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Geobacillus stearothermophilus is used for the validation of moist heat sterilization (121oc)

Bacillu subtilis varniger and bacillus atropheus are used for the validation of dry heat sterilization (160oc)

Bacillus pumilis is mainly used for the validation of ionization radiation sterilization

Geobacillus sterothermophilus is used for the hydrogen peroxide and peracetic acid.

Bacillu subtilisvarniger and bacillus atropheus are used for the validation of ethylene oxide and formaldehyde (gases and fumigants)

 Geobacillus stearothermophilus: This is most commonly used micro organism for validation of moist heat sterilization (autoclave). The D-Value of this organism is 1-2.8. The organisms are selected in the form of endospores means non vegetative state, in this state the micro organisms are very resistance to the temperature.

Following are some micro organisms respective to their D-Value:



Geobacillus stearothermophilus


Bacillus coagulants


Clostridium sporogenes


Bacillus atropheus



These micro organisms are packed in a protective device of known number, of a known resistance to particular sterilization process.

Example: 1) Fibrous paper impregnated with a bacterial spore suspension contained in a glass envelop.

2) Spore dots: circular pieces of fibrous paper impregnated with the spore suspension.

3) spore suspension: Pure spore suspension of desire challenge micro organism can be inoculated  on the surface of the material.

4) Self contained units containing spore strips or suspensions and the media in which they are to be incubated (simple/convenient to use).

The biological indicators should be placed in throughout load, adjacent to the thermocouples, at cold spots and slowest to heat location.

The most commonly used microbial spores are Geobacilus steriothermophilus (considered worst case), because it is most resistance organism. 

These micro organisms amainly used for validation of sterilization tecniquies. If those sterilization tecniquies kill 100% of micro organisms means should not present one micro organism also.