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1. Describe the design and function of biological indicators.

2. Understand how rapid readout biological indicator systems work.

3. Discuss the recommended uses of biological indicators for sterilization monitoring.


The science of speed

Today’s rapid readout biological indicators by Craig Wallace, Senior Technical Specialist, 3M Infection Prevention Division

iological indicators (BIs) are an impor- tant part of a quality control system for hospital sterilization processes. The information on the quality of the steriliza- tion process supplied by biological indicators, when combined with the information from physical monitors and chemical indicators, provides the basis for the decision on whether or not to release the medical devices for use on patients. Biological indicators are defined as a test system containing viable microorganisms providing a defined resistance to a specified sterilization process.1

A key point in this

definition is “viable microorganisms,” as biological indicators are the only sterilization monitoring device that directly tests the effect of the sterilization process on microorgan- isms. The Centers for Disease Control and Prevention describe the value of biological indicators in their 2008 Guideline: “Biological indicators are recognized by most authorities as being closest to the ideal monitors of the sterilization process because they measure the sterilization process directly by using the most resistant microorganisms (i.e., Bacillus spores), and not by merely testing the physical and chemi- cal conditions necessary for sterilization. Since the Bacillus spores used in biological indicators are more resistant and present in greater numbers than are the common microbial contaminants found on patient-care equipment, the demonstration that the biological indicator has been inactivated strongly implies that other potential pathogens in the load have been killed” 2

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The primary biological indicator design used in healthcare facilities is the self-contained biological indicator, or SCBI (see Figure 1). Self-contained biological indicators con- tain the critical elements of the biological indicator: the bacterial spores on a carrier, and the growth media required to cul- ture the test organisms to determine if the BI is positive or negative. The self-contained design


eliminates the need for a microbiological laboratory to complete the BI test. Before we go much further into biological indicators we need to take a minute and review a little bit of microbiology. The term “spores” is short for bacterial endospores. There are a few types of bacteria that have developed the abil- ity to change from an active, growing cell (or vegetative cell) to a highly protected, dormant cell (endospore), and back again depending on their environment. These bacteria will change to a spore when faced with a shortage of food or other conditions that are harmful to the cell. The spore itself is like a plant seed or hard nut — it is biologically dormant (or “sleeping”), it has a highly protective dry shell, and it is capable of withstanding extreme conditions for prolonged periods of time without ill ef- fect. If the spore senses that conditions have improved and will support life, it goes through a series of biological steps called activation and germination, to shed the hard coat and become a regular, active bacterial cell once again. Bio- logical indicators use the spore form of Bacil- lus bacteria because of the toughness of these spores and the challenge they present to the sterilization process. Each sterilization process requires a specific Bacillus species proven to be the most resistant to that process. For example, steam sterilization processes are tested with Geobacillus stearothermophilus spores. Spores require a source of nutrients and

Photo courtesy of 3M.

Figure 1 — Components of a self-contained rapid readout biological indicator.

optimized temperature and pH to be- gin the activation, germination, and outgrowth processes. Self- contained biological indicators contain growth media that has been specially formulated to support outgrowth of the spores used in that BI. All biological indicators require incubation, during which the spores are ex- posed to the growth media and the biological indicator is heated to the optimum temperature for spore outgrowth. Any surviving spores will first activate and ger- minate to become vegetative cells, and then these cells will begin to “grow,”which means they will replicate (one becomes two, two become four, and so on).

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