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| Bacteria are unicellular microorganisms.
Typically a few micrometres in length, bacteria have a wide range of shapes,
ranging from spheres to rods to spirals. Bacteria are ubiquitous in every
habitat on Earth, growing in soil, acidic hot springs, radioactive waste,
seawater, and deep in the Earth's crust. There are typically 40 million
in a gram of soil and a million bacterial cells in a millilitre of fresh
water; in all, there are approximately five nonillion (5×1030) bacteria
on Earth, forming much of the world's biomass. Bacteria are vital in recycling
nutrients, and many important steps in nutrient cycles depend on bacteria,
such as the fixation of nitrogen from the atmosphere. However, most of
these bacteria have not been characterized, and only about half of the
phyla of bacteria have species that can be cultured in the laboratory.
The study of bacteria is known as bacteriology, a branch of microbiology.
There are approximately ten times as many bacterial cells as human cells in the human body, with large numbers of bacteria on the skin and in the digestive tract. Although the vast majority of these bacteria are rendered harmless by the protective effects of the immune system, and a few are beneficial, some are pathogenic bacteria and cause infectious diseases, including cholera, syphilis, anthrax, leprosy and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people a year, mostly in sub-Saharan Africa. In developed countries, antibiotics are used to treat bacterial infections and in various agricultural processes, so antibiotic resistance is becoming common. In industry, bacteria are important in processes such as sewage treatment, the production of cheese and yoghurt, and the manufacture of antibiotics and other chemicals.
Bacteria are prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotic life consists of two very different groups of organisms that evolved independently from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.
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