A group (in some classifications a phylum) of bacteria whose members are able to withstand extreme environmental conditions. For example, some Deinococcus species are highly resistant to radiation as well as to cold, dehydration, and vacuum. Another hadobacterium, Thermus aquaticus, is an inhabitant of hot springs; it is the source of the very heat-stable enzyme Taq polymerase, which is used in the polymerase chain reaction.