A Biological Indicator (BI) is a well-characterized preparation of a specific microorganism with known resistance to aspesific sterilization process (USP Biological Indicators - Resistance Performance Tests).

BIs are regulated by: ISO 111-1:2017 Sterilization of health care product -- Biological Indicators -- Part 1: General requirements. USP pharmacopeia  Biological Indicator for sterilization. 

A BI is ready for use microbiological test system, consisting of spores that are added to a carrier (suspension, paper strip, glass, plastic or other) packed to maintain the integrity and the viability of inoculated carrier.

Microorganisms widely recognized as suitable for biological indicator are spore-forming bactreia, more resistant than microflora:

- Geobacillus stearothermophillus (steam, hydrogen peroxide sterilization)

- Bacillus athropeus (dry heat, ethylene oxide sterilization)

- Bacillus pumilus (gamma irradiation sterilization).

If the spores grow after a sterilization cycle the process is considered to have failed. On the contrary the sterilization process turns out to be successful if spores are completely killed after the sterilization cycle.

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