Autoclave sterilization at 121°C for 15 minutes is the standard cycle for moist heat sterilization. Monitoring of autoclave efficacy using a biological indicator uses:
- A Clostridium sporogenes spores — most heat-resistant Clostridium species used for monitoring moist heat
- B Bacillus subtilis var. niger spores — used for both moist heat and dry heat autoclave monitoring
- C Geobacillus stearothermophilus spores — spores that survive suboptimal cycles produce acid in the indicator, changing color of the pH indicator from purple to yellow ✓
- D Bacillus anthracis spores — most heat-resistant bacterial spore used to validate sterilization parameters
Explanation
Geobacillus stearothermophilus (formerly Bacillus stearothermophilus) spores are used as the biological indicator (BI) for moist heat (autoclave) sterilization. These thermophilic spores have a D121°C value of approximately 1.4–2.0 minutes, making them sufficiently resistant to validate autoclave performance. In commercial BIs (e.g., Attest vials), the spores are in a growth medium with a pH indicator — if sterilization fails, surviving spores germinate and produce acid (fermentation), turning the indicator from purple to yellow. Bacillus subtilis var. niger (atrophaeus) spores are used for dry heat sterilization and ethylene oxide monitoring. Chemical indicators (Bowie-Dick test, autoclave tape) detect steam penetration but not biological kill.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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