Biological monitoring of an autoclave (steam sterilizer) is performed using Geobacillus stearothermophilus spore strips. The test is considered PASSED when:

• A The spore strips shows NO growth (no colour change in growth medium) after incubation at 56°C for 7 days ✓
• B The spore strip changes colour to indicate acid production (growth positive)
• C The Bowie-Dick tape changes to show complete colour change
• D The Browne's tube changes colour from orange to green

Explanation

Geobacillus stearothermophilus (formerly Bacillus stearothermophilus) spores are the biological indicator for steam autoclaves; they are the most heat-resistant organisms known and serve as the most stringent test. After autoclaving, the spore strip is incubated in growth medium at 56°C for 5–7 days. NO growth (no turbidity or colour change) confirms adequate sterilization — the autoclave passed. Growth (yellow colour change due to acid production) indicates sterilization failure. Bowie-Dick tape and Browne's tubes are chemical/physical indicators, not biological indicators.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.