Ultra Rapid Self-contained Biological Indicator for Steam sterilization processes. Geobacillus stearothermophilus (ATCC 7953) 10^6 spores per vial. Readout: 20 minutes.An optional visual pH color change confirmation could be made after 48 hours of incubation. If sterilization process has not been successful, culture medium will change to a greenish color first, and then to yellow during incubation at 60 ºC, thus showing the presence of living spores. If sterilization process is successful culture medium will remain purple after the incubation process.