Ultramicrobacteria (also known as dwarf bacteria) are bacteria that are smaller than 0.1 μm3 under all growth conditions. They have shrunk to less than a thousandth of their normal volume because they have used up virtually all their internal reserves.
These emaciated microbes are commonly found at great depths under the Earth’s surface (see endoliths) where nutrients are scarce. In their reduced state, dwarf bacteria have an extremely low metabolic rate and may divide only once every century or so, compared with a normal reproduction time measured in minutes, hours or days. The discovery of such extremely small organisms, able to survive without the light on a starvation diet, appears to increase the chance of finding primitive life elsewhere in the solar system and beyond.