Materials and methods
. The Fedegari-manufactured resistometer is a model FOB 3 steam sterilizer (140 L) fitted with a device that allows the rapid
insertion and removal of BIs into the chamber while charged and under pressure (see Figure 4). The device consists of a loading
chamber, a guiding tube through the main chamber, and an exit chamber. The loading chamber is positioned on top of the resistometer.
The biological indicators are gravity-fed through the vessel by the precise coordination of the series of valves. The system
is computer controlled and allows the operator to program the exposure time. The system is capable of running prevacuum cycles
by evacuation of the loading chamber before insertion of the BIs into the main chamber. A Kaye Digistrip datalogger using
10 thermocouples was used to monitor the chamber temperature.
The Steris (Joslyn)-manufactured resistometers (model SB-001 and model JS-BIER-STM, Steris Corp., Mentor, OH) meet the performance
requirements as described in ANSI/AAMI/ISO 18472:2006. Only the load probe of the resistometers was used to monitor chamber
In each resistometer, exposures were performed in triplicate at temperatures from 121 to 129 °C. Parametric data collected
from these cycles were analyzed using the ANSI/AAMI/ISO standard as guidance.
Table I. Description of biological indicators (BIs).
. Three types of BIs—spore strip, self-contained, and glass ampul—from three manufacturers were used (see Table I). Spore-strip
BIs consisted of Geobacillus stearothermophilus spores inoculated onto paper carriers and packaged in glassine envelopes. Self-contained BIs consisted of G. stearothermophilus spores inoculated onto paper carriers placed into a thermoplastic vial that serves as a culture tube. A small glass ampul
containing sterile culture medium and color indicator also was in the vial. Glass-ampul BIs consisted of G. stearothermophilus spores suspended in liquid culture medium and sealed inside a small, thin-walled, pharmaceutical-grade glass ampul. BIs were
loaded into a cassette (see Figure 5) before insertion into the chamber. Upon completion of the cycle, the cassette was removed
from the chamber.
D and Z value analysis. Spore strip survivor curve.
In the Fedegari and Steris (JS-BIER-STM) resistometers, D 121-values were determined (in triplicate) on the spore strip BIs by the survivor-curve method according to ISO 11138 (2).
The survivor-curve method involves subjecting the BIs to a series of incremental exposure times followed by enumeration of
the surviving spores through the performance of a population assay. This method was chosen for the simplest BIs to evaluate
the linearity of the curve in each resistometer. Testing was as follows:
- BI lot# S1—D 121-value determined in triplicate in Fedegari resistometer
- BI lot# S1—D 121-value determined in triplicate in Steris (JS-BIER-STM) resistometer.