Sterilization assurance — Which bacterial species is routinely used as a biological indicator to examine the efficacy of an autoclave cycle?

Introduction / Context:
Autoclaving is the gold standard for sterilization of heat-stable materials. Verifying sterilization requires biological indicators that are harder to kill than typical contaminants. Knowing the correct indicator organism ensures reliable quality control in clinical, research, and industrial environments.

Given Data / Assumptions:

• Biological indicators rely on highly heat-resistant bacterial spores.
• The organism selected must challenge steam sterilization at 121°C for an appropriate holding time.
• Nomenclature note: Bacillus stearothermophilus is now known as Geobacillus stearothermophilus.

Concept / Approach:
Geobacillus (Bacillus) stearothermophilus produces spores with exceptional resistance to moist heat, making them ideal for biological indicator strips or self-contained vials used to validate autoclave cycles. If these spores are inactivated, one can be confident the load reached sterilizing conditions. Other Bacillus species listed do not serve as the standard biological indicator for steam sterilization.

Step-by-Step Solution:

Verification / Alternative check:
After a completed cycle, incubation of the indicator shows no growth or color change if sterilization was effective; failure indicates inadequate parameters.

Why Other Options Are Wrong:

• B. polymyxa, B. brevis, B. megaterium: not the standard for moist-heat sterilization validation.

Common Pitfalls:
Confusing indicators for dry heat or ethylene oxide with those for steam; different processes use different indicator organisms.

Final Answer:
Bacillus stearothermophilus