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

Ethylene oxide gas exposures. All EO exposures were performed with self-contained BIs with 106 spores of B. atrophaeus inoculated onto paper carriers (see Table I).


Table IV: Time-to-growth positive results for Bacillus atrophaeus biological indicators (BIs) exposed to ethylene oxide gas.
Unexposed controls: Three lots of EO BIs were incubated at 37 2 C in a conventional incubator to determine a baseline value for the time required for BIs to exhibit nonsterile results (see Table IV). The average time for the first nonsterile BI in the three lots was 7.5 hours. Ninety-five percent of all BIs were nonsterile in 10.2 hours. The incubation time between the first nonsterile BI and 95% nonsterile BIs was 2.7 hours; no delayed nonsterile BIs were observed.

Calculated survival time exposures: The calculated survival time exposures were performed using three lots of 100 BIs. The average time for the first nonsterile BI in the three lots was 18.2 hours. Ninety-five percent of all BIs exposed to these conditions were nonsterile in 21.3 hours. The duration of incubation between the first nonsterile BI and 95 % nonsterile was 3.1 hours. This duration was approximately 15% longer than that found with the unexposed controls. No delayed nonsterile BIs were observed.


Table V : Time-to-nonsterile results for Bacillus atrophaeus biological indicators (BIs) exposed to chlorine dioxide gas.
FDA RIT protocol exposures: Six lots of BIs were exposed to yield 30 to 80 BIs nonsterile per lot (see Table IV). The average time for the first nonsterile BI for the six lots tested was 21.96 hours. Ninety-five percent of all BIs exposed to these conditions were nonsterile in 62.9 hours. The duration of incubation from the first nonsterile BI to 95% nonsterile was 40.94 hours, which was approximately 13 times longer than that found with the calculated survival time exposures.

Delayed nonsterile BIs were observed with two of the lots. Two BIs were observed in one lot and one BI was observed in the second lot. The delayed nonsterile BIs grew out between 72 hours and 168 hours of incubation. Using the probability table in Part I of this study, the authors projected that 66% of the nonsterile BIs would contain only one surviving CFU (1). However, only 0.97% of the nonsterile BIs exhibited a delayed grow-out time.

The grow-out time results for all BIs exposed to EO gas are illustrated in the vertical scatter plots in Figure 1, Panel C. A graphical expression of these data appears in Figure 2, Panel C.


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