[The significance of heat activation for the testing of bioindicators on surviving microorganisms exposed to formaldehyde].

Heat activation is a special phenomenon: After an additional heat treatment, a larger share of the bacterial spores which had been exposed to formaldehyde proves to be viable than without such heat activation. Model studies have been performed to test the effects of heat activation on the examination of bioindicators and test objects for surviving organisms. Test objects (cotton threads of 1 cm length) contaminated with spores of Bacillus stearothermophilus were used for these trials. The test objects were exposed to a 2% formaldehyde solution at 60 degrees C. After periods of action of 30, 45, 60 ... and 105 min, formaldehyde adhering to the test objects was neutralized. For testing these objects for surviving organisms, they were placed into a nutrient medium and incubated for 40 days at 56 degrees C. The investigation consisted of 2 parallel test series which only differed in one single point. In one series, the test objects were incubated at 56 degrees C as soon as they had been placed into the nutrient solution. In the other series, the test objects were exposed to a temperature of 95 degrees C for 1 h (heat activation) before starting incubation. The culture tubes were checked daily to see whether signs of growth (turbidity and deposits) could be observed. The frequencies of test objects with surviving organisms depending on the period of action of formaldehyde and the period of incubation determined in this way are based on the examination of 72 test objects each. Without heat activation, the share of test objects on which surviving test organisms could be detected, increased slowly with the period of incubation. Only after 30 days the counts did not increase any more when continuing the incubation (cf. Fig. 1). In the test series in which the spores had been subjected to heat activation before the incubation period, useful results were obtained already after 3 days. They only changed slightly when incubation was continued. Moreover, the frequency of test objects on which surviving organisms could be detected was always considerably higher than without heat activation. When the frequency of test objects with surviving organisms was plotted against the period of action of formaldehyde (cf. Fig. 2A), S-shaped curves resulted.(ABSTRACT TRUNCATED AT 400 WORDS)