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This study used biological indicators containing Geobacillus stearothermophilus spores and a new technology to continuously monitor incubated BIs and record nonsterile results.
Figure 2: Vertical scatter plots of incubation time of biological indicators (BIs) tested.
Grow-out time. The data presented in the left panel of Figure 2 give the results for the 11 sets of BIs that were not exposed to moist heat
sterilization prior to incubation; this set had a population of approximately 105 CFU/BI at the start of incubation. The first nonsterile BI was recorded at 2:16; the last nonsterile BI was recorded at
4:33.
Figure 3: Horizontal scatter plots of incubation time for the three ranges of surviving biological indicators.
The data presented in the center panel of Figure 2 give the results for the 10 sets of 100 BIs exposed to moist heat sterilization
processes that resulted in approximately 300 surviving CFU/BI at the start of incubation. The first nonsterile BI was recorded
at 2:53; the last nonsterile BI was recorded at 7:14 (see Figure 4).
Figure 4: Incubation times for biological indicators with 105 and 300 spores.
The data presented in the right panel of Figure 2 give the results for all 18 sets of 100 BIs exposed to moist heat sterilization
processes, where 30 to 80 nonsterile BIs were observed per 100 tested. At the start of incubation, the number of surviving
CFU ranged from 0 to ~6/BI. The first nonsterile BI was recorded at 3:35; the last nonsterile BI was recorded at 18:41. These
data are arranged in increasing order with respect to the absolute number of nonsterile BIs per test group. The left-most
data set in the panel reflects 33 nonsterile BIs; the right-most data set had 80 nonsterile outcomes.
Scatter plots of all of the grow-out times for each of the three sets of BIs are shown in Figure 3.
As can be seen in Figures 2 and 3, the BIs that were not exposed to moist heat sterilization (left panel of Figure 2 and bottom
plot of Figure 3) exhibited the shortest overall grow-out times (time recorded by the incubator system when growth was detected)
with relatively little intra- or inter-lot variability.
When the BIs were exposed to moist heat sterilization in a manner that resulted in ~300 surviving CFU/BI (center panel of
Figure 2 and center plot of Figure 3), the overall grow-out times were somewhat increased with an apparent increase in intra-lot
variability. The cause of the increase in grow-out time appears to be related to the lower number of surviving spores at the
start of incubation. These BIs began incubation with ~300 CFU versus the ~105 CFU for the unexposed BIs, a difference of approximately nine generations.
Figure 5: Summary of all biological indicator incubation times yielding 30–80 positives/100 units tested.
The BIs that were exposed to moist heat sterilization with an outcome of 30 to 80 nonsterile BIs/100 exposed (right panel
of Figure 2 and top plot of Figure 3) exhibited grow-out times that again increased with a concomitant large increase in inter-
and intra-lot variability. As above, the increase in overall grow-out time is due to the larger number of generations required
to produce enough of a pH change to be scored as nonsterile. The large increase in variability is undoubtedly related to the
large number of BIs in each test group that would be expected to have only one surviving CFU with a great potential to be
impaired with respect to speed of germination and/or generation time (see Figure 5).
John B. Kowalski, PhD, is principal consultant of SteriPro Consulting at Sterigenics International, Inc., in Oak Brook, IL.
Articles by John B. Kowalski
