animals with robust bodies, short limbs, long tails, they use their sharp incisors to gnaw food, excavate burrows, defend themselves. Most eat seeds or other plant material, they tend to be social animals and many species live in societies with complex ways of communicating with each other.
Mating among rodents can vary from monogamy, to polygyny, to promiscuity. Many have litters of altricial young, while others are precocial at birth; the rodent fossil record dates back to the Paleocene on the supercontinent of Laurasia. Rodents diversified in the Eocene, as they spread across continents, sometimes crossing oceans. Rodents reached both South America and Madagascar from Africa and were the only terrestrial placental mammals to reach and colonize Australia. Rodents have been used as food, for clothing, as pets, as laboratory animals in research; some species, in particular, the brown rat, the black rat, the house mouse, are serious pests and spoiling food stored by humans and spreading diseases. Accidentally introduced species of rodents are considered to be invasive and have caused the extinction of numerous species, such as island birds isolated from land-based predators; the distinguishing feature of the rodents is their pairs of continuously growing, razor-sharp, open-rooted incisors.
These incisors little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull; as the incisors grind against each other, the softer dentine on the rear of the teeth wears away, leaving the sharp enamel edge shaped like the blade of a chisel. Most species have up to 22 teeth with no canines or anterior premolars. A gap, or diastema, occurs between the cheek teeth in most species; this allows rodents to suck in their cheeks or lips to shield their mouth and throat from wood shavings and other inedible material, discarding this waste from the sides of their mouths. Chinchillas and guinea pigs have a high-fiber diet. In many species, the molars are large, intricately structured, cusped or ridged. Rodent molars are well equipped to grind food into small particles; the jaw musculature is strong. The lower jaw is pulled backwards during chewing. Rodent groups differ in the arrangement of the jaw muscles and associated skull structures, both from other mammals and amongst themselves.
The Sciuromorpha, such as the eastern grey squirrel, have a large deep masseter, making them efficient at biting with the incisors. The Myomorpha, such as the brown rat, have enlarged temporalis muscles, making them able to chew powerfully with their molars; the Hystricomorpha, such as the guinea pig, have larger superficial masseter muscles and smaller deep masseter muscles than rats or squirrels making them less efficient at biting with the incisors, but their enlarged internal pterygoid muscles may allow them to move the jaw further sideways when chewing. The cheek pouch is a specific morphological feature used for storing food and is evident in particular subgroups of rodents like kangaroo rats, hamsters and gophers which have two bags that may range from the mouth to the front of the shoulders. True mice and rats do not contain this structure but their cheeks are elastic due to a high degree of musculature and innervation in the region. While the largest species, the capybara, can weigh as much as 66 kg, most rodents weigh less than 100 g.
The smallest rodent is the Baluchistan pygmy jerboa, which averages only 4.4 cm in head and body length, with adult females weighing only 3.75 g. Rodents have wide-ranging morphologies, but have squat bodies and short limbs; the fore limbs have five digits, including an opposable thumb, while the hind limbs have three to five digits. The elbow gives the forearms great flexibility; the majority of species are plantigrade, walking on both the palms and soles of their feet, have claw-like nails. The nails of burrowing species tend to be long and strong, while arboreal rodents have shorter, sharper nails. Rodent species use a wide variety of methods of locomotion including quadrupedal walking, burrowing, bipedal hopping and gliding. Scaly-tailed squirrels and flying squirrels, although not related, can both glide from tree to tree using parachute-like membranes that stretch from the fore to the hind limbs; the agouti is antelope-like, being digitigrade and having hoof-like nails. The majority of rodents have tails, which can be of many shapes and siz
Quantitative genetics is a branch of population genetics that deals with phenotypes that vary continuously —as opposed to discretely identifiable phenotypes and gene-products. Both branches use the frequencies of different alleles of a gene in breeding populations, combine them with concepts from simple Mendelian inheritance to analyze inheritance patterns across generations and descendant lines. While population genetics can focus on particular genes and their subsequent metabolic products, quantitative genetics focuses more on the outward phenotypes, makes summaries only of the underlying genetics. Due to the continuous distribution of phenotypic values, quantitative genetics must employ many other statistical methods to link phenotypes to genotypes; some phenotypes may be analyzed either as discrete categories or as continuous phenotypes, depending on the definition of cut-off points, or on the metric used to quantify them. Mendel himself had to discuss this matter in his famous paper with respect to his peas attribute tall/dwarf, "length of stem".
