Biological indicator for moist-heat sterilization validation: spores of which organism are commonly used as the control challenge in steam (autoclave) sterilization processes?

Introduction:
Validation of sterilization cycles relies on biological indicators (BIs) containing highly resistant spores. For moist-heat (steam) sterilization around 121°C, a standardized thermophilic spore former is used to ensure adequate lethality across loads and penetration-limited locations.

Given Data / Assumptions:

• Steam sterilization cycles target 121°C (or equivalent F0) conditions.
• Biological indicators must be more resistant than routine contaminants to build safety margin.
• Organism taxonomy has evolved, but legacy names appear in practice.

Concept / Approach:
The classical BI for steam is Bacillus stearothermophilus (now Geobacillus stearothermophilus). Its spores exhibit high heat resistance at elevated temperatures typical of autoclaves, making them ideal for cycle challenge. Successful inactivation of these spores validates cycle lethality in worst-case positions.

Step-by-Step Solution:
Identify the sterilization modality: moist heat.Match the appropriate thermophilic spore former used as BI.Select the option listing Bacillus stearothermophilus.

Verification / Alternative check:
Industry standards and pharmacopeias specify spore populations of this organism on carrier strips or self-contained BIs for steam cycles, confirming its status as the control organism.

Why Other Options Are Wrong:

• Bacillus subtilis: Used for dry heat or gas in some cases, not the primary steam BI.
• Clostridium botulinum: Pathogenic anaerobe; spores are not standard BIs.
• Aspergillus niger: Mold; not the BI for steam sterilization.
• Escherichia coli: Vegetative cells are far less heat resistant.

Common Pitfalls:
Confusing BI organisms across modalities; always pair the BI with the sterilant (steam, dry heat, EtO, radiation).

Final Answer:
Bacillus stearothermophilus