Evaluation of Biological Indicators and Sterilization Process

A biological indicator is defined as a characterized preparation of a specific microorganism that provides a defined and stable resistance to a specific sterilization process. Biological indicators (BI) can be used to:

  • assist in the performance evaluation of the sterilization equipment such as the autoclave
  • monitor established sterilization cycles and in periodic re-validation of sterilization process.

Evaluation of Biological Indicators

Each new batch of biological indicators that are used in good manufacturing practices (GMP) operations should be qualified by performing a population verification count. A count ensures the count recovered meets manufacturer’s claims of the Bacillus population inoculated on the carrier. Counts are performed by resuscitation of the Bacillus, serial dilution and pour plate method.

Biological indicators evaluation and count tests are conducted in accordance with manufacturer’s instructions or USP <55> Biological Indicators – Resistance Performance.

Monitoring Steam Sterilization Cycles Using Biological Indicators

Biological indicator

Biological indicator

Generally a biological indicator for evaluating moist heat sterilization will contain viable spores of Bacillus subtilis, Bacillus atrophaeus or Geobacillus stearothermophilus.  

BI growth evaluation determines the viability of Bacillus spores after exposure to the sterilization process. This method is performed by placing spore strips or inoculated process control devices (PCD) into sterile broth; or in the case of BI’s contained with their own medium system, the medium is released to contact the BI. All results are compared against an untreated spore strip or BI control.

For routine use, it is normal to examine for presence or absence of growth to ensure the sterilization process was performed adequately. It is important to include a positive control with the group of process indicator BI.


  1. USP, Chapter 55
  2. ISO 11138-1:2006 Sterilisation of Healthcare Products – Biological Indicators – Part 1: General Requirements.

Sample Requirements

Viable BI sent for verification counting are recommended to be sent in triplicate. BI and PCD should be sent in contained vessels.

Glass vials should be wrapped to prevent cracking. Untreated BI to be sent and tested in conjunction with sample as a control

Biological Indicators Testing Services
Test Description Turnaround Time
Spore strip counts 3 days
Biological Indicator Evaluation 3-7 days (as per manufacturer’s instructions)

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