The bacteria ( [bækˈtɪərɪə] (help·info); singular: bacterium)^[α] are a large group of unicellular microorganisms A microorganism or microbe is an organism that is microscopic (usually too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design. Typically a few micrometres A micrometre or micron is one millionth of a metre, or equivalently one thousandth of a millimetre. It can be written in scientific notation as 1×10−6 m, meaning 1⁄1000000 m in length, bacteria have a wide range of shapes, ranging from spheres A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point. This distance r is known as the radius of the sphere. The to rods and spirals. Bacteria are ubiquitous in every habitat A habitat is an ecological or environmental area that is inhabited by a particular animal or plant species. It is the natural environment in which an organism lives, or the physical environment that surrounds (influences and is utilized by) a species population.[citation needed] on Earth Earth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density. It is also referred to as the World, the Blue Planet,[note 3] and Terra.[note 4], growing in soil, acidic hot springs A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are hot springs all over the earth, on every continent and even under the oceans and seas, radioactive waste Radioactive waste is a waste product containing radioactive material. It is usually the product of a nuclear process such as nuclear fission. However, industries not directly connected to the nuclear industry may produce quantities of radioactive waste. The majority of radioactive waste is "low-level waste", meaning it contains low,^[2] water, and deep in the Earth's crust In geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 in a gram of soil and a million bacterial cells in a millilitre of fresh water Freshwater refers to naturally occurring water on the surface such as bogs, ponds, lakes, rivers and streams, and underground in aquifers and underground rivers. Freshwater is characterized by having low concentrations of dissolved salts. The term specifically excludes seawater and brackish water; in all, there are approximately five nonillion The following table lists those names of large numbers which are found in many English dictionaries and thus have a special claim to being "real words". The "Traditional British" values shown are unused in American English and are obsolete[citation needed] in British English, but are dominant in many non-English-speaking areas, (5×10³⁰) bacteria on Earth,^[3] forming much of the world's biomass Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms, plants or animals. The mass can be expressed as the.^[4] Bacteria are vital in recycling nutrients, with many steps in nutrient cycles In ecology and Earth science, a biogeochemical cycle or nutrient cycle is a pathway by which a chemical element or molecule moves through both biotic and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. In effect, the element is recycled, although in some cycles there may be places (called reservoirs) where the element is depending on these organisms, such as the fixation of nitrogen Nitrogen fixation usually refers to the biological process by which nitrogen in the atmosphere is converted into ammonia. This process is essential for life because fixed nitrogen is required to biosynthesize a basic building block of life, e.g. nucleotides for DNA and amino acids for proteins. Formally, nitrogen fixation also refers to other from the atmosphere The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95 and putrefaction. However, most bacteria have not been characterized, and only about half of the phyla In biology, a phylum [note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division of bacteria have species that can be grown in the laboratory.^[5] The study of bacteria is known as bacteriology Microbiology is the study of microorganisms, which are unicellular or cell-cluster microscopic organisms. This includes eukaryotes such as fungi and protists, and prokaryotes. Viruses, though not strictly classed as living organisms, are also studied. In short; microbiology refers to the study of life and organisms that are too small to be seen, a branch of microbiology Microbiology is the study of microorganisms, which are unicellular or cell-cluster microscopic organisms. This includes eukaryotes such as fungi and protists, and prokaryotes. Viruses, though not strictly classed as living organisms, are also studied. In short; microbiology refers to the study of life and organisms that are too small to be seen.

