Bacteria

Group of microorganisms that share the fundamental characteristics of a prokaryotic, unicellular mode of existence. Bacteria - organisms that represent some of Earth's earliest living beings. The bacteria are a very diverse group of more than 2,000 species that differ from one another with respect to their shape, cellular composition, nutritional requirements, metabolic capabilities, and preferred habitats.

Morphologically, the bacteria average about 1-2 micrometers across. Bacteria possess a cell wall, which lies outside the cell membrane and is responsible for giving the organism its characteristic shape. Within the membrane is enclosed the bacterial cytoplasm, which contains various substances, including ribosomes, but which lacks membrane-bound cytoplasmic organelles such as chloroplasts or mitochondria. As prokaryotes, these organisms lack a defined nucleus but contain a single, circular DNA molecule - also called the bacterial chromosome - which contains all the necessary genes. Some species may also contain a smaller piece of DNA called a plasmid that encodes non-essential functions. Bacteria may be either stationary or motile due to the presence of flagella. Organisms divide by a simple process of binary fission, whereby a single cell (the mother) doubles in size and quantity of different components before splitting into two roughly equal progeny cells.

Different bacteria possess a vast repertoire of metabolic enzymes, which enables them to live under a wide spectrum of environmental conditions and derive food and energy from a variety of sources, including sunlight and organic and inorganic chemicals. Bacteria obtain their nutrition by absorbing food molecules in various forms; they do not ingest large food particles or other organisms. One common feature among all bacteria, regardless of their ability to perform photosynthesis, is their inability to produce oxygen. Most bacteria are free-living in nature and, due to their diverse metabolic capabilities, occupy almost every environmental niche.

The bacteria perform many vital functions for human life-e.g., bacteria produce vitamins in the gut and they ferment milk into yogurt and cheese. Yet bacteria also pose some of the worst threats to our health and well-being, and are among the principal causes of human disease.

Bacteria have different shapes. Bacteria that have an approximately round shape are referred to as cocci (singular: coccus). This name is derived from a Latinized Greek word kokkos, meaning a berry. Cocci are not necessarily perfectly spherical, and may be quite markedly deformed. Cells of Streptococcus pneumoniae generally occur in pairs, known as diplococci, that are flattened to appear like small lancets. For this reason they are sometimes referred to as lanceolate diplococci. Another diplococcus, Neisseria gonorrhoeae, has cells that appear flattened against one another. The cells of Streptococcus pneumoniae are aligned on their long axis, with their short axes in parallel, whereas those of Neisseria gonorrhoeae are aligned along their short axis, with their long axes in parallel. Cocci may also associate in clusters of more than two cells. Streptococci appear in chains as a result of regular cell divisions in one plane (Greek: streptos, twisted). Staphylococci bunch like grapes as a result of cell division along irregular planes (Greek: staphule, grapes). Bacteria may grow in cuboidal packets as a result of regular cell divisions along three planes.

The term bacillus (plural: bacilli) is derived from the Latin word bacillus, meaning a stick. Bacilli are rod-shaped and come in a variety of forms. Even within a single bacterial genus, considerable variation in shape is seen. For example, cells of Clostridium peifringens are relatively short and fat, and the vegetative cell shape is not deformed by the presence of a spore, whereas cells of Clostridium tetani are long and slender. Often the end of the cell is swollen to accommodate its spore. This gives the cells of Clostridium tetani the appearance of drumsticks.

Just as many cocci arrange themselves in specifically shaped groups, some bacilli are associated with particular arrangements. Bacillus anthracis, the causative agent of anthrax, grows as a long chain of cells aligned along the long axis. Cells of the genus Beggiatoa, and many cyanobacteria form chains in which the cells are in intimate contact. Cells align in long strands in which individual cells are compartmentalized by cross-walls. Such chains are called trichomes because they resemble hairs (Greek: trikhos, hair). Bacteria of the genus Corynebacterium have cells that align along their long axis in parallel, in a manner somewhat similar to fence posts. This arrangement is referred to as a palisade, the Latin word for a stake being palus.

Streptomycetes form long, multinucleate hyphae, that become branched. A collection of hyphae is called a mycelium. The terms hyphae and mycelia are also applied to analogous fungal structures.

Curved rods are referred to as vibrios. This name reflects the vibrational motility seen among these bacteria. Bacterial cells may grow in a twisted, helical shape. Spirilla have rigid cells whereas spirochaetes have a very flexible structure. The Greek word for a coil is speira, Latinized as spira, and khaite is Greek for long hair.

Pleomorphic bacteria have variable shapes (Greek: pleon, more; morphe, shape). Many bacteria display a degree of pleomorphism. In cultures of Proteus mirabilis cells occasionally grow as long filaments, whereas the majority of cells grow as short rods. Similarly Haemophilus inftuenzae cells may grow as filaments, rods or even as coccobacilli. Corynebacteria, although they appear as bacilli are very irregular in shape. In some cases pleomorphism is the result of the environment in which the bacteria are grown. In artificial culture, bacteria of the genus Rhizobium grow as regularly shaped bacilli of fairly uniform dimensions, but when the same bacteria are seen in microscopic preparations of the root nodules of nitrogen fixing plants, cells have a highly degenerate and irregular appearance and are referred to as bacteroids.

