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Bacteria (singular: bacterium) are microscopic, unicellular organisms. They are often coccus- (spherical) or rod-shaped and 0.5-5 µm in the longest dimension, although the wide diversity of bacterial diversity can display a huge variety of morphologies. The study of bacteria is known as bacteriology, a branch of microbiology. Bacteria are ubiquitous in the environment, living in every possible habitat (including, but by no means limited to, soil, underwater, deep in the earth's crust, and even such environments as sulfuric acid and nuclear waste) on the planet. There are typically ten billion bacterial cells in a gram of soil, and one hundred thousand bacterial cells in a millilitre of sea water. Bacteria play an important role in the cycling of nutrients in the environment, and many important steps in the nutrient cycle are catalysed exclusively by bacteria, such as the fixation of nitrogen from the atmosphere. There are more bacterial cells on each of our bodies than there are our cells of our own and bacteria are a natural component of the human body, particularly on the skin and in the mouth and intestinal tract. Bacteria are important to human health, as they are the causative agent of many infectious diseases, including cholera and tuberculosis. Historically, bacteria have been responsible for such diseases as bubonic plague and leprosy, but after the discovery of antibiotics many bacterial diseases are able to be controlled. Bacteria are also important to numerous industrial processes, such as wastewater treatment and more recently the industrial production of antibiotics and other chemicals. The term "bacteria" has traditionally been generally applied to all microscopic, single-celled prokaryotes. Although this term remains in everyday use, the scientific nomenclature changed after the discovery that prokaryotic life actually consists of two very different lines of evolution (see three-domain system). Originally called Eubacteria and Archaebacteria, these evolutionary domains are now called Bacteria and Archaea.
History The first bacteria were observed by Anton van Leeuwenhoek in 1674 using a single-lens microscope of his own design. The name bacterium was introduced much later, by Ehrenberg in 1828, derived from the Greek word βακτηριον meaning "small stick". Because of the difficulty in describing individual bacteria and the importance of their discovery to fields such as medicine, biochemistry, and geochemistry, the history of bacteriology is generally described as the history of microbiology. Cell Structure Cell Morphology and Arrangement Bacteria display a wide diversity of shapes and sizes, called morphologies. Bacterial cells are typically 0.5-5 μm in length, however some species, for example Thiomargarita namibiensis and Epulopiscium fishelsoni, may be up to 500 µm (0.5 mm) long and are visible to the unaided eye. Among the smallest bacteria are members of the genus Mycoplasma which measure just 0.2 µm; approximately the same size as the largest viruses. Despite this diversity, each bacterial species tends to display a characteristic morphology. Most bacteria are either spherical, called coccus (pl. cocci, from Greek kókkos, grain, seed) or rod-shaped, called bacillus (pl. baccili, from Latin baculus, stick). Some rod-shaped bacteria, called vibrio, are slightly curved or comma-shaped, while others, called spirilla, form twisted spirals. Many bacterial species exist simply as single cells, while other tend to associate in diploids (pairs), characteristic for example Neisseria, or chains, such as Streptococcus, while members of the genus Staphylococcus, form a characteristic "bunch of grapes" clusters. Bacteria can also be elongated to form filaments, for example the Actinomycetes. Filamentous bacteria are often surrounded by a sheath which contains many individual cells, and certain species, such as the genus Nocardia, form complex, branched filaments, similar in appearance to fungal mycelia. Despite their apparent simplicity bacteria can also form more complex associations. Bacteria also attach to surfaces and form dense aggregations or biofilms. Bacteria living in biofilms can display a complex arrangement cells, forming secondary structures such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients. In natural environments a majority of bacteria are found associated with surfaces in biofilms. Under nutrient starvation, Myxobacteria aggregate and form multicellular fruiting bodies which can be up to 500 µm long and contain in the order of 100,000 cells. Cells in these fruiting bodies differentiate into a dormant state to become myxospores, which are more resistant to desiccation and other adverse environmental conditions than the ordinary cells. Certain bacteria form close spatial associations that are essential for their survival. One such association, called interspecies hydrogen transfer, occurs between clusters of anaerobic bacteria that consume organic acids and produce hydrogen, and methanogenic Archaea that consume hydrogen. These bacteria are unable to consume the organic acids and grow when all but the smallest concentration of hydrogen is present in their