Archaebacteria, simple organisms that resemble ordinary bacteria in that they lack a well-formed nucleus and can therefore be characterized as procaryotes in the classification of living organisms. Their biochemistry differs in important ways from that of other bacteria, however, and some biologists place them in a kingdom of their own. According to these theories, archaebacteria may be ancestral to the main cellular body of eucaryotes, or organisms with well-formed cell nuclei, whereas ordinary bacteria are generally thought to be ancestral to the mitochondria and chloroplasts within eucaryotic cells.(Greek bakterion, "little staff"), large group of mostly microscopic, unicellular organisms that lack a distinct nucleus and that usually reproduce by cell division.
Bacteria are tiny, most ranging from 1 to 10 micrometers (1 micrometer equals 1/25,000 in), and are extremely variable in the ways they obtain energy and nourishment. They can be found in nearly all environments-from air, soil, water, and ice to hot springs; even the hydrothermal vents on the deep ocean floor are the home of sulfur-metabolizing bacteria (see MARINE LIFE). Certain types are found in nearly all food products, and bacteria also occur in various forms of symbiosis with most plants and animals and other kinds of life.the currently used five-kingdom scheme of classification, bacteria constitute the kingdom Monera, also known as Procaryotae-organisms in whose cells the nucleus is not enclosed by a membrane (see CELL). About 1600 species are known. Generally, bacteria are classified into species on the basis of characteristics such as shape-cocci (spheres), bacilli (rods), spirochetes (spirals); cell-wall structure; differential staining GRAM'S STAIN; ability to grow in the presence or absence of air (aerobes and anaerobes, respectively); metabolic or fermentative capabilities; ability to form dormant spores under adverse conditions SPORE; serologic identification of surface components; and nucleic-acid relatedness.
The most widely used reference for taxonomic classification of bacteria divides them into four major groups based on cell-wall characteristics. The division Gracilicutes encompasses bacteria with thin, gram-negative-type cell walls; the Firmicutes have thick, gram-positive cell walls; the Tenericutes lack cell walls; and the Mendosicutes have unusual cell walls made of material other than typical bacterial peptidoglycan. Among the Mendosicutes are the archaebacteria, a group of unusual organisms that includes methanogens, strict anaerobes that produce methane from carbon dioxide and hydrogen; halobacteria, which grow at high salt concentrations; and thermoacidophiles, which are sulfur-dependent extreme thermophiles. It has been argued that the archaebacteria should be classified into a separate kingdom because recent biochemical studies have shown that they are as different from other bacteria as they are from eucaryotes (the nucleii of which are membrane-bound). The four major bacterial divisions are further subdivided into about 30 numbered sec tions, some of which are divided into orders, families, and genera. Section 1, for example, is made up of spirochetes-long, corkscrew-shaped bacteria with gram-negative cell walls and internal (between the cell wall and cell membrane) filamentous flagella that provide the organisms with motility (ability to move). Treponema pallidum, causing syphilis, is a spirochete, a member of the order Spirochaetales, and the family Spirochaetaceae.
Not all bacteria can move, but the mobile ones are generally propelled by screwlike appendages-flagella-that may project from all over the cell or from one or both ends, singly or in tufts. Depending on the direction in which the flagella rotate, the bacteria either move forward or tumble in place. The duration of runs versus tumbling is linked to receptors in the bacterial membrane; variations enable the bacteria to move toward attractants such as food sources and away from unfavorable environmental conditions. In some aquatic bacteria that contain iron-rich particles, locomotion has been found to be oriented to the earth's magnetic field.
