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Bacteria are microscopic unicellular organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. They were once considered part of the plant kingdom, but eventually they were placed in a separate kingdom, Monera, also referred to as Eubacteria in recent taxonomic schemes. Monera includes bacteria and cyanobacteria (blue-green algae). The term "bacterium" (plural bacteria) was introduced by German scientist C.G. Ehrenberg in 1828 as a representative name for some bacterial types. The name bacteria comes from the Greek word, bakterion meaning "small stick". In 1878, French surgeon Charles Emmanuel Sedillot coined the term "microbe", which is also used to describe a bacterial cell. Since bacteria are unicellular microscopic organisms, they are not visible with the naked eye and require the use of a microscope to be seen. In 1683, Antony van Leeuwenhoek was the first to report viewing bacteria with the aid of a single ball lens microscope of his design, at a magnification of about 200 times actual size. Louis Pasteur (1822-1895) and Robert Koch (1843-1910) described the role of bacteria in causing disease, and in 1866, E.H. Haeckel, a German zoologist, suggested the name "Protista" to include all unicellular organisms (bacteria, algae, fungi, and protozoa). 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 milliards 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. Not all bacteria are harmful, and in some cases, their presence is a necessity for the human body-for example, their presence in the large intestine can help prevent the growth of potentially harmful microbes. Bacteria are minute, with physical dimensions typically in the range of 0.5 to 5.0 micrometers (one micrometer is about 1/25,400 inch).
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 the Danish biologist who developed the technique of gram staining.) Some bacterial cells have capsules outside their cell walls, which are made up of polysacharides, 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. Bacteria move from one place to another with the help of thin, hair-like structures called flagella. Bacteria that possess flagella are categorized as motile; those without flagella are categorized as immotile. 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. Some bacteria have peritrichous flagella scattered all over the cell. A unique group of bacteria, the spirochaetes, have structures similar to flagella, called axial filaments, between two membranes in the periplasmic space. The growth of bacterial populations has four different phases: lag phase, exponential or log phase, stationary phase, and death phase. During lag phase, bacteria adapt themselves to growth conditions. At exponential phase, bacteria are reproducing at their maximum rate; therefore, their number increases during this phase. During stationary phase, the growth rate slows due to depletion of nutrients. At death phase, bacteria run out of nutrients and die.
Bacteria reproduce both asexually and by genetic recombination. The primary means of reproduction in bacteria is binary fission, an asexual process. In binary fission, one bacterial 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 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. In terms of evolution, bacteria are thought to be very old organisms, appearing about 3.7 milliard years ago.
The nutritional requirements of bacteria are quite diverse. Some bacteria require only carbon dioxide for their carbon source and are called autotrophs; those that obtain their energy in the form of light are called photoautotrophs; and 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. 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.
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. 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. 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 group 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.