surroundings. Only the intimate spatial association with the hydrogen-consuming Archaea can keep the hydrogen concentration low enough to enable them to grow. In some instances bacteria have become so closely associated with another cell that they actually inside another cell. Such intracellular symbioses are the origin of the mitochondria and chloroplasts, which are descended from bacteria that entered into symbiotic associations with eukaryotic cells early in the evolution of life. Chlamydia are a phylum of bacteria that have evolved such that they can grow and reproduce only as symbionts in the cells of other organisms. Cellular structure The bacterial cell is bound by a lipid membrane, or plasma membrane, which encompasses the contents of the cell, or cytoplasm, and acts as a barrier that holds nutrients, proteins and other essential molecules within the cell. Bacteria do not have membrane-bound organelles in the cytoplasm and thus contain few intracellular structures (see prokaryotes). They lack mitochondria, chloroplasts and the other organelles present in eukaryotic cells, such as the golgi apparatus and endoplasmic reticulum. Many important biochemical reactions, such as energy generation, occur due to concentration gradients of certain molecules across membranes that create a potential difference, analogous to a battery. The absence of internal membranes in bacteria means that these reactions, such as electron transport occur across the plasma membrane, between the cytoplasm and the periplasmic space. Bacteria do not have a membrane-bound nucleus and their genetic material is typically a single chromosome located in the cytoplasm as an irregularly-shaped body called the nucleoid. The nucleoid consists mainly of the chromosome but has also associated proteins and RNA. Like all living organisms bacteria contain ribosomes for the production of proteins, but the structure of the ribosome is uniquely different from Eukarya and Archaea. The order Planctomycetes are an exception to the general absence of internal membranes in bacteria. This lineage of bacteria have a membrane bound nucleoid and other membrane-bound structures that give them unique capabilities. External to the cell membrane is the bacterial cell wall. Bacterial cell walls are composed of peptidoglycan (called Murein in older sources), are are thus different from the cell walls of plants and fungi which have cell walls of cellulose and chitin, respectively. The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of bacteria; the antibiotic penicillin is able to effectively kill bacteria by inhibiting a step in the synthesis of peptidoglycan and stopping the production of the cell wall. There are broadly speaking two different arrangements of the cell wall in bacteria, called Gram positive and Gram negative. The names originate from the reaction of cells to the Gram stain, a test long employed for the laboratory classification of bacterial species. Gram positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram negative bacteria have a relatively thin cell wall consisting of a a few layers of peptidoglycan surrounded by an outer lipid membrane containing lipopolysaccharides and lipoproteins. Most bacteria have the Gram negative cell wall; only organisms from the Phyla Firmicutes and Actinobacteria (previously know as the or low G+C and high G+C Gram positive bacteria, respectively). Flagella are rigid protein structures, about 20 nm in diameter and up to 20 µm in length, that are used for motility (see Motility, below). Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum (monotrichous), a flagellum at each end (amphitrichous), clusters of flagella at the poles of the cell (lophotrichous), while others have flagella distributed over the entire surface of the cell (peritrichous). Fimbriae are fine appendages of protein, just 2-10 nm in diameter and up to several micrometers in length. They are distributed over the surface of the cell and resemble fine hairs when the cells are visualised under the electron microscope. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells. Pili (sing. pilus) are cellular appendages, slightly larger than fimbriae, that enable the transfer of genetic material between bacterial cells, called conjugation (see Bacterial Genetics, below). Capsules or slime layers are produced by many bacteria to surround their cells and have differing degrees of structural complexity; from disorganised slime layers of extra-cellular polymer to the highly structured capsule or glycocalyx. These structures can protect cells from environmental conditions such as predation by eukaryotic cells, they can act as antigens and be involved in cell recognition, they can resit the infection of bacterial cells by viruses and they can also play a role in bacterial attachment to surfaces and biofilm formation. Certain genera of gram-positive bacteria bacteria, such as Bacillus and Clostridium, are capable of forming highly resistant dormant structures called endospores. Endospores have a distinct structure which is different from plant and fungal spores. Endospores can survive extreme environmental and chemical stresses. When we step on a rusty nail, for example, we receive