Which thermally resistant bacterium is used as the model organism for designing heat sterilization systems for growth media?

• A. Escherichia coli
• B. Saccharomyces cerevisiae
• C. Bacillus stearothermophilus
• D. Bacillus subtilis

Answer: (C)

When validating heat sterilization for growth media, you need a benchmark organism that is extremely resistant to moist heat, so you can be sure the sterilization process is truly lethal. Bacillus stearothermophilus forms spores that are highly resistant to moist heat, making it the standard model for autoclave validation and for designing heat sterilization systems. If the sterilization cycle can inactivate these spores, it will also kill less resistant organisms that might contaminate media. The other organisms listed are less heat-resistant or not suitable for this benchmark: Escherichia coli is a mesophile, Saccharomyces cerevisiae is a yeast, and Bacillus subtilis, while a spore-former, does not challenge the process as strongly as B. stearothermophilus.