Bacillus stearothermophilus is best described as which type of organism?

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

Answer: (C)

Organisms that are thermophilic grow best at elevated temperatures. Bacillus stearothermophilus fits this description because it thrives at warm temperatures, with optimal growth around 55–65°C and it forms heat-resistant spores. This organism is a Bacillus species, which are Gram-positive, rod-shaped, and capable of forming endospores. It is not considered a typical human pathogen, and it is not an anaerobe; Bacillus species are generally aerobic or facultatively anaerobic, and this particular bacterium stains positive with Gram stain rather than negative. A practical reason this organism is highlighted in lab settings is its spores’ resistance to heat, which is why Bacillus stearothermophilus is used as a biological indicator to validate steam sterilization processes.