an immunization shot for tetanus. This disease is caused by infection of the bacterium Clostridium tetani, which produce endospores which can survive in the environment on surfaces, like rusty nails, and cause infection to wounds when the skin surface is pierced and the endospores can enter and germinate. Some bacteria also produce nutrient storage granules, such as glycogen, polyphosphate, sulfur or polyhydroxyalkanoates. These storage compounds enable bacteria to store compounds for later use. Certain bacterial species, such as the photosynthetic Cyanobacteria, produce internal gas vesicles which they use to regulate their buoyancy to regulate the optimal light intensity or nutrient levels. Metabolism Main article: Microbial metabolism In contrast to higher organisms, bacteria exhibit an extremely wide variety of metabolic types. In fact, it is widely accepted that eukaryotic metabolism is largely a derivative of bacterial metabolism with mitochondria having descended from a lineage within the α-Proteobacteria and chloroplasts from the Cyanobacteria by ancient endosymbiotic events. Bacterial metabolism can be divided broadly on the basis of the kind of energy used for growth, electron donors and electron acceptors and by the source of carbon used. Most bacteria are heterotrophic; using organic carbon compounds as both carbon and energy sources. In aerobic organisms, oxygen is used as the terminal electron acceptor. In anaerobic organisms other inorganic compounds, such as nitrate, sulfate or carbon dioxide as terminal electron acceptors leading to the environmentally important processes of denitrification, sulfate reduction and acetogenesis, respectively. Non-respiratory anaerobes use fermentation to generate energy and reducing power, secreting metabolic by-products (such as ethanol in brewing) as waste. Facultative anaerobes can switch between fermentation and different terminal electron acceptors depending on the environmental conditions in which they find themselves. As an alternative to heterotrophy many bacteria are autotrophic, fixing carbon dioxide into cell mass. Energy metabolism of bacteria is either based on phototrophy or chemotrophy, i. e. the use of either light or exergonic chemical reactions for fueling life processes. Lithotrophic bacteria use inorganic electron donors for respiration (chemolithotrophs) or biosynthesis and carbon dioxide fixation (photolithotrophs), opposed by organotrophs which need organic compounds as electron donors for biosynthetic reactions (and mostly as well as carbon sources). Common inorganic electron donors are hydrogen, carbon monoxide, ammonia (leading to nitrification), ferrous iron, other reduced metal ions or even elemental iron and several reduced sulfur compounds. Additionally, methane metabolism, although formally counted as organotrophic, is actually more related to lithotrophic metabolic pathways. In both aerobic phototrophy and chemolithotrophy oxygen is used as a terminal electron acceptor, while under anaerobic conditions inorganic compounds (see above) are used instead. Most photolithotrophic and chemolithotrophic organisms are autotrophic, meaning that they obtain cellular carbon by fixation of carbon dioxide, whereas photoorganotrophic and chemoorganotrophic organisms are heterotrophic. In addition to carbon, some organisms also fix nitrogen gas (nitrogen fixation). This environmentally important trait can be found in bacteria of nearly all the metabolic types listed above but is not universal. The distribution of metabolic traits within a group of organisms has traditionally been used to define their taxonomy, although these traits often do not correspond with genetic techniques (see groups and identification below). Growth and reproduction
Genetic variation Bacteria, as asexual organisms, inherit an identical copy of their parent's genes (i.e. are clonal). All bacteria, however, have the ability to evolve through selection on changes to their genetic material (DNA) which arise either through mutation or genetic recombination. Mutation occurs as a result of errors made during the replication of DNA. It occurs naturally and as a result of the presence of mutagens. Mutation rates can vary among different species of bacteria. The most frequent genetic changes in bacterial genomes come from random mutation. Some bacteria can also undergo genetic recombination. This can occur when bacteria take-up exogenous environmental DNA from closely related genera in a process called transformation. In the process of transduction, a virus can alter the DNA of a bacterium by becoming lysogenic and introducing foreign DNA into the host chromosome, which can then be transcribed and replicated. The generic term for gene acquisition from the environment is horizontal gene transfer. Because of their ability to quickly grow, and the relative ease with which they can be manipulated, bacteria have historically been the workhorses for the fields of molecular biology, genetics and biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists have been able to determine the function of many different genes and enzymes. Lessons learned from bacteria can then be applied to more complex organisms which are