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Biological Indicators for Sterilization Processes

Biological indicators (BIs) are microorganisms that are used to validate sterilization processes. They are typically composed of a known population of resistant microorganisms, such as bacterial spores, which are considered the most resistant to sterilization methods.

Biological Indicators

Different types of sterilization methods utilize specific biological indicators to monitor and validate the effectiveness of the sterilization process. Here are the commonly used biological indicators for various sterilization methods:

  1. Dry Heat Sterilization:
    • Biological Indicator: Bacillus subtilis spores
    • Purpose: Dry heat sterilization typically involves high temperatures for a prolonged period. Bacillus subtilis spores are resistant to dry heat and commonly used as biological indicators to ensure the adequacy of the sterilization process.
  2. Steam Sterilization (Autoclaving):
    • Biological Indicator: Geobacillus stearothermophilus (formerly Bacillus stearothermophilus) spores
    • Purpose: Steam sterilization relies on high temperature and moisture to achieve sterilization. Geobacillus stearothermophilus spores are heat-resistant and widely used as biological indicators in autoclaves to assess the efficacy of the steam sterilization process.
  3. Ethylene Oxide (EtO) Sterilization:
    • Biological Indicator: Bacillus atrophaeus (formerly Bacillus subtilis var. niger) spores
    • Purpose: Ethylene oxide is a low-temperature sterilization method commonly used for heat-sensitive and moisture-sensitive materials. Bacillus atrophaeus spores are resistant to EtO and are employed as biological indicators to verify the sterilization effectiveness.
  4. Vaporized Hydrogen Peroxide (VHP) Sterilization:
    • Biological Indicator: Geobacillus stearothermophilus spores
    • Purpose: VHP sterilization uses vaporized hydrogen peroxide to achieve sterilization. Geobacillus stearothermophilus spores are heat and moisture resistant, making them suitable as biological indicators for VHP sterilization processes.
  5. Gamma Sterilization:
    • Biological Indicator: Bacillus pumilus (formerly Bacillus subtilis var. globigii) spores
    • Purpose: Gamma sterilization utilizes ionizing radiation to achieve sterilization. Bacillus pumilus spores are highly resistant to radiation and commonly used as biological indicators to validate the effectiveness of gamma sterilization.

It's important to note that the selection of the appropriate biological indicator should be based on the specific sterilization method, its resistance characteristics, and relevant industry standards and guidelines. The use of biological indicators provides a reliable means of assessing the sterility assurance level and ensuring the successful sterilization of products or materials.

Qualification of Biological Indicators

The qualification of biological indicators involves ensuring the viability and resistance of the population of microorganisms within the indicator. This includes verifying the population count and determining the D-value, which is a measure of the resistance or lethality of the microorganisms to the sterilization process.

The population of biological indicators is typically determined by manufacturing or supplier specifications. It is important to verify that the population falls within the specified range to ensure the reliability of the indicators. This can be done through microbial enumeration methods, such as plate counting or other suitable techniques.

To qualify biological indicators, it is important to ensure that the population is within the specified range and that the D-value is determined accurately. This involves conducting appropriate validation studies using validated methods and equipment, following relevant guidelines and standards, and documenting the results for review and approval.