Abstract

The exponential inactivation of homogeneous populations of microorganisms by means of sterilisation is pre-conditioned in (industrial) microbiology in order to experimentally prove a required sterility assurance level (SAL) of ≤ 10^-6 for a “sterile” product [1]. In praxi, biological indicators (BIs) with a spore concentration of approx. 10⁶/BI are used to prove experimentally that this SAL can be reached by the chosen sterilisation method. An achieved inactivation from 10⁶ spores m. a. to around 1 survivor will be extrapolated to the required 10^-6 in order to prove the desired SAL (see above). It has to be kept in mind, that the experimentally proven inactivation of 10⁶ spores represents only 0.0001  % of the theoretically required inactivation of 10¹² spores in order to gain a sterile product: From 10⁶ initial spore population to 10^-6 theoretically surviving spores, a total of 10¹² (12 decimal steps) spores are to be inactivated. By using quantal response data, this inactivation curve could – depending on how many BIs are involved – be extended for another 1–2 decimal steps (in biology, “quantal” means “all or nothing”).

When the obtained quantal results are calculated according to mathematical guidance given in normative guidelines, e. g. [2], it turns out that the pre-conditioned exponential spore inactivation cannot be proven within the range of quantal response. Thus, the postulate of an exponential spore inactivation from initial spore concentration of 10⁶ to theoretically 10^-6 surviving spores might be questioned.

In the present publication, slight modifications of the mathematical “tools” to be used to assess quantal data response should enable the user to prove that quantal sterilisation responses “behave” exponentially, too.

Korrespondenz:

Dr. Michael Pfeiffer, Boehringer Ingelheim Pharma GmbH & Co. KG, Div. Human Resources Germany (HPZ 4329-EG-03), Binger Str. 173, 55216 Ingelheim am Rhein (Germany); e-mail: michael.pfeiffer@boehringer-ingelheim.com