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Bacteria are the domain of prokaryotic microorganisms. Bacteria usually have several microns in length, their cells can have a varied shape: from spherical to rod-shaped and spiral-shaped.
Bacteria is one of the first forms of life on Earth and occurs in almost all terrestrial habitats. They inhabit the soil, fresh and marine reservoirs, sour hot springs, radioactive waste and deep layers of the earth’s crust. Bacteria are often symbionts and parasites of plants and animals. Most bacteria are not described to date, and representatives of only half of the bacterial types can be grown in the laboratory. Bacteria are studying the science of bacteriology – a section of microbiology.
One gram of soil, on average, contains 40 million bacterial cells, and in a milliliter of fresh water one can find a million cells of bacteria. On Earth there are about 5 × 1030 bacteria and their biomass exceeds the total biomass of animals and plants. They play an important role in the circulation of nutrients, for example, it is the bacteria that fix atmospheric nitrogen. They also decompose the remains of animals and plants through rotting. Extremophilic bacteria, living near cold and hot hydrothermal sources, generate energy from insoluble compounds such as hydrogen sulfide and methane. It is assumed that the bacteria also live in the Mariana Cavity, having a depth of 11 kilometers. There are reports of bacteria living in stony rocks 580 meters deep below the seabed at a depth of 2.6 kilometers near the north-east of the United States.
The human microflora is 39 trillion bacterial cells (the human body itself consists of about 30 trillion cells). The most numerous intestinal microflora, the skin is also inhabited by many bacteria. Most bacteria living in the human body are harmless due to the immune system’s containment, and some even benefit. A number of bacteria are pathogenic to humans. Infectious diseases such as cholera, syphilis, anthrax, leprosy and bubonic plague are caused by bacteria. The largest number of deaths is caused by bacterial respiratory infections, and only tuberculosis kills 2 million people a year (mainly in sub-Saharan Africa). In developed countries antibiotics are used not only for the treatment of human diseases, but also in livestock breeding, due to which the problem of resistance to antibiotics is becoming more and more relevant. In the industry, bacteria are used in the treatment of sewage, to eliminate oil spills, to obtain cheese and yogurt, to restore gold, palladium, copper and other metals from ores, as well as in biotechnology, to produce antibiotics and other compounds.