Strain 121 (Geogemma barossii) is a single-celled microbe of the domain Archaea. First discovered 320 km (200 mi) off Puget Sound near a hydrothermal vent, it is a hyperthermophile, able to reproduce at 121 °C (250 °F), hence its name. It was (at the time of its discovery) the only known form of life that could tolerate such high temperatures. A temperature of 130 °C (266 °F) is biostatic for Strain 121, meaning that although growth is halted, the archaeon remains viable, and can resume reproducing once it has been transferred to a cooler medium.
The ability to grow at 121 °C (250 °F) is significant because medical equipment is exposed to this temperature for sterilization in an autoclave. Prior to the 2003 discovery of Strain 121, a fifteen-minute exposure to autoclave temperatures was believed to kill all living organisms. However, Strain 121 is not infectious in humans, because it cannot grow at temperatures near 37 °C (99 °F).