Bacteria (singular, bacterium) are a major group of living organisms. They are microscopic and mostly unicellular, with a relatively simple cell structure lacking a cell nucleus, cytoskeleton, and organelles such as mitochondria and chloroplasts. Such organisms are called prokaryotes, in contrast to organisms with more complex cells, called eukaryotes. The term bacteria has variously applied to all prokaryotes or to a major group of them, depending on ideas about their relationships.
Bacteria are the most abundant of all organisms. They are ubiquitous in soil, water, and as symbionts of other organisms. Many pathogens, including those responsible for many if not most non-hereditary diseases, are bacteria. Most are minute, usually only 0.5-5.0 μm in size, though one type may reach 0.3 mm in diameter (Thiomargarita). They generally have cell walls, like plant and fungal cells, but with a very different composition (peptidoglycans). Many move around using flagella, which are different in structure from the flagella of other groups.
History and taxonomy
The first bacteria were observed by Antony van Leeuwenhoek in 1683 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". Louis Pasteur (1822-1895) and Robert Koch (1843_1910) described the role of bacteria as conveyors and causes of disease or pathogens.
Originally the bacteria were considered microscopic fungi (called Schizomycetes), except for the photosynthetic cyanobacteria, which were considered a group of algae (called Cyanophyta or blue-green algae). It was only with the study of detailed cell structure that it was realized they formed a fundamental group, separate from the other organisms. In 1956 Copeland gave them their own kingdom Mychota, later renamed Monera, Prokaryota, or Bacteria. During the 1960s the concept was refined and bacteria (now including cyanobacteria) were recognized as one of two major divisions of the living world, together with the eukaryotes. Eukaryotes were generally believed to have evolved from bacteria, later from assemblies of bacteria.
The advent of molecular systematics challenged this view. In 1977, Woese divided the prokaryotes into two groups based on 16S rRNA sequences, called the kingdoms Eubacteria and Archaebacteria. He argued that each of these and the eukaryotes all evolved separately and in 1990 emphasized this by promoting them to domains, which were renamed the Bacteria, Archaea, and Eucarya. This redefinition has generally been accepted by molecular biologists but criticized by some others, who maintain that he over_emphasized a few genetic differences and that both archaebacteria and eukaryotes probably developed from within the eubacteria.
Bacteria reproduce only asexually, not sexually. Specifically they reproduce by binary fission, or simple cell division. During this process, one cell divides into two daughter cells with the development of a transverse cell wall.
However, independent of sexual reproduction, genetic variations can occur within individual cells through recombinant events such as mutation (random genetic change within a cell's own genetic code). Similar to more complex organisms, bacteria also have mechanisms for exchanging genetic material. Although not equivalent to sexual reproduction, the end result is that a bacterium contains a combination of traits from two different parental cells. Three different modes of exchange have thus far been identified in bacteria:
- transformation (the transfer of naked DNA from one bacterial cell to another in solution, this can include dead bacteria),
- 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. Heterotrophs depend on an organic source of carbon, while autotrophs are able to synthesize organic compounds from carbon dioxide and water. Autotrophs that obtain energy by oxidizing chemical compounds are called chemotrophs, and those that obtain their energy from light, via photosynthesis, are called phototrophs. There are many variations on this terminology such as chemoautotrophs and photosynthetic autotrophs and so on. In addition, bacteria are distinguished based on the source of reducing equivalents they are using. Those using inorganic compounds (e. g. water, hydrogen, sulfide or ammonia) for this purpose are called lithotrophs and others needing organic compounds (e. g. sugars or organic acids) and are called organotrophs. The metabolic modes of energy metabolism (phototrophy or chemotrophy), reducing equivalent sources (lithotrophy or organotrophy) and carbon sources (autotrophy or heterotrophy) can be combined differently in any single microorganism, and even shifting between different modes frequently occurs in many species.
The photolithoautotrophs include the cyanobacteria, which are some of the oldest organisms known from the fossil record and probably played an important role in creating the Earth's oxygen atmosphere. They apparently pioneered the use of water as (lithotrophic) electron source and were the first to use the photosynthetic water splitting apparatus. Other photosynthetic bacteria use different electron sources and therefore do not produce oxygen. These anoxygenic phototrophs fall into four phylogenetic groups: the green sulfur bacteria, green non-sulfur bacteria, purple bacteria, and heliobacteria.