There are approximately ten times as many bacterial cells in the human flora The human flora is the assemblage of microrganisms that constantly and consistently inhabit the human body. They include bacteria, fungi and archaea. Some of these organisms are known to perform tasks that are useful for the human host, while the majority have no known beneficial or harmful effect. Those that are expected to be present, and that of bacteria as there are human cells in the body, with large numbers of bacteria on the skin The skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds. Human skin is not unlike that of most other mammals and as gut flora Gut flora consists of microorganisms that live in the digestive tracts of animals, and is the largest reservoir of human flora. "Gut" is synonymous with intestinal and "flora" with microbiota and microflora.^[6] The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system An immune system is a collection of biological processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own healthy cells and tissues in order to function properly, and a few are beneficial Probiotics are dietary supplements of live microorganisms thought to be healthy for the host organism. According to the currently adopted definition by FAO/WHO, probiotics are: "Live microorganisms which when administered in adequate amounts confer a health benefit on the host". Lactic acid bacteria and bifidobacteria are the most common. However, a few species of bacteria are pathogenic Pathogenic bacteria are bacteria that cause infectious diseases. This article deals with human pathogenic bacteria and cause infectious diseases An infectious disease is a clinically evident disease resulting from the presence of pathogenic microbial agents, including pathogenic viruses, pathogenic bacteria, fungi, protozoa, multicellular parasites, and aberrant proteins known as prions. These pathogens are able to cause disease in animals and/or plants. Infectious pathologies are usually, including cholera Cholera, sometimes known as Asiatic or epidemic cholera, is an infectious gastroenteritis caused by enterotoxin-producing strains of the bacterium Vibrio cholerae. Transmission to humans occurs through eating food or drinking water contaminated with Vibrio cholerae from other cholera patients. The major reservoir for cholera was long assumed to be, syphilis Syphilis is a sexually transmitted disease caused by the spirochetal bacterium Treponema pallidum subspecies pallidum. The route of transmission of syphilis is almost always through sexual contact, although there are examples of congenital syphilis via transmission from mother to child in utero, anthrax Anthrax is an acute disease caused by Bacillus anthracis. It affects both humans and animals and most forms of the disease are highly lethal. There are effective vaccines against anthrax, and some forms of the disease respond well to antibiotic treatment, leprosy Leprosy , or Hansen's disease (HD), is a chronic disease caused by the bacteria Mycobacterium leprae and Mycobacterium lepromatosis. Leprosy is primarily a granulomatous disease of the peripheral nerves and mucosa of the upper respiratory tract; skin lesions are the primary external symptom. Left untreated, leprosy can be progressive, causing and bubonic plague Bubonic plague is the best known manifestation of the bacterial disease plague, caused by the Gram-negative bacterium Yersinia pestis . It belongs to the family Enterobacteriaceae. The term "bubonic plague" was often used synonymously for plague, but it does in fact refer specifically to an infection that enters through the skin and. The most common fatal bacterial diseases are respiratory infections, with tuberculosis Tuberculosis is a common and often deadly infectious disease caused by mycobacteria, in humans mainly Mycobacterium tuberculosis . Tuberculosis usually attacks the lungs (as pulmonary TB) but can also affect the central nervous system, the lymphatic system, the circulatory system, the genitourinary system, the gastrointestinal system, bones, alone killing about 2 million people a year, mostly in sub-Saharan Africa Sub-Saharan Africa is a geographical term used to describe the area of the African continent which lies south of the Sahara, or those African countries which are fully or partially located south of the Sahara. It contrasts with North Africa, which is considered a part of the Arab world.^[7] In developed countries The term developed country is used to describe countries that have a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue and there is fierce debate about this. Economic criteria have tended to dominate discussions. One such criterion is income per, antibiotics In common usage, an antibiotic is a substance or compound that kills or inhibits the growth of bacteria. Antibiotics belong to the broader group of antimicrobial compounds, used to treat infections caused by microorganisms, including fungi and protozoa are used to treat bacterial infections An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host's resources to multiply, usually at the expense of the host. The infecting organism, or pathogen, interferes with the normal functioning of the host and can lead to chronic wounds, gangrene, loss and in agriculture, so antibiotic resistance Antibiotic resistance is the ability of a microorganism to withstand the effects of antibiotics. It is a specific type of drug resistance. Antibiotic resistance evolves via natural selection acting upon random mutation, but it can also be engineered by applying an evolutionary stress on a population. Once such a gene is generated, bacteria can is becoming common. In industry, bacteria are important in sewage treatment Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce a waste stream (or treated effluent) and a solid, the production of cheese Cheese is a food consisting of proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. It is produced by coagulation of the milk protein casein. Typically, the milk is acidified and addition of the enzyme rennet causes coagulation. The solids are then separated and pressed into final form. Some cheeses also contain molds, and yoghurt Yoghurt or yogurt is a dairy product produced by bacterial fermentation of milk. Fermentation of lactose produces lactic acid, which acts on milk protein to give yoghurt its texture and its characteristic tang. Soy yoghurt, a non-dairy yoghurt alternative, is made from soy milk through fermentation Fermentation is the process of deriving energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. This is in contrast to cellular respiration, where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain, as well as in biotechnology Biotechnology is technology based on biology, agriculture, food science, and medicine. Modern use of the term usually refers to genetic engineering as well as cell- and tissue culture technologies. However, the concept encompasses a wider range and history of procedures for modifying living things according to human purposes, going back to, and the manufacture of antibiotics and other chemicals.^[8]

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes The prokaryotes are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles. The word prokaryote comes from the Greek πρό- (pro-) &. Unlike cells of animals and other eukaryotes A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Almost all species of large organisms are eukaryotes, including animals, plants and fungi, although most species of eukaryotic protists are microorganisms. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the, bacterial cells do not contain a nucleus In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes and rarely harbour membrane-bound The cell membrane is the biological membrane separating the interior of a cell from the outside environment organelles In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane. Although the term bacteria traditionally included all prokaryotes, the scientific classification Biological classification or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis. Modern biological classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved In biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a process that can culminate in the emergence of new independently from an ancient common ancestor. These evolutionary domains In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya. The arrangement of taxa reflects the fundamental differences in the genomes. There are some are called Bacteria and Archaea The Archaea [ɑrˈkiə] are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon (sometimes spelled "archeon"). They have no cell nucleus or any other organelles within their cells. In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the.^[9]

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