Bacterium

Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell, and they constitute a large domain of prokaryotic microorganisms. The cited article says they are typically a few micrometres in length, were among the first life forms to appear on Earth and are present in most of its habitats, inhabiting the air, soil, water, acidic hot springs, radioactive waste and the deep biosphere of the crust of the Earth. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and fixing nitrogen from the atmosphere. Pests and diseases recorded in the atlas as caused by a bacterium are grouped under this concept.