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The formation and storage temperatures of Clostridium botulinum spores are shown to influence their subsequent ability to germinate. Spores were formed at 10 degrees , 20 degrees, 30 degrees and 37 degrees C and following harvest were stored as aqueous suspensions at 20 degrees C (ambient temperature), 4 degrees C (refrigerated) or -20 degrees C (frozen) for periods of up to 1 month. The spores formed at 20 degrees C germinated most rapidly and to the greatest extent. When the spores were germinated immediately after harvest (fresh), there was no difference in the germinability of those spores formed at 20 degrees or 30 degrees C, whether or not they had been heat-shocked before use. However, following storage overnight or longer, differences in the relative germinabilities of the different spore samples were seen. Spores which had been stored at ambient temperature overnight germinated significantly faster and to a greater extent than did those which had been stored for up to 1 month. Similar differences were also observed between spores germinated fresh and those stored overnight, when the spores were stored refrigerated or frozen. Germinability was also influenced by the temperature of storage, since there were differences between spores formed at the same temperature but stored at different temperatures for the same period of time: for example, when spores which had been formed at 20 degrees C were germinated at 10 degrees C following a heat-shock, those which had been stored at ambient temperature germinated faster and to a greater extent than did those which had been stored refrigerated or frozen. It is concluded that there is a complex interaction between formation, storage and germination temperatures, which determines spore germinability. The fact that the changes are time-dependent and can occur in the frozen state is taken to mean that they are physico-chemical rather than metabolic. It is also significant in relation to refrigerated foods which are at risk from Cl. botulinum in that changes which occur during cool or frozen storage can enhance the germinability of spores if the temperature rises above that of chill cabinets.