Bacteria (Bacteria) are unicellular prokaryotic organisms (characterized by an absence of nucleus and organelles).

Most bacteria possess a cell wall carbohydrate, peptidoglycan. The bacteria most large measure over 2 mu.m and was considered until recently that the smallest measured 0.2 mu.m, but has recently discovered "ultramicrobacteries, including freshwater.
They have many forms: spherical (cocci), or elongated rods (bacilli), form more or less spiral. The study of bacteria is bacteriology, a branch of microbiology.

Bacteria are ubiquitous and are present in all habitat types encountered on Earth. They can be isolated from soil, freshwater, marine or brackish water, air, ocean depths, radioactive waste, the crust on the skin and the intestines of animals. There are approximately 40 million bacterial cells in a gram of soil and 1 million bacterial cells in a milliliter of fresh water. It is estimated that there would be (at a given time) four to six quintillion (4 × 1030 to 6 × 1030) between 4 and 6 trillion trillion trillion bacteria in the world, representing a large part of the world's biomass. However, many of these bacteria are not yet characterized as unculturable in the laboratory. The bacteria are of considerable importance in biogeochemical cycles as the carbon cycle and nitrogen fixation from the atmosphere.

In humans, it has been calculated in 1012 that bacteria colonize the skin, 1010 bacteria colonizing the mouth and 1014 live bacteria in the intestine, making it ten times more bacterial cells than human cells in the body human. Most of these bacteria are harmless or beneficial to the organism. However, there are many species of pathogens that cause many infectious diseases such as cholera, syphilis, plague, anthrax, tuberculosis. The most common fatal bacterial diseases are respiratory infections: tuberculosis alone killing about 2 million people per year, mostly in sub-Saharan Africa. Bacteria can cause respiratory or intestinal disorders while others may be responsible for wound infection. Bacterial infections can be treated with antibiotics, which often inhibit one of their vital functions (eg, penicillin inhibits the synthesis of cell wall).

The bacteria can be very useful to man in the process of wastewater treatment in food processing during the manufacture of yoghurt or cheese and industrial production of many chemicals.

Bacteria are microscopic, so they are visible with a microscope. Antony van Leeuwenhoek was the first to observe bacteria through a microscope of its production in 1668. He called them "animalcules" and published his observations in a series of letters he sent to the Royal Society.

The word "bacteria" appears for the first time with the German microbiologist Christian Gottfried Ehrenberg in 1828. This word derives from Greek βακτηριον, which means "stick".

In the nineteenth century, the work of Louis Pasteur revolutionized bacteriology. He showed in 1859 that fermentation is caused by micro-organisms and that this growth was not due to spontaneous generation. It also demonstrated the role of microorganisms as infectious agents. Pasteur also conceived of culture, processes of destruction of micro-organisms such as autoclaving and pasteurization.

The German physician Robert Koch and his colleagues devised techniques to culture bacteria on solid medium. Robert Koch was a pioneer in medical microbiology, he has worked on cholera, anthrax (anthrax) and tuberculosis. It clearly showed that bacteria could be the causative agent of infectious disease and he proposed a series of postulates (Koch's postulates, still used today) confirming the etiologic role of micro organism in a disease. He won the Nobel Prize in Physiology or Medicine in 1905.

Although we knew the nineteenth century that bacteria are the cause of many diseases, there was no disinfectant available. In 1910, Paul Ehrlich developed the first antibiotic, by changing dyes selectively stained Treponema pallidum, the spirochaete that causes syphilis-into compounds that kill the pathogen selectively. Ehrlich received the 1908 Nobel Prize for their work on immunology, and pioneered the use of dye to detect and identify bacteria, his work is the basis of Gram stain and Ziehl-Neelsen.

The microbiologist Martinus Beijerinck and Sergei Winogradsky initiated the first studies of environmental microbiology and microbial ecology by studying soil microbial communities and water and relationships between these micro-organisms.

If bacteria were known in the nineteenth century, there is still no antibacterial treatment. In 1909, Paul Ehrlich devised a treatment against syphilis before the use of penicillin in therapy suggested by Ernest Duchesne in 1897 and studied by Alexander Fleming in 1929.

In 1977, Carl Woese's work through molecular phylogeny divided the prokaryotes into two groups: the Bacteria and Archaea.

Cell Structure
As prokaryotic (no nucleus organization), the bacteria are relatively simple cells, characterized by an absence of nucleus and organelles such as mitochondria and chloroplasts, they are not over the endoplasmic reticulum or device golgi.

An important characteristic of bacteria is the cell wall. Bacteria can be divided into two groups (Gram positive and Gram negative) based on the difference in the structure and chemical composition of cell wall highlighted by the Gram stain. Bacteria Gram positive have a cell wall containing peptidoglycan (or murein) thick and teichoic acid bacteria while Gram negative peptidoglycan present an end located in the periplasm between the cytoplasmic membrane and outer cell membrane. The wall gives the bacterium its shape and protects against bursting under the effect of very high osmotic pressure of cytoplasm. The peptidoglycan provides the rigidity of the wall. However, there are bacteria without walls: it is the mycoplasma.

The bacteria were seen as simple bags of cytoplasm, but many levels of structural complexity have been discovered since, as the discovery of the prokaryotic cytoskeleton, and the localization of specific proteins in the bacterial cytoplasm. These subcellular compartments have been named "Hyperstructure bacteria (" bacterial hyperstructures "in English).

Bacteria possess a chromosome form a filament of DNA, the heredity. The bacterial chromosome is generally circular. In addition to this genomic DNA, bacterial cells often contain circular DNA molecules called extra-chromosomal plasmids. The cells also contain numerous ribosomes for protein synthesis through the mechanism of translation. The cytoplasm of prokaryotes often contains substances intracellular reserves are stocks of nutrients in the form of glycogen, starch or poly-b-hydroxybutyrate (PBH). Some species of aquatic bacteria possess gas vesicles that provide buoyancy cells. Other species, magnetotactic bacteria have the peculiarity of having a magnetosome.

Many bacteria possess extra-cellular structures such as flagella used for movement of cells, allowing the attachment fimbriae and pili sexual essential to the phenomenon of conjugation. Heterotrophic bacteria can use their flagella to move to areas rich in organic substances (nutrients) through the phenomenon known as chemotaxis.

Some bacteria can produce thin layers outside the cell wall, usually composed of polysaccharides (sugars). When the layer is compact, it is called capsule. The capsules are such a barrier protecting the cell against the external environment and also against phagocytosis. It also facilitates attachment to surfaces and biofilm formation. Klebsiella, Bacillus anthracis, Streptococcus pneumoniae are examples of encapsulated bacteria. When the layer is diffuse, it is called mucoid layer. When the layer is thicker, it is called glycocalyx. The glycocalyx allows bacteria to adhere to a substrate.
Some bacteria described as sheathed bacteria produce a dense outer layer and rigid sheath. This phenomenon is common among bacteria of the water forming channels filamentous (Sphaerotilus natans example). The sheath protects cells against the turbulence of the water. The bacteria group Cytophaga - Flavobacterium produce a mucous layer that allows them to remain in close contact with a solid medium. Other bacteria such as Spirillum can wrap a protein layer called the stratum S.

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