Which biological indicator (BI) is used to test ethylene oxide sterilization?

• A. Bacillus subtilis
• B. Geobacillus stearothermophilus
• C. Clostridium difficile
• D. Mycobacterium smegmatis

Answer: (A)

Ethylene oxide sterilization is validated with a biological indicator that uses spores highly resistant to EO to confirm the cycle achieves the required lethality. The standard organism for this purpose is Bacillus atrophaeus spores, which historically were associated with Bacillus subtilis. This spore form is particularly resistant to EO, so if the BI is inactivated after exposure, you can be confident the sterilization cycle was effective. Geobacillus stearothermophilus is used for steam and dry-heat sterilization, not EO, while Clostridium difficile and Mycobacterium smegmatis are not standard EO indicators.