The Deinococcus-Thermus (Hadobacteria) are a small group of bacteria comprised of cocci highly resistant to environmental hazards. There are two main groups. The Deinococcales include a single genus, Deinococcus, with several species that are resistant to radiation; they have become famous for their ability to eat nuclear waste and other toxic materials, survive in the vacuum of space and survive extremes of heat and cold. The Thermales include several genera resistant to heat. Thermus aquaticus was important in the development of the polymerase chain reaction where repeated cycles of heating DNA to near boiling make it advantageous to use a thermo-stable DNA polymerase enzyme. These bacteria have thick cell walls that give them gram-positive stains, but they include a second membrane and so are closer in structure to those of gram-negative bacteria. Cavalier-Smith calls this clade Hadobacteria (from Hades, the Greek underworld).