Bacteria are prokaryotic and unicellular organisms. Bacteria have a simple organization; they contain an external cell wall, a plasma membrane, circular DNA within the cytoplasm and ribosomes for protein synthesis. Some bacteria are encapsulated, meaning that they have a polysaccharide capsule outside their cell wall.
Prokaryotic organisms are classified into two main groups: archaebacteria and bacteria (the latter is also known as eubacteria).
Compared to bacteria, archaebacteria present basic differences, such as the chemical composition of their plasma membrane and cell wall and different enzymes related to DNA and RNA metabolism.
These are three types of archaebacteria. Halophilic archaebacteria only survive in salt-rich environments (even the salinity of the sea is not enough for them). Thermoacidophilic archaebacteria are characterized by living in high temperatures and low pHs. Methanogen archaebacteria are those that release methane gas (CH₄). They are found in swamps.
Bacteria are responsible for the decomposition process at the end of food chains and food webs. In this process, they also release useful gases and nutrients for other living organism. Bacteria that live within the digestive tracts of ruminants and some insects digest cellulose for these animals. Some bacteria also participate in the nitrogen cycle, carrying out the fixation of nitrogen, nitrification and denitrification, almost always in a mutualistic ecological interaction with plants. Bacteria present within living organisms, such as those that live inside the bowels, compete with other pathogenic bacteria, therefore helping to control the population of noxious agents. There are also bacteria that cause diseases and bacteria used in the production of medical drugs.
The excessive Growth or mass destruction of bacteria can impact entire ecosystems. For example, when a river is polluted by organic material, the population of aerobic bacteria increases since the organic material is food for them. This large number of bacteria then exhausts the oxygen dissolved in the water and other aerobic organisms (like fish) experience mass death.
Some human diseases caused by bacteria are tuberculosis, pertussis, diphtheria, bacterial meningitis, gonorrhea, syphilis, the bubonic plague, leptospirosis, cholera, typhoid fever, Hansen’s disease, trachoma, tetanus and anthrax.
Bacteria are used by industry in various ways. Some vaccines are made of attenuated pathogenic bacteria or antigens present in bacteria. One of the most ancient uses of bacteria is the fermentation of milk to produce yogurt, cheese and curds (even before people knew of the existence of bacteria, these microorganisms were already used to make those products). Some methods of antibiotic production involve bacteria. Recombinant DNA technology (genetic engineering) allows the industrial production and commercialization of human proteins, such as insulin for diabetics, which is synthesized by mutant bacteria. Some bacteria can produce fuel, like methane gas.
Pathogenic bacteria have characteristics known as virulence factors, which help them to parasite their hosts. Some bacteria have fimbriae, cilium-like structures that hook the bacterial cell onto the host tissue. Some bacteria are specialized in intracellular parasitism. Others secrete toxins, molecules that cause disease. In some cases, bacterial population growth causes food poisoning by toxins. Generally, bacterial disease is caused by bacterial population growth resulting in the invasion and destruction of tissues or by bacterial toxins that contaminate an organism.
Bacteria can be found in various environments all over the planet. There are bacteria in the air, in fresh water, on the surface, in the intermediate depth and on the bottom of the sea, in soil, on our skin and practically in all environments on the planet in which air circulates freely. Some bacteria can be found in volcanic craters under extremely high temperatures.
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Most bacteria are heterotrophic, meaning that they do not produce their own food. There are also autotrophic bacteria, such as chemosynthetic bacteria or photosynthetic bacteria.
Some photosynthetic bacteria, such as cyanobacteria, use photosynthesis like plants do, using water. Others, such as the sulfur photosynthetic bacteria, use hydrogen sulfide (H₂S) instead of water.
According to their need for oxygen, bacteria are classified into anaerobic (those that survive without oxygen) and aerobic (those that do not survive without oxygen).
Obligate anaerobes are living organisms that do not survive in the presence of oxygen. For example, the bacteria Clostridium tetani, the agent of tetanus, is an obligate anaerobe.
In superficial wounds, it is common to use hydrogen peroxide to expose anaerobic microorganisms to oxygen to kill them.
Bacteria come in different shapes. A bacterium can be classified as coccus, bacillus, vibrion or spirochete.
The bacterial cell wall is made of peptidoglycans.
Heterotrophic bacteria have ribosomes, essential for protein synthesis.
Plasmids are circular fragments of DNA that are accessories to the main bacterial DNA. Plasmids are important for genetic engineering because genes from other organisms are inserted into them to produce recombinant organisms, such as mutant bacteria. These bacteria are made to produce useful proteins for humans on an industrial scale, for example.
Bacterial Cell Review - Image Diversity: plasmid
Bacteria reproduce through binary fission (scissiparity). However, some bacteria use types of sexual reproduction (transformation, transduction or conjugation) with a combination of genetic material from different specimens.
Sexual reproduction occurs when bacteria incorporate genetic material into other bacteria of the same species; the inserted genetic fragment then becomes a part of the genetic material of the second bacterium. This kind of reproduction can happen by means of transformation, transduction or conjugation.
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