[Suitability of Bacillus subtilis and Bacillus stearothermophilus spores as test organism bioindicators for detecting superheating of steam].

Biological indicators used to test sterilisation procedures for their efficacy consist of a so-called germ carrier to which the microorganisms used as test organisms adhere. In previous papers we demonstrated that carriers made of filter paper on contact with saturated steam show superheating while carriers made of glass fibre fleece as well as wetted filter paper do not. Using spores of Bacillus subtilis and Bacillus stearothermophilus as test organisms we have now investigated whether and to what extent carrier superheating affects the characteristic values (t50%) of these biological indicators. The indicators were exposed to saturated steam at 100 degrees C (B. subtilis) or 120 degrees C (B. stearothermophilus) under three different exposure conditions: 1. dry (i.e. conditioned to 45% relative humidity before introduction into the sterilising chamber), freely accessible; 2. dry with a substratum and a cover of filter card-board; 3. wet (moistened with twice distilled water before introduction into the sterilising chamber), freely accessible. For previously selected exposure periods, the incidence of indicators with surviving test organisms was determined. The reaction pattern of bioindicators with spores of B. stearothermophilus was different from that of bioindicators with spores of B. subtilis. For B. subtilis, the incidence of bioindicators exhibiting surviving test organisms depended on the nature of the carries as well as on the exposure conditions. On filter paper carriers, t50% increased in the order "wet, freely accessible", "dry, freely accessible", "dry, between filter card-board". On dry and wetted glass fibre fleece, resistance was approximately the same; when the indicators were sandwiched between layers of filter card-board, t50% increased. For B. stearothermophilus, t50% was largely dependent on the carrier material alone. The values obtained for filter paper were invariably much lower than those for glass fibre fleece. As the results show, using spores of B. subtilis it is possible to detect superheating, but the steam resistance of the spores is relatively low. Spores of B. stearothermophilus are of high steam resistance but they are practically unsuitable for detecting superheating. It is imperative to search for a test organism the resistance of which against steam is sufficiently high and which at the same time is capable of reacting to superheating (equivalent to reduced humidity) by a sufficiently large increase in resistance.