The genetic material of the bacterial cell is in the form of a circular double strand of DNA (see NUCLEIC ACIDS). Many bacteria also carry smaller circular DNAs called plasmids, which encode genetic information but are generally not essential for reproduction. Many of these plasmids can be transferred to other bacteria by conjugation, a mechanism of genetic exchange. Other mechanisms whereby bacteria can exchange genetic information include transduction, in which bacterial viruses (see BACTERIOPHAGE) transfer DNA, and transformation, in which DNA is taken into the bacterial cell directly from the environment. Bacterial cells multiply by binary fission; the genetic material is duplicated and the bacterium elongates, constricts near the middle, and then undergoes complete division, forming two daughter cells essentially identical to the parent cell. Thus, as with higher organisms, a given species of bacteria reproduces only cells of the same species. Some bacteria divide every 20 to 40 minutes. Under favorable conditions, with one division every 30 minutes, after 15 hours a single cell will have produced roughly 1 billion progeny. This mass, called a colony, may be seen with the naked eye. Under adverse conditions some bacteria may undergo a modified division process to produce spores, dormant forms of the cell that can withstand extremes of temperature and humidity.
Work of Bacteria
Two main groups of bacteria exist: the saprophytes, which live on dead animal and vegetable matter; and the symbionts, which live on or in living animal or vegetable matter. Saprophytes are important because they decompose dead animals and plants into their constituent elements, making them available as food for plants. Symbiotic bacteria are a normal part of many human tissues, including the alimentary canal and the skin, where they may be indispensable to physiological processes. Such a relationship is called mutualistic. Other symbionts gain nutrients from their living host without causing serious damage; this is commensalism. The third type, parasites, can destroy the plants and animals on which they live. See also PARASITE.
Bacteria are involved in the spoilage of meat, wine, vegetables, and milk and other dairy products. Bacterial action may render such foods unpalatable by changing their composition. Bacterial growth in foods can also lead to food poisoning such as that caused by Staphylococcus aureus or by Clostridium botulinum (see BOTULISM). On the other hand, bacteria are of great importance in many industries. The fermentative capabilities of various species are manipulated for the production of cheese, yogurt, pickles, and sauerkraut. Bacteria are also important in the production of tanned leather, tobacco, ensilage, textiles, pharmaceuticals and various enzymes, polysaccharides, and detergents.
Bacteria are found in virtually all environments, where they contribute to various biological processes. For example, they may produce light, such as the phosphorescence of dead fish; and they may produce enough heat to induce spontaneous combustion in haystacks or in hop granaries. By decomposing cellulose, certain anaerobic forms evolve marsh gas in stagnant pools; by oxidizing processes, other bacteria assist in forming deposits of bog iron ore, ocher, and manganese ore. See BIOLUMINESCENCE.
Bacteria have an immense influence on the nature and composition of the soil. One result of their activities is the complete disintegration of organic remains of plants and animals and of inorganic rock particles. This action produces in the aggregate vast quantities of plant food. In addition, the leguminous plants that enrich soil by increasing its nitrogen content do so with the help of Rhizobium radicicola and other bacteria that infect the roots of the plants and cause nitrogen-fixing nodules to grow (see NITROGEN FIXATION). The photosynthetic process on which plant life itself is based was almost certainly first established in bacteria; the recent discovery of an unusual photosynthesizing bacterium called Heliobacterium chlorum may help in understanding this fundamental development in the history of life.
About 200 species of bacteria are pathogenic, or disease causing, for humans. Pathogenicity varies widely among various species and is dependent on both the virulence of the particular species and the condition of the host organism. Among the more invasive bacteria responsible for human disease are those that cause cholera, lockjaw, gas gangrene, leprosy, plague, bacillary dysentery, tuberculosis, syphilis, typhoid fever, diphtheria, undulant fever, and several forms of pneumonia. Until the discovery of viruses, bacteria were considered the causative agents of all infectious diseases.
The pathogenic effects of bacteria on body tissues may be grouped in four classes as follows: (1) effects of the direct local action of the bacteria on the tissues, as in gas gangrene, caused by Clostridium perfringens; (2) mechanical effects, as when a mass of bacteria blocks a blood vessel, causing an infectious embolus; (3) effects of the body's response to certain bacterial infections on body tissues, as in the forming of lung cavities in tuberculosis, or destruction of heart tissue by the body's own antibodies in rheumatic fever; (4) effects of bacterial-produced toxins , chemical substances that act as poisons to certain tissues. Toxins are generally species specific; for example, the toxin responsible for diphtheria is different from the one responsible for cholera.