Analysis of quantitative trait loci, or QTL, is a more recent addition to quantitative genetics, linking it more directly to molecular genetics. In diploid organisms, the average genotypic "value" may be defined by the allele "effect" together with a dominance effect, by how genes interact with genes at other loci; the founder of quantitative genetics - Sir Ronald Fisher - perceived much of this when he proposed the first mathematics of this branch of genetics. Being a statistician, he defined the gene effects as deviations from a central value—enabling the use of statistical concepts such as mean and variance, which use this idea; the central value he chose for the gene was the midpoint between the two opposing homozygotes at the one locus. The deviation from there to the "greater" homozygous genotype can be named "+a"; this is the "allele" effect mentioned above. The heterozygote deviation from the same midpoint can be named "d", this being the "dominance" effect referred to above; the diagram depicts the idea.
However, in reality we measure phenotypes, the figure shows how observed phenotypes relate to the gene effects. Formal definitions of these effects recognize this phenotypic focus. Epistasis has been approached statistically as interaction, but epigenetics suggests a new approach may be needed. If 0<d<a, the dominance is regarded as partial or incomplete—while d=a indicates full or classical dominance. D>a was known as "over-dominance". Mendel's pea attribute. Mendel stated that the tall true-breeding parents ranged from 6–7 feet in stem length, giving a median of 198 cm; the short parents ranged from 0.75 -- with a rounded median of 34 cm. Their hybrid ranged from 6 -- with a median of 206 cm; the mean of P1 and P2 is 116 this being the phenotypic value of the homozygotes midpoint. The allele affect is = 82 cm = -; the dominance effect is = 90 cm. This historical example illustrates how phenotype values and gene effects are linked. To obtain means and other statistics, both quantities and their occurrences are required.
The gene effects provide the framework for quantities: and the frequencies of the contrasting alleles in the fertilization gamete-pool provide the information on occurrences. The frequency of the allele causing "more" in the phenotype is given the symbol p, while the frequency of the contrasting allele is q. An initial assumption made when establishing the algebra was that the parental population was infinite and random mating, made to facilitate the derivation; the subsequent mathematical development implied that the frequency distribution within the effective gamete-pool was uniform: there were no local perturbations where p and q varied. Looking at the diagrammatic analysis of sexual reproduction, this is the same as declaring that pP = pg = p; this mating system, dependent upon these assumptions, became known as "panmixia". Panmixia actually occurs in nature, as gamete distribution may be limited, for example by dispersal restrictions or by behaviour, or by chance sampling, it is well-known that there is a huge wastage of gametes in Nature, why the diagram depicts a potential gamete-pool separately to the actual gamete-pool.
Only the latter sets the definitive frequencies for the zygotes: this is the true "gamodeme". But, under Fisher's assumptions, the gamodeme can be extended back to the potential gamete-pool, back to the parental base-population; the random sampling arising when small "actual" gamete-pools are sampled from a large "potential" gamete-pool is known as genetic drift, is considered subsequently. While panmixia may not be extant, the potential for it does occur, although it may be only ephemeral because of those local perturbations, it has been shown, for example, that the F2 derived from random fertilization of F1 individuals, following hybridization, is an origin of a new panmictic population. It has been shown that if panmictic random fertilization occurred continually, it would maintain the
Mathematics includes the study of such topics as quantity, structure and change. Mathematicians use patterns to formulate new conjectures; when mathematical structures are good models of real phenomena mathematical reasoning can provide insight or predictions about nature. Through the use of abstraction and logic, mathematics developed from counting, calculation and the systematic study of the shapes and motions of physical objects. Practical mathematics has been a human activity from as far back; the research required to solve mathematical problems can take years or centuries of sustained inquiry. Rigorous arguments first appeared in Greek mathematics, most notably in Euclid's Elements. Since the pioneering work of Giuseppe Peano, David Hilbert, others on axiomatic systems in the late 19th century, it has become customary to view mathematical research as establishing truth by rigorous deduction from appropriately chosen axioms and definitions. Mathematics developed at a slow pace until the Renaissance, when mathematical innovations interacting with new scientific discoveries led to a rapid increase in the rate of mathematical discovery that has continued to the present day.
