[How many biological indicators have to be tested to get reliable information on their resistance?]

Biological indicators are used in the efficacy test of microbicidal procedures. The indicators consist of an object carrying or holding micro-organisms which exhibit resistance to microbicidal agents. The biological indicators are exposed to the procedure to be tested and afterwards examined for viable germs. If test germs are still found to grow in the cultures, the microbicidal effect of the procedure is considered as insufficient. Biological indicators are suitable for such tests only if it is known how intensive the action of the microbicide has to be to destroy the test germs. The individuals of a germ population do not die at the same time under the action of a microbicide. This phenomenon can also be observed with germs simultaneously grown as a pure culture under identical conditions. Therefore, the biological indicators do not become sterile after one and the same period of action or dose of the microbicide but within a certain period or dose range. At the beginning of this transition range, sterile biological indicators will be found very rarely. With increasing period of action or dose, the frequency of indicators carrying viable germs decreases until, eventually, hardly any biological indicators with viable germs are detectable. When samples of identical biological indicators equal in size are exposed to one and the same period of action or dose of a microbicide, the number of indicators carrying viable germs will vary from one sample to another in the transition range. The number of biological indicators that has to be exposed to the resistance test per period of action and dose, respectively, in order to obtain reliable results, can be estimated only if the regularities are known by which the findings vary from one sample to another. Twelve different batches of biological indicators were employed to determine the variation of the resistance values obtained. Spores of Bacillus subtilis served as test germs. The batches differed in the number of spores per biological indicator. The microbicide used was saturated steam of 100 degrees C with a 9 min period of action. Forty-eight samples of five indicators each were taken per batch. The number of indicators carrying viable germs (n+) varied more or less and characteristic frequency distributions were observed (Table 2, columns 3 and 4). Afterwards, the mean relative frequency of indicators with viable germs was calculated for the different batches (Q; Table 2, column 2).(ABSTRACT TRUNCATED AT 400 WORDS)