Bacteria (Bacterium)

A bacterium (plural: bacteria) is either an individual one-cell organism or a type of organism belonging to one of the three major branches of life. Traditionally classified as one of the five kingdoms, bacteria are microscopic and relatively simple cells. They lack the nucleus and organelles of the more complex cells called "eukaryotes;" however, like the cells of plants, most possess a carbohydrate-based cell wall. In common speech, "bacteria" still refers also to archaeabacteria, although the latter recently have been classified as an independent branch or "domain" of life.

After the "archae," true bacteria are the oldest kind of organism on Earth, and they are also the most abundant. Bacteria exist in soil, water, and as parasites of other organisms. Species and strains of bacteria cause many if not most non-hereditary diseases. They are the target of the drugs known as antibiotics.

History
Bacteria are minute, with physical dimensions typically in the range of 0.5 to 5.0 micrometers. Because of this, they are not visible without a microscope. Antony van Leeuwenhoek was the first to report viewing bacteria, which he did in 1683 with the aid of a single-lens microscope of his own design.

The word bacterium was coined by the German microbiologist C.G. Ehrenberg in 1828. It derives from the Greek word, βακτηριον meaning "small stick". Louis Pasteur (1822-1895) and Robert Koch (1843-1910) described the role of bacteria as the conveyors and cause of disease (more at pathogen).

Reproduction
Bacteria may reproduce either asexually or sexually. Primarily they reproduce asexually, which they accomplish 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, genetic variations can occur within individual cells through recombinant events such as mutation (random genetic change within a cell's own genetic code), transformation (the transfer of naked DNA from one bacterial cell to another in solution), transduction (the transfer of viral, bacterial, or both bacterial and viral DNA from one cell to another via bacteriophage) and bacterial conjugation (the transfer of DNA from one bacterial cell to another via a special protein structure called a conjugation pilus). 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.

Metabolisms
Bacteria show a wide variety of different metabolisms. Some bacteria require only carbon dioxide for their carbon source and are called autotrophs. Those that obtain their energy in the form of light, via photosynthesis, are called photoautotrophs. Those that obtain energy by oxidizing chemical compounds are called chemoautotrophs. Another group of bacteria is dependent on an organic form of carbon and they are called heterotrophs. 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 lithotrophs, the others need organic compounds (e. g. sugars or organic acids) and are called organotrophs. 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 invented 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, phosphorous, vitamins and metallic elements such as sodium, potassium, calcium, magnesium, manganese, iron, zinc, and cobalt for normal growth.

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 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, repectively; and still others inhabit alpine glaciers and are called psychrophiles.

Movement
Motile bacteria can move about, either using flagella, bacterial gliding, or changes of buoyancy. A unique group of bacteria, the spirochaetes, have structures similar to flagella, called axial filaments, between two membranes in the periplasmic space. They have a distinctive helical body which twists about as it moves.

Bacterial flagella are arranged in many different ways. Bacteria can have a single polar flagellum at one end of a cell, or they can have clusters of many flagella at one end. Peritrichous bacteria have flagella scattered all over the cell.

Motile bacteria are attracted or repelled by certain stimuli, behaviors called taxes - for instance, chemotaxis, phototaxis, and mechanotaxis. In one peculiar group, the myxobacteria, individual bacteria attract to form swarms and may differentiate to form fruiting bodies.

Taxonomy
The classification of bacteria has changed radically to reflect thoughts about phylogeny, and many groups and even species undergo frequent alteration or renaming. However, this places bacteriology in an ideal position to exploit recent advances in gene sequencing, genomics, bioinformatics and computational biology.

Originally the bacteria were considered a group of fungi, except the cyanobacteria, which were not considered bacteria at all but rather blue-green algae. The discovery of their common prokaryotic cell structure, as distinct from all other organisms (all of them eukaryotes), led to their treatment as a single and separate group, variously called Monera, Bacteria, and Prokaryota. It was generally believed that this was a grade, in that the eukaryotes arose from prokaryotes.