Various microorganisms, including certain fungi and some bacteria, produce chemical substances that are toxic to specific bacteria. Such substances, which include penicillin and streptomycin, are known as antibiotics; they either kill the bacteria or prevent them from growing or reproducing. In recent years antibiotics have played an increasingly important role in medicine in the control of bacterial diseases. , Oswald Theodore (1877-1955), Canadian-born American physician and bacteriologist, who is best known for his discoveries in the field of genetics. He was born in Halifax, Nova Scotia, and earned his medical degree at Columbia University's College of Physicians and Surgeons. Avery was the first to show that the agent responsible for transferring genetic information was not a protein, as biochemists of his time believed, but the nucleic acid deoxyribonucleic acid, or DNA (see NUCLEIC ACIDS). Avery and his coworkers extracted a substance from a type of bacterium with a smooth surface and introduced the substance into a rough-surfaced type of bacterium. When the rough-surfaced bacteria transformed into the smooth-surfaced type, he knew the substance he had extracted contained the gene that coded for the smooth surface. Avery's team purified this substance and found it was pure DNA. Avery published the results of his research in 1944. The paper led to more intensive studies of DNA, which eventually revealed it to be the common agent of heredity, present in all animal cells., Ferdinand Julius (1828-98), German botanist and bacteriologist, born at Breslau (now Wroclaw, Poland), and educated at the universities of Breslau and Berlin. In 1859 he became professor of botany at Breslau, serving in that position until his death. Often called the founder of the science of bacteriology, Cohn studied microscopic organisms and demonstrated that bacteria are plants. He studied the morphology of algae and fungi and analyzed the bacterial causes of infectious plant and animal diseases. He discovered the nature and principal properties of bacterial spores, and he assisted the German physician and bacteriologist Robert Koch in the preparation of his famous treatise on anthrax. In 1872 Cohn published the first classification of bacteria based on morphology., Sir Alexander (1881-1955), British bacteriologist and Nobel laureate, best known for his discovery of penicillin. Born near Darvel, Scotland, and educated at Saint Mary's Hospital Medical School of the University of London, he served as professor of bacteriology at St. Mary's Hospital Medical School from 1928 to 1948, when he became professor emeritus.
Fleming conducted outstanding research in bacteriology, chemotherapy, and immunology. In 1922 he discovered lysozyme, an antiseptic found in tears, body secretions, albumen, and certain fish plants. His discovery of penicillin came about accidentally in 1928 in the course of research on influenza. His observation that the mold contaminating one of his culture plates had destroyed the bacteria laid the basis for the development of penicillin therapy (see ANTIBIOTIC).
Fleming was knighted in 1944. In 1945 he shared the Nobel Prize in physiology or medicine with the British scientists Howard Walter Florey and Ernst Boris Chain for their contributions to the development of penicillin.
, Robert (1843-1910), German scientist and Nobel laureate, who founded modern medical bacteriology, isolated several disease-causing bacteria, including those of tuberculosis, and discovered the animal vectors of a number of major diseases.
Born in Klausthal-Zellerfeld, on December 11, 1843, Koch enrolled at the University of Gottingen in 1862, where he studied botany, physics, and mathematics and began his lifelong medical career. After a brief tenure at the Hamburg General Hospital and at an institute for retarded children, he started private practice. His professional activities did not deter him from developing outside interests in archaeology, anthropology, occupational diseases such as lead poisoning, and the newly emerging field of bacteriology.
Koch's first major breakthrough in bacteriology occurred in the 1870s, when he demonstrated that the infectious disease anthrax developed in mice only when the disease-bearing material injected into a mouse's bloodstream contained viable rods or spores of Bacillus anthracis. Koch's isolation of the anthrax bacillus was of momentous import, because this was the first time that the causative agent of an infectious disease had been demonstrated beyond a reasonable doubt. It now became clear that infectious diseases were not caused by mysterious substances but instead by specific microorganisms-in this case, bacteria. Koch also showed how the investigator must work with such microorganisms, how to obtain them from infected animals, how to cultivate them artificially, and how to destroy them. He revealed these observations to the great German pathologist Julius Friedrich Cohnheim and his associates, one of whom was the German bacteriologist Paul Ehrlich, the founder of modern immunology.
