Which characteristic applies to Bacillus stearothermophilus?

• A. Pathogenic
• B. Anaerobic
• C. Thermophilic
• D. Gram-negative staining

Answer: (C)

This organism is a thermophile, meaning it thrives at elevated temperatures. Bacillus stearothermophilus grows best around 55–65°C and is widely used for steam-sterilization validation because its heat-resistant spores challenge autoclaves. Its endospore-forming, Gram-positive nature explains its resilience and why it’s chosen for this purpose. It is not a human pathogen in routine use, and it is not anaerobic, nor does it stain Gram-negative. Therefore, thermophilic is the best-fitting characteristic.