Mathematics is essential in many fields, including natural science, medicine and the social sciences. Applied mathematics has led to new mathematical disciplines, such as statistics and game theory. Mathematicians engage in pure mathematics without having any application in mind, but practical applications for what began as pure mathematics are discovered later; the history of mathematics can be seen as an ever-increasing series of abstractions. The first abstraction, shared by many animals, was that of numbers: the realization that a collection of two apples and a collection of two oranges have something in common, namely quantity of their members; as evidenced by tallies found on bone, in addition to recognizing how to count physical objects, prehistoric peoples may have recognized how to count abstract quantities, like time – days, years. Evidence for more complex mathematics does not appear until around 3000 BC, when the Babylonians and Egyptians began using arithmetic and geometry for taxation and other financial calculations, for building and construction, for astronomy.
The most ancient mathematical texts from Mesopotamia and Egypt are from 2000–1800 BC. Many early texts mention Pythagorean triples and so, by inference, the Pythagorean theorem seems to be the most ancient and widespread mathematical development after basic arithmetic and geometry, it is in Babylonian mathematics that elementary arithmetic first appear in the archaeological record. The Babylonians possessed a place-value system, used a sexagesimal numeral system, still in use today for measuring angles and time. Beginning in the 6th century BC with the Pythagoreans, the Ancient Greeks began a systematic study of mathematics as a subject in its own right with Greek mathematics. Around 300 BC, Euclid introduced the axiomatic method still used in mathematics today, consisting of definition, axiom and proof, his textbook Elements is considered the most successful and influential textbook of all time. The greatest mathematician of antiquity is held to be Archimedes of Syracuse, he developed formulas for calculating the surface area and volume of solids of revolution and used the method of exhaustion to calculate the area under the arc of a parabola with the summation of an infinite series, in a manner not too dissimilar from modern calculus.
Other notable achievements of Greek mathematics are conic sections, trigonometry (Hipparchus of Nicaea, the beginnings of algebra. The Hindu–Arabic numeral system and the rules for the use of its operations, in use throughout the world today, evolved over the course of the first millennium AD in India and were transmitted to the Western world via Islamic mathematics. Other notable developments of Indian mathematics include the modern definition of sine and cosine, an early form of infinite series. During the Golden Age of Islam during the 9th and 10th centuries, mathematics saw many important innovations building on Greek mathematics; the most notable achievement of Islamic mathematics was the development of algebra. Other notable achievements of the Islamic period are advances in spherical trigonometry and the addition of the decimal point to the Arabic numeral system. Many notable mathematicians from this period were Persian, such as Al-Khwarismi, Omar Khayyam and Sharaf al-Dīn al-Ṭūsī. During the early modern period, mathematics began to develop at an accelerating pace in Western Europe.
The development of calculus by Newton and Leibniz in the 17th century revolutionized mathematics. Leonhard Euler was the most notable mathematician of the 18th century, contributing numerous theorems and discoveries; the foremost mathematician of the 19th century was the German mathematician Carl Friedrich Gauss, who made numerous contributions to fields such as algebra, differential geometry, matrix theory, number theory, statistics. In the early 20th century, Kurt Gödel transformed mathematics by publishing his incompleteness theorems, which show that any axiomatic system, consistent will contain unprovable propositions. Mathematics has since been extended, there has been a fruitful interaction between mathematics and science, to
Doctor of Medicine
A Doctor of Medicine is a medical degree, the meaning of which varies between different jurisdictions. In the United States and other countries, the MD denotes a professional graduate degree awarded upon graduation from medical school. In the United Kingdom and other countries, the MD is a research doctorate, higher doctorate, honorary doctorate or applied clinical degree restricted to those who hold a professional degree in medicine. In 1703, the University of Glasgow's first medical graduate, Samuel Benion, was issued with the academic degree of Doctor of Medicine. University medical education in England culminated with the MB qualification, in Scotland the MD, until in the mid-19th century the public bodies who regulated medical practice at the time required practitioners in Scotland as well as England to hold the dual Bachelor of Medicine and Bachelor of Surgery degrees. North American medical schools switched to the tradition of the ancient universities of Scotland and began granting the MoD title rather than the MB beginning in the late 18th century.
