European Pharmacopoeia - Biological Indicators of Sterilization for steam sterilization -> 15 minutes kill time - Why?

GOOD VALIDATION PRACTICE (cGVP) EQUIPMENT VALIDATION
1

Dear all,

Biological indicators are meant to show a worst case resistance against a sterilant such as saturated steam, etox etc…

EP 5.1.2 states:

It is verified that exposing the biological indicators to steam at 121 ± 1 °C for 6 min leaves revivable spores, and that there is no growth of the reference micro-organisms after the biological indicators have been exposed to steam at 121 ± 1 °C for 15 min.

I understand that the European Pharmacopoeia requires a minimal survival time of 6 minutes at 121 ± 1 °C (to show that the Biological indicator is resistant enough).

Why is there a requirement for killing time of 15 minutes at 121 ± 1 °C? Hard to kill biological indicators would be still OK to use during validation (i.e. worst case)?

USP does not have such a requirement it seems, or do they?

Thanks,

2

Here is a good article about it. It has to do with assurance level (think overkill method) and the D-value of the spore strips.