Looking at RNA, Woese found that the prokaryotes comprised two separate groups. These he called the Eubacteria and Archaebacteria, but they have since become renamed the Bacteria and Archaea, which is the usage followed here. Woese argued that these two groups, together with the eukaryotes, comprised separate domains which had originated separately from a primordial organism. Researchers have abandoned this model, but the three-domain system has gained general acceptance. In this case the Bacteria, so restricted, may be divided into several kingdoms, though in other systems they are treated as a single kingdom. They are generally considered a monophyletic group, though this has been disputed.

Groupings of bacteria

Bacteria are grouped in a number of different ways. Bacteria exist in a number of shapes. Most bacteria are of one of three shapes: The Bacillus is rod-shaped; the Coccus is spherical in shape (e.g. Streptococcus or Staphylococcus); and the Spirillum is spiral-shaped. An additional group, the Vibrio, is comma-shaped.

The structure of bacteria is very simple--that of a prokaryotic cell, which does not have membrane-bound organelles such as mitochondria and chloroplasts, but does have cell walls. On the basis of the composition of the cell walls, that is, the number and placement of cell membranes, bacteria are divided into two groups, gram positive and gram negative. The name gram comes from Hans Christian Gram, who developed the technique of gram staining.

The cell wall usually includes a second membrane surrounding the cell, but in a few groups this is absent, and instead the cell wall is composed mostly of glycoproteins. A few bacteria without the second membrane, however, lack the glycoproteins and show up as gram-negative despite belonging to gram-positive groups.

Some bacterial cells have capsules outside their cell walls, which are made up of polysaccharides, and form a covering or envelope around the cell. These capsules help the bacteria to remain dormant during dry seasons and to store food and dispose of waste substances.

Many bacteria move from one place to another with the help of thin, hair-like structures called flagella.

Benefits and dangers
Bacteria are both harmful and useful to the environment, humans, and animals. Some bacteria act as pathogens and cause tetanus, typhoid fever, pneumonia, syphilis, cholera, influenza, and tuberculosis. In plants, bacteria cause leaf spot, fire blight, and wilts. The mode of infection includes contact, air, food, water, and insect-borne microorganisms. Pathogens may be treated by antibiotics, which can be classified as bacteriocidal and bacteriostatic, which at concentrations that can be reached in bodily fluids either kill bacteria or hamper their growth, respectively.

In soil, microorganisms help in the transformation of nitrogen to ammonia with enzymes secreted by these microbes, which reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking). Some bacteria are able to use molecular nitrogen as their source of nitrogen, converting it to nitrogenous compounds, a process known as nitrogen fixation. Many other bacteria are found as symbionts in humans and other organisms. For example, their presence in the large intestine can help prevent the growth of potentially harmful microbes.

The ability of bacteria to degrade a variety of organic compounds is remarkable. Highly specialized groups of microorganisms play important roles in the mineralization of specific classes of organic compounds. For example, the decomposition of cellulose, which is one of the most abundant constituents of plant tissues, is mainly brought about by aerobic bacteria that belong to the genus Cytophaga.

Bacteria, often in combination with yeasts and molds, are used in the preparation of fermented foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yoghurt. Using biotechnology techniques, bacteria can be bioengineered for the production of medical compounds, like insulin, or for the bioremediation of toxic wastes.

Miscellaneous
In terms of evolution, bacteria are thought to be very old organisms, appearing about 3.7 billion years ago.

Two organelles, mitochondria and chloroplasts, are generally believed to have been derived from endosymbiotic bacteria.

Microorganisms are widely distributed and are most abundant where they have food, moisture, and the right temperature for their multiplication and growth. Bacteria can be carried by air currents from one place to another. The human body is home to billions of microorganisms; they can be found on skin surfaces, in the intestinal tract, in the mouth, nose, and other body openings. They are in the air one breathes, the water one drinks, and the food one eats.

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