Biological indicators (BIs), commonly referred to as “spore tests,” are standardized laboratory test systems used to monitor and validate the effectiveness of sterilization processes. They contain a high population of live, highly resistant bacterial spores. If the sterilization cycle is powerful enough to kill these resilient spores, lab technicians can safely assume all other common microbial contaminants have been destroyed as well.
Types of Biological Indicators Available
- Self-Contained Biological Indicators (SCBIs): These vials feature a plastic casing that contains a spore strip and a sealed glass ampoule filled with nutrient growth media and a pH color-indicator dye. After a cycle, you simply crush the vial to mix the spores with the fluid and incubate them. Brand options include Mesa Labs EZTest and 3M Attest 24-Hour Biological Indicators.
- Spore Strips: These are conventional filter paper strips inoculated with bacterial spores and sealed inside protective glassine envelopes. They require a sterile laboratory transfer into an external test tube containing growth media after the autoclave process. Examples include Mesa Labs Spore Strips.
- Liquid Spore Ampoules: These are entirely sealed glass ampoules filled with liquid media and spores, specifically designed to be submerged inside liquid loads (like liquid laboratory media) to ensure the liquid center reaches sterilization temperatures. Variants include Mesa Labs MagnaAmp®.
Biological Indicators by Application
Biological indicators use specific bacterial strains tailored to survive and challenge different sterilization methodologies:
| Sterilization Method | Microorganism Used | Example Indicator Products |
|---|---|---|
| Steam Autoclaves | Geobacillus stearothermophilus | 3M Attest Steam Indicators, Mesa Labs EZTest Steam |
| Ethylene Oxide (EO) | Bacillus atrophaeus | Mesa Labs Crosstex Spore Strips, NAMSA SCBIs |
| Dry Heat Ovens | Bacillus atrophaeus | MesaStrip Paper Biological Indicator, Mesa Labs DriAmp® |
| Vaporized Hydrogen Peroxide (VPHP) | Geobacillus stearothermophilus | Mesa Labs Apex® BIs |
How to Evaluate the Test (Pass vs. Fail)
To verify a cycle, the processed biological indicator is placed inside a dedicated Biological Indicator Incubator at a specific temperature (usually around 57°C to 60°C for steam organisms).
- PASS (Successful Sterilization): The growth medium retains its original color (e.g., purple). No spores survived to replicate.
- FAIL (Sterilization Failure): The growth medium turns from purple to yellow. This happens because surviving spores multiplied and metabolized the nutrients, causing an acid shift in the pH indicator.
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