Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. The most common fatal bacterial diseases are respiratory infections. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. Most are in the gut, and there are many on the skin. Humans and most other animals carry vast numbers (approximately 10 13 to 10 14) of bacteria. The study of bacteria is known as bacteriology, a branch of microbiology.
Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. Bacteria also live in symbiotic and parasitic relationships with plants and animals.
In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. The nutrient cycle includes the decomposition of dead bodies bacteria are responsible for the putrefaction stage in this process. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. They constitute a large domain of prokaryotic microorganisms.
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