In 1880, after completing important work on the bacteriology of wound infections, Koch was appointed government adviser with the Imperial Department of Health in Berlin, where he carried out most of his research for the rest of his career. In 1881 he launched his studies of tuberculosis, and the following year he announced that he had isolated a bacillus that was the causative agent of the dreaded disease. Koch's findings were confirmed by investigators around the world. The discovery led to an improvement in diagnosis by means of finding evidence of the bacilli in bodily excretions, especially sputum.
Koch now focused his attention on cholera, which had reached epidemic levels in India by 1883. Traveling there, he identified the bacillus that caused the disease and found that the bacillus was transmitted to human beings primarily through water. Koch later traveled in Africa, where he studied the causes of insect-borne diseases.
In 1891 Koch became director of Berlin's Institute for Infectious Disorders (the institute now bears his name), which had been organized for specialized medical research, and remained there until he retired in 1904. In 1905 he won the Nobel Prize in physiology or medicine. On May 27, 1910, Koch died at the German health resort of Baden-Baden.
Lederberg, Joshua (1925- ), American geneticist and Nobel laureate, born in Montclair, New Jersey. He received a Ph.D. degree from Yale University in 1947 and joined the faculty of the Stanford University School of Medicine in 1959. While engaged in research at Yale, he discovered (1947) that bacteria have an elementary sex life; that is, they reproduce by conjugation, the mutual exchange of genes between sexually undifferentiated one-celled organisms. This discovery considerably expanded the possibilities of genetic research. Considered even more important was Lederberg's later discovery that some viruses carry hereditary materials from one bacterial cell to another and thereby change the heredity of their hosts. For these discoveries, Lederberg was a cowinner of the 1958 Nobel Prize in physiology or medicine. From 1978 until 1990 he was president of Rockefeller University in New York., Louis (1822-95), world-renowned French chemist and biologist, who founded the science of microbiology, proved the germ theory of disease, invented the process of pasteurization, and developed vaccines for several diseases, including rabies.
Pasteur was born in Dole on December 7, 1822, the son of a tanner, and grew up in the small town of Arbois. In 1847 he earned a doctorate at the Ecole Normale in Paris, with a focus on both physics and chemistry. Becoming an assistant to one of his teachers, he began research that led to a significant discovery. He found that a beam of polarized light (see OPTICS) was rotated to either the right or the left as it passed through a pure solution of naturally produced organic nutrients, whereas when polarized light was passed through a solution of artificially synthesized organic nutrients, no rotation took place. If, however, bacteria or other microorganisms were placed in the latter solution, after a while it would also rotate light to the right or left.
Pasteur concluded that organic molecules can exist in one of two forms, called isomers (that is, having the same structure and differing only in mirror images of each other), which he referred to as "left-handed" and "right-handed" forms. When chemists synthesize an organic compound, both of these forms are produced in equal proportions, canceling each other's optical effects. Living systems, however, which have a high degree of chemical specificity, can discriminate between the two forms, metabolizing one and leaving the other untouched and free to rotate light.
Work on Fermentation
After spending several years of research and teaching at Dijon and Strasbourg, Pasteur moved in 1854 to the University of Lille, where he was named professor of chemistry and dean of the faculty of sciences. This faculty had been set up partly to serve as a means of applying science to the practical problems of the industries of the region, especially the manufacture of alcoholic beverages. Pasteur immediately devoted himself to research on the process of fermentation. Although his belief that yeast plays some kind of role in this process was not original, he was able to demonstrate, from his earlier work on chemical specificity, that the desired production of alcohol in fermentation is indeed due to yeast and that the undesired production of substances (such as lactic acid or acetic acid) that make wine sour is due to the presence of additional organisms such as bacteria. The souring of wine and beer had been a major economic problem in France; Pasteur contributed to solving the problem by showing that bacteria can be eliminated by heating the starting sugar solutions to a high temperature.
Pasteur extended these studies to such other problems as the souring of milk, and he proposed a similar solution: heating the milk to a high temperature and pressure before bottling. This process is now called pasteurization.
