A biological indicator (BI) is a test device containing live bacterial spores that verifies whether a sterilization process actually killed microorganisms. It’s the most direct way to confirm sterilization worked, because instead of just measuring temperature or chemical exposure, it answers a simple question: did the process kill the toughest organisms we could throw at it?
How Biological Indicators Work
A biological indicator contains a known quantity of bacterial spores chosen specifically because they’re extremely hard to kill. These endospores are among the most resilient life forms on Earth. They can survive extreme heat, pressure, and chemical exposure that would destroy virtually all other microorganisms. The logic is straightforward: if the sterilization process can kill these spores, it can kill anything else that might be present on surgical instruments, medical devices, or other items being sterilized.
Each type of sterilization process is paired with a spore species known to resist that specific method. For steam sterilization (the most common method in healthcare), the indicator uses spores of Geobacillus stearothermophilus, which are highly resistant to moist heat. For dry heat sterilization and ethylene oxide gas sterilization, the indicator uses Bacillus atrophaeus spores instead. Vaporized hydrogen peroxide sterilization also uses G. stearothermophilus, since those spores are particularly resistant to that process as well.
Physical Formats
Biological indicators come in several physical forms depending on the application. The most common types in healthcare settings are spore strips and self-contained biological indicators (SCBIs). A spore strip is a small paper strip inoculated with spores, which must be carefully transferred into growth media after the sterilization cycle. This transfer requires aseptic technique, because if the strip or the media gets contaminated during handling, you can get a false positive result that incorrectly suggests the sterilizer failed.
Self-contained indicators solve this problem by packaging the spores and growth media together in a single sealed unit. After sterilization, you simply crush an internal vial to release the media and incubate the whole device. There’s no transfer step, so no opportunity for contamination. For situations where the indicator needs to fit into a tight space inside a medical device, miniaturized versions are available, including threads, micro-strips, and paper discs. For sterilizing containers of liquid, glass ampoule indicators that can be submerged directly in the liquid are used.
Reading the Results
After a sterilization cycle, the biological indicator is placed in an incubator at a specific temperature. For steam sterilization indicators using G. stearothermophilus, the incubation temperature is typically 56°C. For ethylene oxide indicators, it’s 37°C. The traditional method requires 24 to 48 hours of incubation, during which the indicator is checked daily for signs of bacterial growth.
Growth shows up as turbidity (cloudiness) in the media and a color change from purple to yellow. If the spores survived the sterilization process, they germinate and produce acid as they grow, triggering that color shift. A yellow result means sterilization failed. A result that stays purple with no cloudiness means the spores were killed and the cycle was successful.
Every test requires a positive control: an untreated indicator from the same manufacturing lot that was never exposed to the sterilizer. This control must show growth (turn yellow) for the test to be valid. If the control doesn’t grow, something is wrong with the spores or media themselves, and the entire test series is invalid. A negative control, an ampoule without spores, is also run to confirm the media itself isn’t contaminated.
Rapid Readout Technology
Waiting 48 hours for sterilization results creates a practical problem: items are often needed sooner than that. Rapid readout biological indicators dramatically shorten this timeline. Instead of waiting for spores to visibly grow, these systems detect an enzyme produced by the spores. If the enzyme is still active after sterilization, it means live spores survived. For steam sterilization, rapid readout systems paired with an auto-reader can deliver results in as little as 20 minutes. For ethylene oxide, rapid results take about four hours.
Studies comparing rapid readout results with conventional 48-hour incubation have found strong agreement between the two methods. In one study at a cardiac hospital in Bangladesh, 100% of ethylene oxide results and all steam sterilization results matched between the rapid and conventional methods.
How BIs Differ From Chemical Indicators
Chemical indicators are strips or tapes that change color when exposed to certain physical conditions like temperature or steam. They confirm that items were exposed to sterilization conditions, but they don’t prove those conditions were sufficient to kill microorganisms. A chemical indicator might change color even when the exposure time was too short to achieve sterilization.
Research comparing the two has highlighted this gap. In one study, certain chemical indicators failed to flag inadequate sterilization at specific exposure times in a gravity displacement sterilizer, which could lead to falsely passing loads that weren’t fully sterilized. Biological indicators, because they directly measure whether organisms survived, provide a fundamentally different and more reliable level of assurance. Chemical indicators are useful as a quick check on every load, but they’re not a substitute for biological testing.
How Often They Should Be Used
The CDC recommends using a biological indicator at least once per week to monitor sterilizers in dental and healthcare settings, with a matching control from the same lot number. Any load containing an implantable device should be tested with a biological indicator before the implant is used. Many facilities test more frequently than the weekly minimum, particularly high-volume surgical centers where sterilizer failure would have serious consequences.
Beyond routine weekly monitoring, biological indicators are also used when a sterilizer is first installed, after major repairs, and during validation of new sterilization processes. They serve as the gold standard for sterilization assurance because they’re the only monitoring method that directly confirms lethality rather than inferring it from physical or chemical conditions.