[Superheating of germ carriers falsifies the steam resistance of bioindicators]

In the course of experiments on the resistance of bioindicators in saturated steam the authors noticed that carriers made of filter paper showed superheating by hygroscopic condensation. The differences in temperature between the bioindicators and the saturated steam amounted to about 5 Kelvin. Even after 20 minutes temperature equilibrium has not been reached. As a result of overheating and reduced water activity the frequency of bioindicators with surviving test organisms was increased. Beyond that, the microbiologic results depended on storage conditions of the bioindicators before putting them into the resistometer. The lower (higher) the-relative humidity, the higher (lower) the superheating, and the higher (smaller) the frequency of indicators with test organisms having survived. Carriers made of glass fibre fleece did not show any superheating and related effects. But carriers onto which small amounts of a suspension of test organisms in blood have been dried on, gave a superheating of about 1.7 Kelvin. Under identical conditions t50% values of bioindicators made of filter paper and bioindicators made of glass fibre fleece differed considerably. Experiments with Enterococcus faecium as test organism at 75 degrees C resulted in 14.4 and 1.7 minutes, respectively. Therefore, the high resistance of test organisms dried onto filter paper is an artefact caused by superheating. The experimental results should have consequences on the manufacture and use of bioindicators as well as chemoindicators and on the evaluation of results got with them.