A microbiology lab uses an autoclave set at 121°C and 15 psi for 20 minutes. A batch of culture media is prepared and a Bacillus stearothermophilus biological indicator strip is included. After autoclaving, the strip shows no growth on incubation at 55–60°C. What does this confirm?

• A That the autoclave achieved pyrogen-free conditions for parenteral preparations
• B That the media are sterile only from vegetative bacteria; Bacillus spores from the environment could still contaminate
• C That the autoclave achieved sterilisation conditions (121°C, 15 psi, 20 min killed highly heat-resistant B. stearothermophilus spores), confirming adequate moist heat sterilisation ✓
• D That dry heat of the autoclave was effective since B. stearothermophilus is only sensitive to dry heat

Explanation

Biological indicators are the GOLD standard for monitoring autoclave sterilisation efficacy. Bacillus stearothermophilus (now called Geobacillus stearothermophilus) spores are highly resistant to moist heat and are the preferred indicator for autoclaving (121°C/15 psi/20 min). No growth post-autoclaving confirms that lethal temperature-time conditions were achieved. For dry heat ovens (160–180°C), B. subtilis var. niger spores are used. For ETO sterilisation, B. subtilis var. globigii is used. Pyrogen-free status requires depyrogenation (250°C for 30 min dry heat) and cannot be confirmed by biological spore indicators.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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