Diversity of bacterial nutrition

Bacteria have an extremely wide distribution. They are found from pole to pole. They have been isolated from the icy wastes of the Arctic and the Antarctic, and are found in the waters of hot springs, and near volcanoes on the ocean bed. The diversity of habitats in which bacteria may be found is staggering. It reflects the variety of energy supplies that bacteria can utilize, and also the success with which bacteria have evolved to exploit their environments.

There are bacteria that obtain energy from light; others require a source of chemical energy. Some bacteria utilize inorganic compounds; others require a supply of organic matter. The diversity of nutrition displayed by bacteria is essential for the cycling of elements through biological systems and is a prerequisite for the continued maintenance of life on this planet.

All forms of life require nutrients in order to grow and multiply. Nutrients are the fundamental building blocks from which all living beings are constructed. In order to assemble these building blocks, living organisms require a source of energy. Those bacteria that obtain energy from light are known as phototrophs. Literally this means light-nourished (Greek: photos, light; trophe, nourishment). Bacteria relying upon chemical energy sources are likewise termed chemotrophs. The division of bacteria into these two classes is not necessarily absolute, and the energy source exploited may depend upon the prevalent environmental conditions. When growing under anaerobic conditions Rhodospirillum rubrum is a phototroph and requires a source of light. However, if oxygen is present, then the bacterium can grow in the dark as a chemotroph.

Autotrophic bacteria

All life on this planet is carbon based, and a supply of carbon compounds is essential for the growth and multiplication of organisms. Autotrophs, self nourishing organisms, have the ability to elaborate organic compounds by fixing environmental carbon dioxide. Autotrophic bacteria can be cultured in simple salt solutions, required to provide trace elements for the growing cells. They require no carbon source other than dissolved carbon dioxide. Autotrophs are a vital component of food webs since they produce a renewable supply of organic carbon that is needed by heterotrophs, organisms that cannot elaborate all their own organic compounds and that must therefore be nourished by others.

Autotrophic bacteria may utilize light energy, photoautotrophs, or may obtain energy from the oxidation of inorganic chemical compounds, chemoautotrophs. Photoautotrophs are represented by the purple sulphur bacteria, such as Chromatium spp., the green sulphur bacteria such as Chlorobium spp. and the cyanobacteria. Chemoautotrophic bacteria include hydrogen bacteria, iron bacteria, nitrifying bacteria and sulphur-oxidizing bacteria. Hydrogen bacteria such as Hydrogenobacter spp. use hydrogen as an electron donor and oxygen as an electron acceptor to produce water from the reduction of molecular hydrogen and oxygen. Iron bacteria such as Gallionella spp. convert ferrous ions to ferric ions. Nitrifying bacteria oxidize ammonia. Those bacteria that oxidize ammonia to nitrite are referred to as nitrosifying bacteria. True nitrifying bacteria can further oxidize nitrite to nitrate. These are exemplified by bacteria like Nitrobacter spp. There are bacteria such as Thiothrix spp. that are aerobic and obtain energy from the oxidation of hydrogen sulphide.

All of these bacteria obtain energy from the metabolism of inorganic compounds. Because of this they are described as lithotrophs (Greek: lithos, stone); lithotrophs are nourished from stones. Consequently, photoautotrophs may be referred to as photolithotrophs and chemoautotrophs are also called chemolithotrophs.

Autotrophs have the ability to elaborate all the organic compounds required for their growth and multiplication from carbon dioxide, which they have the ability to fix. Most bacteria are heterotrophic and cannot achieve this feat. Instead, heterotrophs rely on an exogenous supply of organic carbon compounds.

Heterotrophic bacteria

Bacteria of the genus Beggiatoa were the organisms first described as chemolithotrophs, but many strains are incapable of growth unless they are supplied with organic compounds. Thus, although they obtain their energy from the oxidation of sulphur, and may be considered lithotrophic, they are also heterotrophic because they require a fixed carbon source such as acetate so that they can grow. These bacteria are mixotrophs or mixed feeders.

Some photosynthetic bacteria use simple organic compounds such as acetate, formate and methanol as a sole carbon source rather than fixing carbon dioxide. These bacteria are photoorganotrophs, and are exemplified by the purple non-sulphur bacteria such as the Rhodospirillaceae when grown anaerobically in the light. Rhodospirillaceae cannot grow anaerobically unless they are illuminated. However, in the presence of oxygen they may grow in the dark as chemoorganotrophs. Chemoorganotrophs utilize organic compounds as nutrients, and require a chemical energy supply. The vast majority of non-photosynthetic bacteria are chemoorganotrophs.

