Bacillus stearothermophilus

Bacillus stearothermophilus (or Geobacillus stearothermophilus) is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes. The bacteria is a thermophile and is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It is commonly used as a challenge organism for steam sterilization validation studies and periodic check of sterilization cycles. The biological indicator contains spores of the organism on filter paper inside a vial. After autoclaving/sterilizing the cap is closed, an ampule of growth medium inside of the vial is crushed and the whole vial is incubated. A color change indicates the results of sterility, no growth of the spores or non-sterility, growth of the spores.

An example of this type of biological indicator using this bacillus is Getinge's Biosign Steam-24.