Several Aspects of Biological Indicators for Sterility Assurance

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A biological indicator (Bl) for use in thermal or chemical sterilization processes consists of standardized bacterial spore preparations which are usually in the form either of suspensions or of spores dried on carriers. Bl is usually placed in dummy packs located at strategic sites in the sterilizer. After the sterilization process, the aqueous suspensions or spores on carriers are aseptically transferred to an appropriate culture medium, which is then incubated and periodically examined for signs of growth. Spores of Geobacillus stearothermophilus ATCC 7953 or 12980 are used for steam, formaldehyde, hydrogenperoxide, ozone sterilization monitoring, and these are incubated at 55°C. Spore of Bacillus atrophaeus ATCC 9372 is used for ethylene oxide and dry heat sterilization monitoring, and incubated at 30-35°C. When the paper strip type Bl is used, aseptic transfer is indispensable. Aseptic transfers can be avoided by the use of self-contained Bl where the spore strip and nutrient medium are present in the same device ready for mixing after use. This Bl is, however, not recommended for use in a validation study. Resistance of Bl is adjudged from the spore destruction curve obtained upon exposure to the sterilization process, as well as the recommended Bl spores and their decimal reduction times. Great care must be taken in the preparation and storage to ensure a reproducible response to sterilization processes to assure that Bl is a reliable monitor for steriliza tion validation. One of the drawbacks of Bl is that a long period of incubation is required to confirm a satisfactory sterilization process, which imposes an undesirable delay on the release of the product. To shorten the incubation period is now being discussed at ISO TC 198, but as sterilized Bls are injured spores, a short cultivation time may result in an inaccurate result.