[Application of a new method for the calculation and description of the resistance of microbiological indicators. I. Testing of several common microbiological sterilization indicators (author's transl)].
The method described by SPICHER and PETERS (1975) for the calculation and description of the resistance of microbiological indicators was tested. As test objects served spore-containing earth according to DIN 58946, Attest indicators (3 M Company, Minnesota) and Oxoid Spore Strips (Oxoid Ltd., London). The tests were performed not only for different batches of indicators but also for preparations of different age. After application of steam (120 degrees C), the indicators were examined for the presence of surviving germs capable of multiplication. When plotting the frequency of indicators with surviving germs (q) against the duration of steam action, S-shaped curves were obtained as expected. By altering the scale of the ordinate (y = lg (-ln(1 - q))), the S-shaped curves could be transformed into straight lines. Thus, the experimentally established paired values could be used for a calculation of regression. This method of calculation proved to be suitable in all cases studied. By indicating the position and the slope of these straight regression lines, the resistance of microbiological indicators can be exactly described (cf. Table 2). This method is applicable not only to indicators containing culture spores but also for native spore-containing earth. The indicators examined differed in their resistance and stability. Seven out of eight batches of Attest indicators (cf Figs. 1 and 2 and Table 1) fulfilled the requirements of DIN 58946, Part 4, for the resistance of bio-indicators for steam sterilization. One of the batches had a slightly higher resistance. The Attest indicators tested were of good stability (see Fig. 1 and Table 1). Where surviving germs were present on the indicators after treatment by steam, their growth was recognizable, in 99% of cases, already after incubation of the cultures for 24 hours. Only two batches of Oxoid Spore Strips were available for testing. One batch was of a higher resistance than required by DIN 58946. The second batch was slightly above the lower limit of the permissible range (see Fig. 3). During storage for 12 months, the resistance of both batches was reduced by 3--4 min. Where the indicators exhibited surviving germs after treatment by steam, growth was recognizable in 87% of the cases after incubation for 24 hours, while for the other indicators, incubation for 48 hours was necessary. The experiments confirmed the good stability of native spore-containing earth (see Fig. 5). Within 4--5 years, the steam resistance of the preparations decreased only by 3--4 min.