Which organism is used in the spore test for steam sterilization?

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

Answer: (D)

Spore testing relies on highly resistant bacterial spores to verify that a sterilization cycle actually achieves lethality. For steam (moist heat) sterilization, Geobacillus stearothermophilus is used because its spores withstand moist heat better than many other organisms, making it a reliable indicator of proper steam sterilization. After running the cycle, the spore-containing indicator is incubated; no growth means the cycle was effective, while growth indicates the cycle did not achieve the required lethal effect. The other organisms listed aren’t standard steam indicators because they’re not the most resistant to moist heat or aren’t used as spores for this purpose. Geobacillus stearothermophilus is chosen specifically for its strong steam resilience, ensuring a meaningful validation of the process.