Understanding Biological Indicators and Endospore Selection in Sterile Processing

 

Martin Li, MA, CRCST, CER, CIS, CHL

Sterile processing plays a critical role in healthcare settings by ensuring that surgical instruments and medical devices are free of harmful microorganisms. Among the tools used to verify the efficacy of sterilization processes are biological indicators (BIs), which are integral for monitoring and validating sterilization methods. This article will explore the definition and role of BIs, their use in different sterilization processes, and the microorganisms selected for this purpose, focusing on endospores as primary indicators of sterilization efficacy.

What Are Biological Indicators?

According to standards set by the American National Standards Institute (ANSI), the Association for the Advancement of Medical Instrumentation (AAMI), and the International Organization for Standardization (ISO), biological indicators (BIs) are test systems that contain viable microorganisms, which are selected for their defined resistance to a specific sterilization process (ANSI/AAMI, 2018). BIs are used to assess whether the conditions of a sterilization process were sufficient to eliminate a specific number of microorganisms, providing a high level of confidence in the effectiveness of the sterilization process.

Endospores, also referred to as bacterial spores, are the microorganisms most commonly used in BIs. These spores are recognized as some of the most resilient biological forms, capable of withstanding extreme environmental conditions. The specific type of bacterial spore selected for a biological indicator is based on its known resistance to a particular sterilization method. Bacterial spores in BIs allow sterile processing departments to verify that the sterilization process has achieved its goal of eliminating microbial contamination.

Biological Indicators in Different Sterilization Processes

Different sterilization methods require distinct biological indicators because certain spores demonstrate varying levels of resistance depending on the sterilization process. Two of the most common sterilization methods are steam sterilization (autoclaving) and vaporized hydrogen peroxide (VHP) sterilization. For both of these processes, Geobacillus stearothermophilus spores are typically selected as biological indicators. This spore is particularly resistant to both high-temperature steam and vaporized hydrogen peroxide environments, making it the ideal choice for monitoring these sterilization processes (ANSI/AAMI, 2018).

In contrast, ethylene oxide (EO) sterilization relies on Bacillus atrophaeus spores to validate its effectiveness. EO sterilization is particularly useful for items that cannot withstand high temperatures, such as plastic, rubber, metal, and various medical materials. BIs used for EO sterilization, such as the Bionova® BT10 Biological Indicator, contain a population of Bacillus atrophaeus spores soaked on a carrier medium. During the sterilization process, if the EO sterilization cycle has failed to eliminate the spores, the growth medium remains + positive, color changes to yellow after 48 hours of incubation at 37°C. If the growth medium shows – negative, indicates that the load was successfully sterilized (Pflug, 2021).

The Importance of Spore Selection in Sterilization Monitoring

Spore selection is fundamental to the efficacy of biological indicators. As mentioned, Geobacillus stearothermophilus and Bacillus atrophaeus are selected for specific sterilization processes based on their resistance characteristics. Geobacillus stearothermophilus is chosen for steam and vaporized hydrogen peroxide sterilization due to its resilience to high temperatures and oxidative conditions. Similarly, Bacillus atrophaeus is chosen for EO sterilization because of its resistance to alkylating agents like ethylene oxide, commonly used for low-temperature sterilization.

By utilizing the correct spores in biological indicators, sterile processing departments can ensure that each sterilization method is functioning correctly. This is essential for patient safety complying with regulatory standards and maintaining the integrity of sterile processing protocols.

Conclusion

Biological indicators are a cornerstone of modern sterilization validation processes in healthcare. By selecting appropriate spores, such as Geobacillus stearothermophilus for steam and VHP sterilization, and Bacillus atrophaeus for EO sterilization, sterile processing departments can confidently verify the effectiveness of their sterilization methods. Regular monitoring using biological indicators ensures that sterilization conditions are met, ultimately safeguarding patient health and preventing infections.

References

ANSI/AAMI. (2018). Comprehensive guide to steam sterilization and sterility assurance in health care facilities (ANSI/AAMI ST79:2017). Association for the Advancement of Medical Instrumentation.

Pflug, I. J. (2021). Principles of Microbiological Sterilization Processes. Academic Press.