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 will grow within a temperature range of 30-75 degrees Celsius. It is commonly used as a challenge organism for sterilization validation studies and periodic check of sterilization cycles. The biological indicator contains spores of the organism on filter paper inside a vial. After sterilizing, the cap is closed, an ampule of growth medium inside of the vial is crushed and the whole vial is incubated. A color and/or turbidity change indicates the results of the sterilization process; no change indicates that the sterilization conditions were achieved, otherwise the growth of the spores indicates that the sterilization process has not been met.

Biological indicators are used in conjunction with chemical indicators and process indicators to validate sterilization processes.

It was first described in 1920 as Bacillus stearothermophilus, but following a reclassification it is officially a member of the genus Geobacillus

Polymerase
Recently, a DNA polymerase derived from these bacteria, Bst polymerase, has become important in molecular biology applications.

Bst polymerase has a helicase-like activity, making it able to unwind DNA strands. Its optimum functional temperature is between 60 and 65°C and it is denatured at temperatures above 70°C. These features make it useful in isothermal Nucleic Acid Amplification. Isothermal amplification is similar to the polymerase chain reaction (PCR) but does not require the high temperature (96°C) step required to denature DNA.

The Bacillus stearothermophilus is the main target of milk sterilization