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Bacteria are unicellular microorganisms. They are typically a few micrometres long and have many different shapes including spheres, rods and spirals. The study of bacteria is bacteriology, a branch of microbiology. Bacteria are ubiquitous, living in every possible habitat on the planet including soil, underwater, deep in the earth's crust and even such environments as acidic hot springs and radioactive waste. In all, there are around five nonillion (5 × 1030) bacteria in the world. There are 10 times more bacterial cells than 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 harmless or beneficial, a few pathogenic bacteria cause infectious diseases, including cholera, syphilis, anthrax, leprosy and bubonic plague. The most common fatal bacterial disease is tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Bacteria are prokaryotes and, unlike animals and other eukaryotes, bacterial cells do not contain a nucleus or other membrane-bound organelles. Although the term bacteria has traditionally been generally applied to all prokaryotes, the scientific nomenclature changed after the discovery that prokaryotic life consists of two very different groups of organisms that evolved independently from an ancient common ancestor. (more...)
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