Commercial biological indicator spore strips in glassine envelopes, produced by three manufacturers, were evaluated by fraction-negative procedures after being heated at 121.0 ± 0.05°C. Only one type of spore strip met the manufacturer's specifications. The strips of one manufacturer were further evaluated by fractionnegative and survivor curve-plate count procedures after being heated under several conditions (enclosed in glassine envelopes, in trypticase soy broth plus 0.0015% bromocresol purple, in Trypticase soy broth alone, in Water for Injection, directly); Trypticase soy broth plus bromocresol purple and tryptic soy agar, respectively, were used as recovery media. The heating condition affected the D-value of the spore strip. Recovery procedures also had an effect; in all cases, the D-values obtained from the survivor curve tests were larger than those obtained from fraction-negative tests carried out under the same conditions. To determine if the differences in D-values between the two evaluation procedures were caused by the recovery media, we evaluated, by both methods, one type of spore strip heated directly and in glassine envelopes, using tryptic soy agar plus bromocresol purple and Trypticase soy broth plus 1.5% agar, respectively, as the recovery media. The survivor curve results showed that for both enclosed and unenclosed spore strips, there was a marked difference between the two recovery media; however, there was no difference when fraction-negative tests were used.

The efficiency of four culture media for recovery of heat-activated and heated Clostridium sporogenes spores was studied. Yeast extract agar gave the highest spore recovery. The effect of the method of preparing the yeast extract agar on the recovery of heated spores was also evaluated. The results indicate that (a) a significantly lower spore recovery was obtained when the dextrose was omitted completely or when added to the medium before autoclaving, and (b) no significant difference in spore recovery was found between yeast extract agar freshly made or prepared and stored at 4 C up to 11 days before use.

SUMMARY— Conidia of Aspergillus flavors were exposed to the gamma radiation of Cobalt‐60 and to 1 Mev electrons generated in a resonance transformer accelerator. When the spores were irradiated in water there was a linear relationship between radiation dose and logarithmic survival of the spores, with a D value equal to 38 Krad for both the gamma rays and the electrons. In the absence of surrounding water the resistance of the spores to radiation was higher than in the presence of water and the dose‐survival curve was not linear. There was no appreciable pH effect in the range of 3 to 7 on the radiation resistance of the spores. When the spores were irradiated in dextrose solution a small increase in their radiation resistance was observed as the sugar concentration was raised from 0 to 40%.