often more difficult to study. Movement Motile bacteria can move about, 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 that 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, clusters of many flagella at one end or flagella scattered all over the cell, as with peritrichous. Many bacteria (such as E. coli) have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and introduces an important element of randomness in their forward movement. (See external links below for link to videos.) Motile bacteria are attracted or repelled by certain stimuli, behaviors called taxes - for instance, chemotaxis, phototaxis, mechanotaxis, and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria attract to form swarms and may differentiate to form fruiting bodies. The myxobacteria move only when on solid surfaces, unlike E. coli which is motile in liquid or solid media. Groups and identification Historically, bacteria as originally studied by botanists were classified in the same way as plants, that is, mainly by shape. Bacteria come in a variety of different cell morphologies (shapes), including bacillus (rod-shape), coccus (spherical), spirillum (helical), and vibrio (curved bacillus). However, because of their small size bacteria are relatively uniform in shape and therefore classification based on morphology was unsuccessful. The first formal classification scheme was developed following the development of the Gram stain by Hans Christian Gram which separates bacteria based on the structural characteristics of their cell walls. This scheme included: Further developments (essentially) based on this scheme included comparisons of bacteria based on differences in cellular metabolism as determined by a wide variety of specific tests. Bacteria were also classified based on differences in cellular chemical compounds such as fatty acids, pigments, and quinones for example. While these schemes allowed for the differentiation between bacterial strains, it was unclear whether these differences represented variation between distinct species or between strains of the same species. It was not until the utilization of genome-based techniques such as guanine cytosine ratio determination, genome-genome hybridization and gene sequencing (in particular the rRNA gene) that microbial taxonomy developed (or at least is developing) into a stable, accurate classification system. It should be noted, however, that due to the existence numerous historical classification schemes and our current poor understanding of microbial diversity, bacterial taxonomy remains a changing and expanding field. Benefits and dangers Bacteria are both harmful and useful to the environment and animals, including humans. The role of bacteria in disease and infection is important. Some bacteria act as pathogens and cause tetanus, typhoid fever, pneumonia, syphilis, cholera, food-borne illness, leprosy, and tuberculosis(TB). Sepsis, a systemic infectious syndrome characterized by shock and massive vasodilation, or localized infection, can be caused by bacteria such as Streptococcus, Staphylococcus, or many gram-negative bacteria. Some bacterial infections can spread throughout the host's body and become systemic. In plants, bacteria cause leaf spot, fireblight, and wilts. The mode of infection includes contact, air, food, water, and insect-borne microorganisms. The hosts infected with the pathogens may be treated with 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. Antiseptic measures may be taken to prevent infection by bacteria, for example, by swabbing skin with alcohol prior to piercing the skin with the needle of a syringe. Sterilization of surgical and dental instruments is done to make them sterile or pathogen-free to prevent contamination and infection by bacteria. Sanitizers and disinfectants are used to kill bacteria or other pathogens to prevent contamination and risk of infection. In soil, microorganisms which reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) help in the transformation of molecular dinitrogen gas as their source of nitrogen, converting it to nitrogenous compounds in a process known as nitrogen fixation. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of the gut flora 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. This ability has also been utilized by humans in industry, waste processing, and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Some beaches in Prince William Sound were fertilized in an attempt to facilitate the growth of such bacteria after the infamous 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil. 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 therapeutic drugs, such as insulin, or for the bioremediation of toxic wastes. "Friendly bacteria" is a term used to refer to those bacteria that offer some benefit to human hosts, such as Lactobacillus species, which convert milk protein to lactic acid in the gut. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion). Other bacteria that are helpful inside the body are many strains of E. coli, which are harmless in healthy individuals and provide Vitamin K. Trivia See also Sources | |||||||||||||||||||||||
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