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Biological Indicators (BI) Evaluation and Count

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Biological indicators (BI) are used to validate and monitor steam sterilization cycles, ethylene oxide and hydrogen peroxide sterilization cycles.  In Australia, it is expected that each new batch of BI that are used in GMP operations are qualified by performing a population verification count.  For routine use it is normal to examine for presence or absence of growth to ensure the sterilization process has performed adequately.  It must be remembered to include a positive control in with the group of process indicator BI. 

These are tested in accordance with USP <55> Biological Indicators – Resistance Performance Tests or manufacturer’s instructions.


Viable spores of B. subtilisB. atrophaeus or G. stearothermophilus are used to validate sterilization cycles.  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.

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.


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

Test Protocols
Method Code Test Description Turnaround Time
TME-155 Spore strip counts 3 days
TME-157 BI evaluation 3-7 days (as per manufacturer's instructions)