Experimental data and theoretical considerations concerning the validity of the SAL concept to characterize non-thermal antimicrobial treatments.

For sterilization processes the pharmacopoeias demand a sterility assurance level (SAL) of 10(-6), i.e. a probability of not more than one viable microorganism among one million sterilized products. This SAL concept is based on the assumption that the inactivation of microorganisms by physical or chemical means generally follows first-order kinetics. In this paper it is demonstrated that this is not absolutely true for non-thermal antimicrobial processes. Using Bacillus subtilis spore test preparations the sporicidal efficacy of gamma and ultraviolet irradiation on the one hand as well as the treatment by glutaraldehyde and hydrogen peroxide containing solutions on the other hand was investigated. A range of mean spore contamination between 10(8) and 10(-2) spores per test item could be supported by experimental data. It was demonstrated that the antimicrobial treatment parameters which are sufficient to reduce a high spore burden were not valid for an adequate reduction of the remaining lower spore burden. It is concluded that any extrapolation of such experimental data to the SAL range as usual in the validation of sterilization process parameters may be not permitted. Possible theoretical explanations of the non-homogeneity of the spore inactivation by non-thermal methods as well as consequences for the safety evaluation of sterilization processes are discussed.