Disproof of Spontaneous Generation
Fully aware of the presence of microorganisms in nature, Pasteur undertook several experiments designed to address the question of where these "germs" came from. Were they spontaneously produced in substances themselves, or were they introduced into substances from the environment? Pasteur concluded that the latter was always the case. His findings resulted in a fierce debate with the French biologist Felix Pouchet-and later with the noted English bacteriologist Henry Bastion-who maintained that under appropriate conditions instances of spontaneous generation could be found. These debates, which lasted well into the 1870s, although a commission of the Academie des Sciences officially accepted Pasteur's results in 1864, gave great impetus to improving experimental techniques in microbiology.
In 1865, Pasteur was summoned from Paris, where he had become administrator and director of scientific studies at the Ecole Normale, to come to the aid of the silk industry in southern France. The country's enormous production of silk had suddenly been curtailed because a disease of silkworms, known as pebrine, had reached epidemic proportions. Suspecting that certain microscopic objects found in the diseased silkworms (and in the moths and their eggs) were disease-producing organisms, Pasteur experimented with controlled breeding and proved that pebrine was not only contagious but also hereditary. He concluded that only in diseased and living eggs was the cause of the disease maintained; therefore, selection of disease-free eggs was the solution. By adopting this method of selection, the silk industry was saved from disaster.
Germ Theory of Disease
Pasteur's work on fermentation and spontaneous generation had considerable implications for medicine, because he believed that the origin and development of disease are analogous to the origin and process of fermentation. That is, disease arises from germs attacking the body from outside, just as unwanted microorganisms invade milk and cause fermentation. This concept, called the germ theory of disease, was strongly debated by physicians and scientists around the world. One of the main arguments against it was the contention that the role germs played during the course of disease was secondary and unimportant; the notion that tiny organisms could kill vastly larger ones seemed ridiculous to many people. Pasteur's studies convinced him that he was right, however, and in the course of his career he extended the germ theory to explain the causes of many diseases.
Pasteur also determined the natural history of anthrax, a fatal disease of cattle. He proved that anthrax is caused by a particular bacillus and suggested that animals could be given anthrax in a mild form by vaccinating them with attenuated (weakened) bacilli, thus providing immunity from potentially fatal attacks. In order to prove his theory, Pasteur began by inoculating 25 sheep; a few days later he inoculated these and 25 more sheep with an especially strong inoculant, and he left 10 sheep untreated. He predicted that the second 25 sheep would all perish and concluded the experiment dramatically by showing, to a skeptical crowd, the carcasses of the 25 sheep lying side by side.
Pasteur spent the rest of his life working on the causes of various diseases-including septicemia, cholera, diphtheria, fowl cholera, tuberculosis, and smallpox-and their prevention by means of vaccination. He is best known for his investigations concerning the prevention of rabies, otherwise known in humans as hydrophobia. After experimenting with the saliva of animals suffering from this disease, Pasteur concluded that the disease rests in the nerve centers of the body; when an extract from the spinal column of a rabid dog was injected into the bodies of healthy animals, symptoms of rabies were produced. By studying the tissues of infected animals, particularly rabbits, Pasteur was able to develop an attenuated form of the virus that could be used for inoculation.
In 1885, a young boy and his mother arrived at Pasteur's laboratory; the boy had been bitten badly by a rabid dog, and Pasteur was urged to treat him with his new method. At the end of the treatment, which lasted ten days, the boy was being inoculated with the most potent rabies virus known; he recovered and remained healthy. Since that time, thousands of people have been saved from rabies by this treatment.
Pasteur's research on rabies resulted, in 1888, in the founding of a special institute in Paris for the treatment of the disease. This became known as the Institut Pasteur, and it was directed by Pasteur himself until he died. (The institute still flourishes and is one of the most important centers in the world for the study of infectious diseases and other subjects related to microorganisms, including molecular genetics.) By the time of his death in Saint-Cloud on September 28, 1895, Pasteur had long since become a national hero and had been honored in many ways. He was given a state funeral at the Cathedral of Notre Dame, and his body was placed in a permanent crypt in his institute.
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