Binary fission
Method of reproduction or replication in which a cell duplicates its DNA and doubles the rest of its contents before dividing itself into two roughly identical progeny cells by the formation of a septum or wall between the two halves. This is the simplest and most common means of replication of most single-celled prokaryotes including bacteria and mycoplasmas. The mitosis that takes place in eukaryotic cells is also a form of symmetrical binary fission. In certain rare instances, e.g., Caulobacter species, binary fission is asymmetrical, resulting in the formation of two progeny of which one looks and behaves considerably differently from the parent cell. Certain cyanobacteria undergo binary fission repeatedly under a common sheath or capsule so that the progeny do not separate but resemble a filament. This process is known as multiple fission.
Chloroplast
Organelle that serves as the site of photosynthesis in eukaryotic cells, such as those of algae and green plants. It is composed of a double membrane, the inside of which is associated with chlorophyll-containing vesicles, not unlike the thylakoids of the cyanobacteria. In fact, some scientists postulate that the chloroplasts evolved from cyanobacterial endosymbionts. Further strengthening this theory is the presence of small, circular, DNA molecules, which lend the organelle some measure of autonomy, though not complete independence, from nuclear control.
Flagella
Thread-like structure protruding from a cell, that functions as the organ of motility in a single-celled organism or in the mobile cells (e.g., sperm) of a multicellular organism. Although the basic function is the same - they act like propellers to enable the organism to swim in liquid environments - the flagella of prokaryotes and eukaryotes have markedly distinct structures.
A bacterial flagellum is made up primarily of a protein called flagellin. It originates inside the cell and is powered by an electrical motor-like apparatus at its base. The number and arrangement of flagella is characteristic for a given bacterial species. Bacteria may be monotrichous (with a single flagellum) or possess several flagella. These may be regularly distributed all over (peritrichous), present in tufts (lophotrichous), or in polar arrangements (amphitrichous). The visualization of flagella presents particular difficulties because they are slender and do not take up ordinary stains. Therefore, these structures are viewed with the help of mordents, such as tannic acid, which coat the flagella and make them wide enough to be perceived under an ordinary light microscope.
Mitochondrion
Often referred to as the "powerhouses" of cells, the mitochondria are semiautonomous organelles that serve as the sites of respiration/energy generation in most eukaryotes. These organelles are suspected to be the modern-day descendants of prokaryotic endosymbionts in eularyotic cells. Partial evidence for this lies in the fact that these organelles contain their own DNA - under autonomous control - as well as ribosomes that resemble prokaryotic ribosomes rather than the eukaryotic types. Mitochondria vary considerably in shape from small rod-like packages to large branched structures. They consist of a double membrane, with the inside layer folded into several cristae, which contain the components of the electron transport chain. The fluid enclosed within the mitochondrial sac contains respiratory (Krebs cycle) enzymes, as well as enzymes of the urea cycle.
Prokaryote
A living organism characterized by the lack of any membrane-bound compartment, such as a nucleus, within its cells. The structure of a prokaryotic cell is thus very simple and consists of different macromolecules required for performing various living processes enclosed within a membrane. The nucleic acid material of these organisms lies free or naked in the cell, usually in the form of a single continuous circular DNA molecule, with no discernible beginning or end. All prokaryotes are single-celled organisms. Examples include bacteria, rickettsiae, and mycolasmas.
Ribosome
Cellular component involved in the translation of the mRNA into DNA in the cytoplasm. It consists of complexes of RNA and proteins and interacts with tRNAs, energy molecules, and other components of the protein synthetic machinery to translate the coding sequence of the mRNA into polypeptides. Ribosomes are present in all living things but have markedly different structures and components depending on whether they are present in prokaryotic or eukaryotic cells. It is interesting to note that the eukaryotic organelles such as mitochondria and chloroplasts contain their own ribosomes which are more akin to the prokaryotic ribosomes. During the active or functional phase of protein synthesis, one may find several ribosomes strung along a length of mRNA to form what is called a polyribosome, which is necessary for protein synthesis to proceed properly.
Vibrio
Named for the characteristic comma or curvy shape of the bacteria when isolated from their natural habitats, this genus of gram-negative bacteria is the only genus within its family known to be associated with human disease. The best-known species of this genus is the causative agent of cholera. Vibrios are facultative anaerobes but show a marked preference for aerobic conditions. Organisms can use a variety of sugars, including glucose, sucrose, galactose, and mannitol, which they metabolize to produce acid but no gas-i.e., fermentation does not proceed to completion. They reduce nitrates and are strongly proteolytic. The vibrios survive adequately outside the body, especially in warm climates and near water, but are easily killed by heating at high temperatures or with chemicals. Organisms are motile due to the presence of usually a single polar flagellum, although some species may contain a tuft of flagella at the poles. Because the organisms often lose their curved shape upon serial passages in laboratory culture, this feature should not be used as an index for identification.

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