Geobacillus stearothermophilus

By | July 25, 2021

Cell structure

Geobacillus stearothermophilus (previously Bacillus stearothermophilus)[1][2] is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes.


The bacterium 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 to 75°C. Some strains are capable of oxidizing carbon monoxide aerobically.

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. Recently a fluorescent-tagged strain, Rapid Readout(tm), is being used for verifying sterilization, since the visible blue fluorescence appears in about one-tenth the time needed for pH-indicator color change, and an inexpensive light sensor can detect the growing colonies.

Geobacillus stearothermophilus

Scientific classification

Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Bacillaceae
Genus: Geobacillus
Species: G. stearothermophilus


cteria are utilised in the biotechnology industry as sources of thermostable enzymes, as platforms for biofuel production, and as potential components of bioremediation strategies.

To date over 60 Geobacillus genomes have been sequenced, mainly to identify genes that could be used in different biotechnological applications.

These bacteria have a self-preservation technique – they can form endospores and can remain in this dormant stage for very long periods of time, until conditions for growth are favourable.

  1. stearothermophilus is used to verify decontamination of many laboratory processes as a biological indicator.


  1. stearothermophilus has not been observed to be pathogenic to any host.
  2. stearothermophilus has a significant role in the spoilage of food, especially milk and dairy products as it can survive the pasteurization process due to its heat-tolerance.


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 loop-mediated isothermal amplification (LAMP).[5] LAMP is similar to the polymerase chain reaction (PCR) but does not require the high temperature (96°C) step required to denature DNA.

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,[3] but, together with Bacillus thermoglucosidasius, it was reclassified as a member of the genus Geobacillus in 2001.

Biological Aspects

The amounts of the three aminopeptidases in Bacillus stearothermophilus vary according to the strain. Obligately thermophilic strains (temperature optimum, 55°C; no growth at 37°C) produce substantial amounts of API and APII but little APIII, whereas obligately mesophilic strains (temperature optimum, 37°C; no growth at 55°C) produce substantial amounts of APIII but very little API or APII. Facultative strains (growth at 37 and 55°C) produce all three of these aminopeptidases in comparable amounts [1]. The gene for API has not been cloned, and the reasons for such temperature-dependent expression of the API gene are still unknown. The enzyme activity may be regulated both by a variable Co/Zn ratio and by certain metabolites or membrane (especially hydrophobic) components.

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