Which type of Bacillus is used to test efficacy of Sterilisation by Autoclave?

Sterility indicator strips containing spores from the bacterium Bacillus stearothermophilus is used to test efficacy of sterilisation by autocalving. It is a thermophilic bacterium with an optimum temperature of 55-60 degree celsius and its spores require an exposure of 12minutes at 121 degree celsius to die. Spores of Clostridium tetani are used as sterilization controls to determine the efficacy of sterilization by hot air oven.