Understanding Biological Indicator Grow-Out Times—Part II - Pharmaceutical Technology

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Understanding Biological Indicator Grow-Out Times—Part II
In Part II of a series, the authors report on the range and distribution of grow-out times for biological indicators exposed to sublethal sterilization processes.

Pharmaceutical Technology
Volume 37, Issue 6, pp. 52-59


The study showed several key findings as outlined below:

  • Grow-out time results, regardless of the mode of sterilization, approximates a normal distribution for all exposure conditions.
  • This study indicates that the dynamics of spore germination and out-growth are very similar regardless of the sterilization mode to which the BIs are exposed. This study included moist heat at 121 C, 132 C, 134 C, and 135 C as well as H2O2 vapor, EO gas, and ClO2 gas. It is unlikely that different bacterial endospores would respond differently to other sterilizing agents.
  • The data continue to support the conclusion that there is an inverse relationship between the number of surviving spores on a BI and the overall range of grow-out time. The time required to obtain an acceptable cell density and/or cumulative metabolic activity requires less time when the starting level of viable spores is higher than when it is lower.
  • Grow-out times were shorter for G. stearothermophilus spores than for B. atrophaeus spores.
  • The timing of the calculated survival time exposures yielded nonsterile results faster and more consistently than the FDA RIT protocol exposures. No delayed nonsterile results were observed in the calculated survival time exposures that provided results with all BIs nonsterile.
  • Delayed nonsterile BIs were observed with all sterilization modes tested. All delayed nonsterile BIs were observed when the exposures yielded dichotomous results (some BI replicates nonsterile and some BI replicates sterile).
  • Delayed nonsterile BIs almost certainly contained a single viable spore. If two or more spores are present on the BI and only one exhibited delayed germination and/or outgrowth, the other spore would mask this condition and yield a nonsterile result in a time similar to a BI with a single spore that was not severely damaged.
  • Delayed nonsterile results were found in a very small portion of the BIs exposed. The total number of BIs exposed that yielded a dichotomous result was approximately 2400. The number of BIs that were nonsterile was 1188. The total number of delayed nonsterile BIs observed was 13 or 1.1% of all nonsterile BIs. Delayed nonsterile results for each sterilization mode evaluated were as follows:

—Moist heat: 0.44% delayed nonsterile BIs
—H2O2: 2.5% delayed nonsterile BIs
—EO: 0.97% delayed nonsterile BIs
—ClO2: 1.2 % delayed nonsterile BIs.

*John R. Gillis, PhD, is retired president of SGM Biotech, Inc., 10 Evergreen Dr., Bozeman, MT,
; Gregg A. Mosley is retired president of Biotest Laboratories, Minneapolis, MN; John B. Kowalski, PhD, is principal consultant of SteriPro Consulting, Sterigenics International, Inc., Oak Brook, IL, Garrett Krushefski is vice-president, biological indicator operations; Paul T. Nirgenau is contract studies coordinator; Kurt J. McCauley is director of R&D and Apex Labs, all at Mesa Laboratories, Bozeman, MT; and Philip M. Schneider is a senior consultant with LexaMed, 705 Front Street, Toledo, OH 43604,
, and is Convener of ISO/TC 198 WG4 Biological Indicators.

*To whom all correspondence should be addressed.

Submitted: July 11, 2012. Accepted: July 18, 2012.


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