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

Chlorine dioxide gas exposures. All ClO2 exposures were performed with paper spore strips and tubed media culture sets. The paper strips contained 106 B. atrophaeus spores and were packaged in Tyvek Mylar envelopes (see Table I).

Unexposed controls: The unexposed controls consisted of three lots of spore strip culture set BIs which were incubated in a conventional incubator at 37 2 C. The average time for the first nonsterile BI for the three lots was 5.5 hours. Ninety-five percent of all BIs were nonsterile in 6.1 hours. The incubation duration between the first nonsterile BI and 95% nonsterile was 0.6 hours. No delayed nonsterile BI's were observed in this series of tests.


Figure 1: Vertical scatter plots of grow-out time results for exposure to moist heat (Panel A), hydrogen peroxide vapor (Panel B), ethylene oxide gas (Panel C), and chlorine dioxide gas (Panel D). RIT = reduced incubation time. (ALL FIGURES COURTESY OF AUTHORS)
The grow-out time for the ClO2 unexposed controls was faster than the time required for the EO gas unexposed controls. The spores, carrier, and incubation conditions were the same. The recovery media used to culture the spore strip had a different formulation for the CIO2 BIs, which resulted in the faster grow-out time.

A vertical scatter plot for each of the exposures is shown in Figure 1, Panel D. A graphical representation of these data is illustrated in Figure 2, Panel D.


Figure 2: Graphical illustration of grow-out time results for exposure to moist heat (Panel A), hydrogen peroxide vapor (Panel B), ethylene oxide gas (Panel C), and chlorine dioxide gas (Panel D). RIT = reduced in incubation time.
Calculated survival time exposures: Two lots of ClO2 spore strip culture sets were used for these exposures. The average time for the first nonsterile BI for the two lots tested was 12.75 hours. Ninety-five percent of all BIs exposed to these conditions were nonsterile in 15.5 hours. The incubation duration from the first nonsterile BI to 95% nonsterile was 2.75 hours, which was 4.5 times longer than that observed with the unexposed controls. No delayed nonsterile BIs were observed.

FDA RIT protocol exposures: The RIT exposures were performed using four lots of spore strip culture set BIs. The average time for the first nonsterile BI for the three lots tested was 15 hours. Ninety-five percent of all BIs exposed to these conditions were observed nonsterile in 32.25 hours. The incubation duration from the first nonsterile BI to the 95% nonsterile was 17.25 hours, which was 6.27 times longer than that observed with the calculated survival time.

Three delayed nonsterile BIs were observed. Two BIs exhibited a delayed response in one lot and one BI in each of the other lots tested. Using the probability table in Part I of this study, the authors projected that 53% of the nonsterile BIs would contain only one surviving CFU (1). However, only 1.2% of the nonsterile BIs exhibited a delayed response. One lot of BIs was tested twice in this series, thus there are four sets of results but only three separate lots of BIs.

The grow-out time results for all BIs exposed to ClO2 gas is illustrated using vertical scatter plots in Figure 1, Panel D. A graphical representation of these data sets is shown in Figure 2, Panel D.


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