The Columbia University College of Physicians and Surgeons in New York was the first American university to grant the MD degree instead of the MB. Early medical schools in North America that granted the Doctor of Medicine degrees were Columbia, Harvard, McGill; these first few North American medical schools that were established were founded by physicians and surgeons, trained in England and Scotland. A feminine form, "Doctress of Medicine" or Medicinae Doctrix, was used by the New England Female Medical College in Boston in the 1860s. In most countries having a Doctor of Medicine degree does not mean that the individual will be allowed to practice medicine. A doctor must go through a residency for at least four years and take some form of licensing examination in their jurisdiction. In Afghanistan, medical education begins after high school. No pre-medicine courses or bachelor's degree is required. Eligibility is determined through the rank applicants obtain in the public university entrance exam held every year throughout the country.
Entry to medical school is competitive, only students with the highest ranks are accepted into medical programs. The primary medical degree is completed in 7 years. According to the new medical curriculum, during the 12th semester, medical students must complete research on a medical topic and provide a thesis as part of their training. Medical graduates are awarded a certificate in general medicine, regarded "MD" and validated by the "Ministry of Higher Education of Afghanistan". All physicians are to obtain licensing and a medical council registration number from the "Ministry of Public Health" before they begin to practice, they may subsequently specialize in a specific medical field at medical schools offering the necessary qualifications. After graduation, students may complete residency; the MD specification: Before the civil wars in Afghanistan, medical education used to be taught by foreign professors or Afghan professors who studied medical education abroad. The Kabul medical institute certified the students as "Master of Medicine".
After the civil wars, medical education has changed, the MD certification has been reduced to "Medicine Bachelor". In Argentina, the First Degree of Physician or Physician Diplomate is equivalent to the North American MD Degree with six years of intensive studies followed by three or four years of residency as a major specialty in a particular empiric field, consisting of internships, social services and sporadic research. Only by holding a Medical Title can the postgraduate student apply for the Doctor degree through a Doctorate in Medicine program approved by the National Commission for University Evaluation and Accreditation. Australian medical schools have followed the British tradition by conferring the degrees of Bachelor of Medicine and Bachelor of Surgery to its graduates whilst reserving the title of Doctor of Medicine for their research training degree, analogous to the PhD, or for their honorary doctorates. Although the majority of Australian MBBS degrees have been graduate programs since the 1990s, under the previous Australian Qualifications Framework they remained categorized as Level 7 Bachelor's degrees together with other undergraduate programs.
The latest version of the AQF includes the new category of Level 9 Master's degrees which permits the use of the term'Doctor' in the styling of the degree title of relevant professional programs. As a result, various Australian medical schools have replaced their MBBS degrees with the MD to resolve the previous anomalous nomenclature. With the introduction of the Master's level MD, universities have renamed their previous medical research doctorates; the University of Melbourne was the first to introduce the MD in 2011 as a basic medical degree, has renamed its research degree to Doctor of Medical Science. In French-speaking Belgium, the medical degree awarded after six years of study is "Docteur en Médecine". Physicians would have to register with the Ordre des Medicins to practice medicine in the country. At the end of the six-year medical programs from Bulgarian medical schools, medical students are awarded the academic degree Master in Medicine and the professional title Physician - Doctor of Medicine.
