A microorganism (also can be spelled as micro organism) or microbe is an organism that is microscopic (too small to be seen by the naked human eye).Bacteria are the most diverse and abundant group of organisms on Earth. Bacteria inhabit practically all environments where some liquid water is available and the temperature is below +140 °C. They are found in sea water, soil, air, animals' gastrointestinal tracts, hot springs and even deep beneath the Earth's crust in rocks. Practically all surfaces which have not been specially sterilized are covered in bacteria. The number of bacteria in the world is estimated to be around five million trillion trillion, or 5 x 1030.Bacteria are practically all invisible to the naked eye, with a few extremely rare exceptions.