Bacteria
Bacteria are one-celled organisms that lack a nucleus defined by a nuclear membrane and the organelles that are associated with eukaryotic cells. Bacteria are prokaryotes that maintain their genetic material, DNA, in a long circular molecule. Bacteria also contain DNA in small circular molecules termed plasmids.
Bacteria (singular, bacterium) are organisms that are larger and more organized than viruses. Although the direct detection of bacteria requires the use of a microscope or specialized tests, bacteria are large in number and variety. Bacteria flourish in every possible habitat, as well as on and within the bodies of living things. Scientists utilize their knowledge of bacterial genetics and the techniques of bacterial genetic engineering to produce important medical and industrial substances, including human insulin, interferon, and other proteins.
With the invention of the powerful electron microscope, bacterial cells have been studied in great detail. They occur in three basic shapes, round cocci,, rod-like bacilli, and helical spirilla and spirochetes . Bacteria are prokaryotes, in which the genetic material is not enclosed in a definite nuclear membrane. Although bacteria lack most organelles found in eukaryotic cells, they do have a type of ribosome that operates during protein synthesis.
The genetic material in bacteria is deoxyribonucleic acid (DNA) formed in a circular arrangement. Additional genes in smaller rings called plasmids also exist within bacterial cells. In 1997, researchers at the University of Wisconsin-Madison determined the complete sequence of genes in Escherichia coli, an intestinal bacteria that has been extensively studied by biologists. E. coli's DNA contains about 4,300 known genes. The gene sequences of other bacteria are also currently under study.
Although simper than eukaryotes, bacteria are genetically sophisticated enough to be able to adapt to a wide variety of environments. Scientists have discovered subsurface populations of bacteria that exist as deep as 2.8 km (appx. 1.7 mi) below Earth's surface, and in locations with temperatures that reach 75 °C.
Although bacteria are often mistakenly associated only with disease, bacteria also perform useful functions. Bacteria are part of chemical cycles during which they release essential elements such as carbon and nitrogen for recycling. Bacteria decompose dead and decaying organic matter, assist animal digestion of food, and produce chemicals such as ethyl alcohol, butyl alcohol, acetic acid, and acetone. Bacteria also aid in the production of food products such as cheese, butter, sauerkraut, coffee, wine, and cocoa. Bacteria can be an integral part of the manufacture of silk, cotton, and rubber. Bacteria synthesize certain antibiotics. Bacteria that can digest and dissolve hydrocarbons are used to clean up oil spills.
Because of their size (diameters in the range of 1-5 um), bacteria were not classified or examined in detail until the nineteenth century. With the subsequent advent of better microscopes, many species of bacteria have been discovered.
As early as 1683, Antoni van Leeuwenhoek, a Dutch scientist, observed bacteria in human saliva using a single lens microscope. Leeuwenhoek termed these organisms "animalcules" (little animals) because, at that time, species were classified as either plants or animals. As a result, until the mid-nineteenth century scientists considered bacteria to be a type of animal life form. In 1857, a Swiss botanist named Karl von Naügeli suggested that bacteria have a class of their own within the plant kingdom. A century later, Otto Friedrich Muller, a Danish biologist, observed bacteria carefully and divided them into groups according to shape.
During the nineteenth century, the nature and function of bacteria was of considerable interest to scientists. French chemist and microbiologist, Louis Pasteur made extensive studies regarding the role of bacteria in fermentation. Pasteur originally set out to study why some of the best of France's wines, an important industrial product, were spoiling. When Pasteur observed wine under a microscope, he noticed that properly aged wine contained yeast cells that are necessary to the process of fermentation and the production of alcohol. Pasteur observed that in soured wines there was an increased number and rate of proliferation of bacteria. Pasteur suggested that heating the wine gently at about 120°F (49° C) killed the bacteria but not the yeast. In addition to his heroic service to the French wine industry, Pasteur identified two microbes that were killing silkworms, thereby saving the French silkworm industry from disaster. Pasteur also suggested that greater cleanliness was need to eliminate bacteria, and so inspired his student, Joseph Lister, to eventually develop the techniques of antiseptic surgery. A few years later the idea of using heat to kill microorganisms was applied to other perishable fluids, especially milk, via what is now termed the process of pasteurization.
