CRYTOBIOTIC STAGES IN BIOLOGY 3 



difference in the biological state of the individual cells. It is also 

 possible that the cells that die secrete substances which help to 

 preserve and protect those that remain. It has been shown both 

 with bacteria and protozoa that a few cycles of repeated drying 

 and rehydration will kill all the cells if no opportunity is given 

 for them to grow and repair damage between cycles. It is also 

 clear from experiments that have been performed on bacteria, 

 that damage can occur both during drying and rehydration as 

 well as during the period of storage in the dry state^. This is 

 clear from the varied quantitative results obtained with different 

 methods of drying and rehydration. The degree of killing during 

 drying is influenced greatly by the composition of the medium 

 in which the cells are dried. An abundance of colloids will 

 generally help to protect the cells; whereas metabolic end- 

 products, such as acids and other small organic compounds will 

 have an opposite effect, particularly as the concentrations are 

 increased with the removal of water. The protective action of 

 colloids, such as proteins, starches, and dextrins, may be due in 

 part to a counteraction of the toxic effect of accumulated meta- 

 bolic byproducts or to an antioxidant effect. The temperature of 

 the material during drying can also have a marked effect. In 

 general, the lower the temperature is during drying, the less the 

 damage. It is for this reason in part, perhaps, that many cells 

 can be preserved best when they are dried from the frozen 

 state. Here, however, one does need to consider the additional 

 damage that may occur during freezing. If cells are dried from 

 the frozen state, it should be remembered that the rate of 

 freezing is important as well as the nature of the menstruum that 

 is frozen. 



Damage is also incurred during rehydration. With dehydrated 

 bacteria and yeasts, the temperature and the rate of rehydration 

 are important factors in controlling the extent of damage. 

 Desiccated cells may sustain injury to their cell walls, so that 

 during rehydration, soluble constituents leach out^. If the 

 temperature and other environmental conditions are such that 

 growth can be initiated at the outset, it may be possible for the 



References p. 13 



