Biological indicators are the only process indicators that directly monitor the lethality of a given sterilization process. Spores used to monitor a sterilization process have demonstrated resistance to the sterilizing agent and are more resistant than the bioburden found on medical devices.  While Bacillus atrophaeus spores are used to monitor ETO and dry heat sterilizers, Geobacillus stearothermophilus spores are used to monitor steam sterilization, hydrogen peroxide gas plasma, and liquid peracetic acid sterilizers.  Geobacillus stearothermophilus spores are incubated at 55-60°C and Bacillus atrophaeus spores are incubated at 35-37°C. 

Weekly testing is minimum:

Steam and low temperature sterilizers (e.g., hydrogen peroxide gas plasma, peracetic acid) should be monitored at least weekly with the appropriate commercial preparation of Geobacillus stearothermophilus spores.  If a sterilizer is used frequently (e.g., several loads per day), daily use of biological indicators allows earlier discovery of equipment malfunctions or procedural errors and thus minimizes the extent of patient surveillance and product recall needed in the event of a positive biological indicator.  Each load should be monitored if it contains implantable objects.  If feasible, implantable items should not be used until the results of spore tests are known to be negative. 

See also www.cdc.gov/oralhealth/infectioncontrol/faq/sterilization_monitoring.htm

Originally, spore-strip biological indicators required up to 7 days of incubation to detect viable spores from marginal cycles (i.e., when few spores remained viable).  The next generation of biological indicator was self-contained in plastic vials containing a spore-coated paper strip and a growth media in a crushable glass ampoule. This indicator had a maximum incubation of 48 hours but significant failures could be detected in ≤24 hours.  A rapid-readout biological indicator that detects the presence of enzymes of Geobacillus stearothermophilus by reading a fluorescent product produced by the enzymatic breakdown of a non-fluorescent substrate has been marketed for the more than 10 years. Studies demonstrate that the sensitivity of rapid-readout tests for steam sterilization (1 hour for 132°C gravity sterilizers, 3 hrs for 121°C gravity and 132°C vacuum sterilizers) parallels that of the conventional sterilization-specific biological indicators and the fluorescent rapid readout results reliably predict 24 and 48-hour and 7-day growth.  The rapid-readout biological indicator is a dual indicator system as it also detects acid metabolites produced during growth of the Geobacillus stearothermophilus spores.  This system is different from the indicator system consisting of an enzyme system of bacterial origin without spores.  Independent comparative data using suboptimal sterilization cycles (e.g., reduced time or temperature) with the enzyme-based indicator system have not been published.  ATS, Inc. also has a rapid readout system available – see Biological Indicators.

ATS, Inc. facilitates high quality and stringent laboratory standards with our Mail-In Services.  We provide a dual-spore strip inoculated with both spores; Geobacillus stearothermophilus and Bacillus atrophaeus spores, allowing users to spore test all sterilizers used in healthcare facilities.