Bacterium
| Bacteria | ||
|---|---|---|
| Scientific classification | ||
| ||
| Phyla/Divisions | ||
|
Aquificae
Thermotogae Thermodesulfobacteria Deinococci Chrysogenetes Chloroflexi Thermomicrobia Nitrospira Deferribacteres Cyanobacteria Proteobacteria Firmicutes Chlorobi Actinobacteria Planctomycetes Chlamydiae Spirochaetes Fibrobacteres Acidobacteria Bacterioidetes Fusobacteria Verrucomicrobia Dictyoglomi | ||
| * or kingdom, see text |
A bacterium (plural form: bacteria) is a certain type of single celled micro-organism characterized by the lack of a membrane-bound nucleus and membrane-bound organelles, the other type being the archaea. Bacteria are among the oldest and most numerous living things and are found in the soil, water, and inside and outside most multicellular organisms.
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). Their study, bacteriology, is part of microbiology.
| Table of contents |
|
2 History 3 Reproduction 4 Metabolisms 5 Movement 6 Taxonomy 7 Benefits and dangers 8 Miscellaneous 9 Further reading 10 Reference |
The term bacterium was introduced by German scientist C.G. Ehrenberg in 1828 as a representative name for some bacterial types. The word comes from the Greek word, βακτηριον meaning "small stick".
In 1866, E.H. Haeckel, a German zoologist, suggested the name Protista to include all unicellular organisms (bacteria, algae, fungi, and protozoa).
In 1878, French surgeon Charles Emmanuel Sedillot coined the term microbe which is used to describe a bacterial cell or, more generally, any micro-organism.
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-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.
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 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.
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.
The photoautotrophs include the cyanobacteria, which are some of the oldest known organisms and probably played an important role in creating the Earth's oxygen atmosphere. Other photosynthetic bacteria undergo different processes which do not produce oxygen. These 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.
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.
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 domainss 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 kingdomss, though in other systems they are treated as a single kingdom. They are generally considered a monophyletic group, though this has been disputed.
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.
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 yogurt. Using biotechnology techniques, bacteria can be bioengineered for the production of medical compounds, like insulin, or for the bioremediation of toxic wastes.
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.
See Also: Bacterial growth, bacteriocin
Etymology
History
Reproduction
Metabolisms
Movement
Taxonomy
Groupings of bacteria
A. Rod-shaped
B. Round-shaped or spherical.
C. Round-shaped in clusters.
D. Round-shaped in twos.
E. Spiral-shaped.
F. Comma-shaped.
Benefits and dangers
Miscellaneous
Further reading
Reference
"Bacteria" is also the fictional name of a warring nation under Benzino Napaloni as dictator, in the 1940 film The Great Dictator.
