Biological monitoring of a steam autoclave uses a Geobacillus stearothermophilus spore strip. A strip processed in a full sterilisation cycle shows growth in the indicator broth after 24 hours at 55°C. What does this indicate?

• A The autoclave cycle was effective — spore growth is the expected result
• B Sterilisation failure — viable spores survived the cycle ✓
• C False positive due to contamination of the unprocessed control strip
• D The temperature indicator tape colour change indicates process failure

Explanation

Biological indicators (BIs) for steam sterilisation contain spores of Geobacillus stearothermophilus (D121 value ~1.5–2 min at 121°C), which are the most resistant organisms used as worst-case surrogates. Growth of the processed strip after incubation indicates spores survived — i.e., sterilisation has failed. The unprocessed control must also show growth (confirming spore viability); if only the processed strip grows, it is a genuine failure. Chemical indicators (Bowie-Dick, autoclave tape) detect process parameters but do not confirm sterility.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.