Other nutritional requirements include nitrogen, sulfur, phosphorus, vitamins and metallic elements such as sodium, potassium, calcium, magnesium, manganese, iron, zinc, cobalt, copper and nickel for normal growth. For some species, additional trace elements such as selenium, tungsten, vanadium or boron are needed.
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 that do not utilize oxygen for respiration but still grow in its presence are called aerotolerant. 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, respectively; 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. 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 (Italian) (http://it.wikipedia.org/wiki/Batteri_magnetotattici). In one peculiar group, the myxobacteria, individual bacteria attract to form swarms and may differentiate to form fruiting bodies.
Groups and identification
Bacteria come in a wide variety of shapes:
B. Round_shaped or spherical.
C. Round_shaped in clusters.
D. Round_shaped in twos.
Phylogenetic Tree of Bacteria
Bacteria come in a variety of different shapes. Most are rod-shaped, sphere-shaped, or helix-shaped; these are respectively referred to as bacilli, cocci, and spirillum. An additional group, vibrios, are comma-shaped. Shape is no longer considered a defining factor in the classification of bacteria, but many genera are named for their shape (e.g. Bacillus, Streptococcus, Staphylococcus) and it is an important part in their identification.
Another important tool is Gram staining, named after Hans Christian Gram who developed the technique. This separates bacteria into two groups, based on the composition of their cell wall. The first formal grouping of bacteria into phyla was based largely on this test:
- Gracilicutes - bacteria with a second cell membrane containing lipids, giving them Gram-negative stains
- Firmicutes - bacteria with a single membrane and thick peptidoglycan wall, giving them Gram-positive stains
- Mollicutes - bacteria with no second membrane or wall, giving them Gram_negative stains
The archeabacteria were originally included as the Mendosicutes. As given, these phyla are no longer believed to represent monophyletic groups. The Gracilicutes have been divided into many different phyla. Most gram_positive bacteria are placed in the phyla Firmicutes and Actinobacteria, which are closely related. However, the Firmicutes have been redefined to include the mycoplasmas (Mollicutes) and certain Gram-negative bacteria.
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, foodborne illness and tuberculosis. 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, fire blight, 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, prior to cutting the skin during surgery or swabbing skin with alcohol when 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 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. This ability has also been utilized by humans for industrial uses and for 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 yoghurt. Using biotechnology techniques, bacteria can be bioengineered for the production of therapeutic drugs, such as 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.
- Some text in this entry was merged with the Nupedia article entitled Bacteria, written by Nagina Parmar; reviewed and approved by the Biology group (editor: Gaytha Langlois, lead reviewer: Gaytha Langlois, lead copyeditors: Ruth Ifcher and Jan Hogle)
- This article contains material from the Science Primer (http://www.ncbi.nlm.nih.gov/About/Primer) published by the NCBI, which, as a US government publication, is in the public domain  (http://www.ncbi.nlm.nih.gov/About/disclaimer.html).
- Alcamo, I. Edward. Fundamentals of Microbiology. 5th ed. Menlo Park, California: Benjamin Cumming, 1997.
- Atlas, Ronald M. Principles of Microbiology. St. Louis, Missouri: Mosby, 1995.
- Holt, John.G. Bergey's Manual of Determinative Bacteriology. 9th ed. Baltimore, Maryland: Williams and Wilkins, 1994.
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- Hugenholtz P, Goebel BM, Pace NR. Impact of Culture-Independent Studies on the Emerging Phylogenetic View of Bacterial Diversity. J Bacteriol 1998;180:4765-4774. Fulltext (http://jb.asm.org/cgi/content/full/180/18/4765?view=full&pmid=9733676) / Medline abstract (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9733676) (PMID 9733676).
- Tree of Life (http://tolweb.org/tree?group=Eubacteria&contgroup=Life_on_Earth)
- Videos (http://www.rowland.harvard.edu/labs/bacteria/index_movies.html) of bacteria swimming and tumbling, use of optical tweezers and other fine videos.