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The first bacteria were observed by Antony van Leeuwenhoek in 1683 using a single-lens microscope of his own design. The name bacterium was introduced much later, by Ehrenberg in 1828, derived from the Greek word βακτηριον meaning "small stick". Louis Pasteur (1822-1895) and Robert Koch (1843-1910) described the role of bacteria as conveyors and causes of disease or pathogens.
Originally the bacteria were considered microscopic fungi (called Schizomycetes), except for the photosynthetic cyanobacteria, which were considered a group of algae (called Cyanophyta or blue-green algae). It was only with the study of detailed cell structure that it was realized they formed a fundamental group, separate from the other organisms. In 1956 Copeland gave them their own kingdom Mychota, later renamed Monera, Prokaryota, or Bacteria. During the 1960s the concept was refined and bacteria (now including cyanobacteria) were recognized as one of two major divisions of the living world, together with the eukaryotes. Eukaryotes were generally believed to have evolved from bacteria, later from assemblies of bacteria.
The advent of molecular systematics challenged this view. In 1977, Woese divided the prokaryotes into two groups based on 16S rRNA sequences, called the kingdoms Eubacteria and Archaebacteria. He argued that each of these and the eukaryotes all evolved separately and in 1990 emphasized this by promoting them to domains, which were renamed the Bacteria, Archaea, and Eucarya. This redefinition has generally been accepted by molecular biologists but criticized by some others, who maintain that he over-emphasized a few genetic differences and that both archaebacteria and eukaryotes probably developed from within the eubacteria.
Bacteria reproduce only asexually, not sexually. Specifically they reproduce by binary fission, or simple cell division. During this process, one cell divides into two daughter cells with the development of a transverse cell wall.
However, independent of sexual reproduction, genetic variations can occur within individual cells through recombinant events such as mutation (random genetic change within a cell's own genetic code). Similar to more complex organisms, bacteria also have mechanisms for exchanging genetic material. Although not equivalent to sexual reproduction, the end result is that a bacterium contains a combination of traits from two different parental cells. Three different modes of exchange have thus far been identified in bacteria:
Bacteria, having acquired DNA from any of these events, can then undergo fission and pass the recombined genome to new progeny cells. Many bacteria harbor plasmids that contain extrachromosomal DNA. Under favourable conditions, bacteria may form aggregates visible to the naked eye, such as bacterial mats.
Bacteria show a wide variety of different metabolisms. Heterotrophs depend on an organic source of carbon, while autotrophs are able to synthesize organic compounds from carbon dioxide and water. Autotrophs that obtain energy by oxidizing chemical compounds are called chemotroph s, and those that obtain their energy from light, via photosynthesis, are called phototroph s. There are many variations on this terminology such as chemoautotrophs and photosynthetic autotrophs and so on. In addition, bacteria are distinguished based on the source of reducing equivalents they are using. Those using inorganic compounds (e. g. water, hydrogen, sulfide or ammonia) for this purpose are called lithotroph s and others needing organic compounds (e. g. sugars or organic acids) and are called organotroph s. The metabolic modes of energy metabolism (phototrophy or chemotrophy), reducing equivalent sources (lithotrophy or organotrophy) and carbon sources (autotrophy or heterotrophy) can be combined differently in any single microorganism, and even shifting between different modes frequently occurs in many species.
The photolithoautotrophs include the cyanobacteria, which are some of the oldest organisms known from the fossil record and probably played an important role in creating the Earth's oxygen atmosphere. They apparently pioneered the use of water as (lithotrophic) electron source and were the first to use the photosynthetic water splitting apparatus. Other photosynthetic bacteria use different electron sources and therefore do not produce oxygen. These anoxygenic phototrophs comprise the green sulfur, green non-sulfur, purple sulfur, purple non-sulfur bacteria and heliobacteria .
Other nutritional requirements include nitrogen, sulfur, phosphorus, vitamins and metallic elements such as sodium, potassium, calcium, magnesium, manganese, iron, zinc, cobalt, copper and nickel for normal growth. For some species, additional trace elements such as selenium, tungsten, vanadium or boron are needed.
Based on their response to oxygen, most bacteria can be placed into one of three groups: Some bacteria can grow only in the presence of oxygen and are called aerobes; others can grow only in the absence of oxygen and are called anaerobes; and some can grow in the presence or absence of oxygen and are called facultative anaerobes. Bacteria that do not utilize oxygen for respiration but still grow in its presence are called aerotolerant . Bacteria also thrive in environments that are considered extreme for mankind. These organisms are called extremophiles. Some bacteria inhabit hot springs and are called thermophiles; others inhabit highly saltine lakes and are called halophiles; yet others inhabit acidic or alkaline environments and are called acidophiles and alkaliphiles, respectively; and still others inhabit alpine glaciers and are called psychrophiles.
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