A biological sterilization indicator used to test autoclave efficiency is which organism?

• A. Virus
• B. Bacillus thuringiensis
• C. Clostridium botulinum
• D. Bacillus stearothermophilus

Answer: (D)

Biological indicators for autoclave validation use organisms that are extremely resistant to moist heat, so they provide a strict test of whether the sterilization cycle is truly effective. The standard organism for this purpose is Bacillus stearothermophilus (Geobacillus stearothermophilus) because its spores survive steam at typical autoclave conditions (like 121°C for a standard cycle) unless the process is proper. If the spores are killed and nothing grows after incubation, you can trust that the sterilization was successful for the whole load.

Other choices aren’t used as the standard indicator for this purpose. Viruses aren’t a reliable benchmark for steam sterilization since their resistance can vary and they’re not the conservative test that spores provide. Bacillus thuringiensis isn’t the reference organism for autoclave checks, and using Clostridium botulinum spores would raise safety concerns and isn’t the standard practice for routine validation. So the best choice is Bacillus stearothermophilus.