The introduction includes a description of the physics of freeze- drying together with the methods used and the variables that have to be controlled. The properties of lyophilised bacteria are discussed, with particular reference to the factors that affect the heat and radiation resistance of freeze-dried spores and the way in which these findings have been used to develop hypotheses concerning the mechanisms of spore inactivation. The experimental work is divisible into three parts. The first is concerned with physical measurements carried out on the drying system. The temperature and weight of the spore sample were monitored during drying. To explain the results obtained, a hypothesis of reversibly and irreversibly dehydrated sites within the spore has been introduced. The second section is concerned with the effects of different drying treatments on the biological response of Bacillus megaterium spores. The criteria used were viability, the characteristics of outgrowth and germination, together with the subsequent growth rate of the vegetative cells produced, and the heat and radiation resistance of the spores. Log survivor/heating time curves obtained in these experiments have been explained on the basis of the shoulder representing the time during which necessary structural changes occur in the spore before the lethal mechanism represented by the heat inactivation constant becomes operative. The effects of different drying treatments have been discussed in relation to this "model". The final section is concerned with a detailed study of the E.P.R. spectra of dried spores. The development of the resonance signal during drying and during subsequent heating of the dried spores has been considered together with the effect of drying conditions, oxygen, and rehydration on the resonance signal. Findings from these experiments have been discussed in the light of current concepts of the mechanisms of spore inactivation and of the influence of water removal on these mechanisms.
|Date of Award||1970|