After 6 years of general medical education, all students will graduate with
Education is the process of facilitating learning, or the acquisition of knowledge, values and habits. Educational methods include storytelling, teaching and directed research. Education takes place under the guidance of educators and learners may educate themselves. Education can take place in formal or informal settings and any experience that has a formative effect on the way one thinks, feels, or acts may be considered educational; the methodology of teaching is called pedagogy. Formal education is divided formally into such stages as preschool or kindergarten, primary school, secondary school and college, university, or apprenticeship. A right to education has been recognized by the United Nations. In most regions, education is compulsory up to a certain age. Etymologically, the word "education" is derived from the Latin word ēducātiō from ēducō, related to the homonym ēdūcō from ē- and dūcō. Education began in prehistory, as adults trained the young in the knowledge and skills deemed necessary in their society.
In pre-literate societies, this was achieved orally and through imitation. Story-telling passed knowledge and skills from one generation to the next; as cultures began to extend their knowledge beyond skills that could be learned through imitation, formal education developed. Schools existed in Egypt at the time of the Middle Kingdom. Plato founded the Academy in the first institution of higher learning in Europe; the city of Alexandria in Egypt, established in 330 BCE, became the successor to Athens as the intellectual cradle of Ancient Greece. There, the great Library of Alexandria was built in the 3rd century BCE. European civilizations suffered a collapse of literacy and organization following the fall of Rome in CE 476. In China, Confucius, of the State of Lu, was the country's most influential ancient philosopher, whose educational outlook continues to influence the societies of China and neighbours like Korea and Vietnam. Confucius gathered disciples and searched in vain for a ruler who would adopt his ideals for good governance, but his Analects were written down by followers and have continued to influence education in East Asia into the modern era.
The Aztecs had a well-developed theory about education, which has an equivalent word in Nahuatl called tlacahuapahualiztli. It means "the art of raising or educating a person" or "the art of strengthening or bringing up men." This was a broad conceptualization of education, which prescribed that it begins at home, supported by formal schooling, reinforced by community living. Historians cite that formal education was mandatory for everyone regardless of social class and gender. There was the word neixtlamachiliztli, "the act of giving wisdom to the face." These concepts underscore a complex set of educational practices, oriented towards communicating to the next generation the experience and intellectual heritage of the past for the purpose of individual development and his integration into the community. After the Fall of Rome, the Catholic Church became the sole preserver of literate scholarship in Western Europe; the church established cathedral schools in the Early Middle Ages as centres of advanced education.
Some of these establishments evolved into medieval universities and forebears of many of Europe's modern universities. During the High Middle Ages, Chartres Cathedral operated the famous and influential Chartres Cathedral School; the medieval universities of Western Christendom were well-integrated across all of Western Europe, encouraged freedom of inquiry, produced a great variety of fine scholars and natural philosophers, including Thomas Aquinas of the University of Naples, Robert Grosseteste of the University of Oxford, an early expositor of a systematic method of scientific experimentation, Saint Albert the Great, a pioneer of biological field research. Founded in 1088, the University of Bologne is considered the first, the oldest continually operating university. Elsewhere during the Middle Ages, Islamic science and mathematics flourished under the Islamic caliphate, established across the Middle East, extending from the Iberian Peninsula in the west to the Indus in the east and to the Almoravid Dynasty and Mali Empire in the south.
The Renaissance in Europe ushered in a new age of scientific and intellectual inquiry and appreciation of ancient Greek and Roman civilizations. Around 1450, Johannes Gutenberg developed a printing press, which allowed works of literature to spread more quickly; the European Age of Empires saw European ideas of education in philosophy, religion and sciences spread out across the globe. Missionaries and scholars brought back new ideas from other civilizations – as with the Jesuit China missions who played a significant role in the transmission of knowledge and culture between China and Europe, translating works from Europe like Euclid's Elements for Chinese scholars and the thoughts of Confucius for European audiences; the Enlightenment saw the emergence of a more secular educational outlook in Europe. In most countries today, full-time education, whether at school or otherwise, is compulsory for all children up to a certain age. Due to this the proliferation of compulsory education, combined with population growth, UNESCO has calculated that in the next 30 years more people will receive formal education than in all of human history thus far.
Formal education occurs in a structured environment. Formal education takes place in a school environme
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
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