Eventually, Pasteur turned his attention to the link between diseases and microorganisms. Pasteur became interested in developing techniques for culturing and examining various disease-causing bacteria such as the bacteria that cause cholera and anthrax and the production of vaccines that successfully prevented them.
Inspired by the work of Pasteur, Ferdinand Julius Cohn, a German botanist, became increasingly interested in bacteria and in 1872 published three volumes in which he created a classification system based on their structure. Cohn divided bacteria into four classes, Sphaerobacteria, Microbacteria, Desmobacteria and Spirobacteria, and provided a basis for bacterial classification. He is credited with coining the term bacterium (from the Latin, meaning "little rod"). Cohn was the first to demonstrate that the external form of bacteria was a fixed characteristic: for example, rod-shaped bacilli can never become round or spiral. Cohn also discovered that some bacteria, such as Bacillussubtilis, form resistant spores which make them difficult to kill with heat. Cohn's work was also instrumental in Robert Koch's discovery of the relationship between specific bacteria and specific diseases.
In 1884, a discovery by Danish bacteriologist Hans Christian Gram further advanced the study of bacteria. He developed a staining technique that distinguishes between two forms of bacteria, termed gram-negative and gram-positive that depends on the structure of the cell wall. This distinction is useful in identifying and classifying bacteria. It became more important years after its discovery when scientists realized that gram-negative bacteria, such as tuberculosis bacilli, resist taking up penicillin and other antibiotics in their cell walls, and thus resist treatment with certain classes of antibiotics.
Bacteria are enclosed by a cell wall that lacks the cellulose found in plants. Some bacteria have another protective layer or capsule outside the cell wall for protection and that is used to adhere to substrate. Certain bacteria also have structures called pilli on the outside that helps them fasten onto surfaces such as host membranes. Microscopy also shows that some bacteriamove by hair-like projections called flagella . Although bacteria are unicellular, some form aggregates of cells called colonies.
Bacteria reproduce by binary fission, and under unfavorable conditions some bacteria form resistant spores called endospores.
Some bacteria undergo cellular respiration (e.g., they use oxygen to help break down food for energy during the process of aerobic respiration. Other bacteria obtain energy by means of fermentation in anaerobic conditions (without oxygen). Bacteria obtain their food in various ways. Certain bacteria are phototrophic, similar to green plants in that they use light as an energy source to make food from carbon dioxide (CO2) and water. These bacteria produce oxygen as a part of the anaerobic reaction. Phototrophic bacteria use hydrogen sulfide gas instead of water, and release sulfur instead of oxygen. Phototrophic bacteria contain pigments that are necessary to carry out food making. Chemolithotrophs, on the other hand, are bacteria that do not get their energy from the Sun. Instead, they oxidize inorganic compounds such as hydrogen, ammonia, or sulfur to obtain energy for the food-making process. These bacteria live near cracks (fissures) in the Earth where molten rock, magma, heats the seawater that seeps in. The heated water dissolves minerals from the magma, before it flows out to the sea, and these elements serve as the raw material for chemolithotrophs.
Disease-causing bacteria are parasites since they benefit at the expense of the host. Referred to as pathogens, these harmful bacteria release toxins or poisons that interfere with some function of the host's body. Examples of more common infectious diseases caused by bacteria include strep throat, tuberculosis, typhoid fever and tooth decay. Bacteria also cause sexually transmitted diseases, such as syphilis and gonorrhea. Bacterial diseases are transmitted from host to host through various means, such as direct contact, food, water, animal bites, contaminated objects, and droplets in the air. The body's immune system is capable of overcoming some bacterial diseases, but the control of bacterial diseases requires preventing transmission, using vaccines, and administering antibiotics (medications that inhibit the synthesis of a functioning cell wall in bacteria, particularly in gram-positive species.
Many scientists argue that bacteria were the earliest life forms on Earth. Australian rocks that are 3.5 billion years old contain prokaryote microfossils.
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