This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc., Subject name: Microbiology
Efficacy of Moist Heat Sterilization is tested by using the Spores of 
Select and explain. 
A. Bacillus stearothermophilus, B. Clostridium tetani, C. Bacillus subtilis, D. Clostridium botulinum
A: Bacillus stearothermophilus
Answer: A: Bacillus stearothermophilus 
Explanation: (A) Bacillus stearothermophilus > For determining the efficacy of moist heat sterilization, (e.g. autoclaving), spores of Bacillus stearothermophilus are used as the test organism. This a thermophilic organism with an optimum growth temp. 55-60degC. Its spores are killed at 121 degC after 12 min exposure.