Which organism is commonly used as a BI for ethylene oxide sterilization in the documentation?

• A. Bacillus subtilis
• B. Geobacillus stearothermophilus
• C. Escherichia coli
• D. Staphylococcus aureus

Answer: (A)

Biological indicators for ethylene oxide sterilization are built around a spore-forming organism because spores are the most resistant life stage to chemical sterilants. Using a spore-forming Bacillus species provides a rigorous test of the sterilization cycle, ensuring that the process can inactivate hardier forms of life under the conditions tested. In EO documentation, this organism is typically Bacillus atrophaeus, which has historically been referred to as Bacillus subtilis (the old naming). The spores of this organism are well characterized, reliably recoverable after testing, and provide a consistent benchmark for pass/fail decisions, making it the standard choice to validate EO sterilization.

Geobacillus stearothermophilus is the usual indicator for steam or dry-heat sterilization because it is particularly resistant to high temperatures rather than to EO. Escherichia coli and Staphylococcus aureus are vegetative bacteria and are far less resistant to ethylene oxide, so they do not provide an appropriate challenge for EO sterilization validation.