To determine the relation between physiological state and viability, the fol- 

 lowing parameters should be examined: 



1. The effect of the particular portion of the growth cycle (i.e., exponential 

 vs. stationary phase cells, including cells maintained for extended periods in the 

 stationary phase) 



2. The effect of the growth medium (i.e., complex vs. simple) 



3. The effect of the type of energy-generating processes (i.e., fermentative 

 vs. respiratory) 



4. Resistance to oxidation (i.e., the presence of superoxide dismutase and/or 

 catalase) 



5. The effect of lyophilization on resistance 



Since exposure to the low temperatures and vacuum of outer space would effec- 

 tively lyophilize microbial cells, an examination of the role of the state of hydra- 

 tion on resistance and survival is of paramount importance. Experiments should 

 be planned so as to consider synergistic effects. The current literature contains 

 much information on the effect of various physical and chemical factors on the 

 survival of microorganisms. However, many of these studies were carried out 

 with "hydrated" cells and are probably irrelevant to the lyophilized state. The 

 degree to which hydrated and dehydrated cells exhibit different sensitivities to 

 various agents should be thoroughly tested. Another particularly important 

 parameter is temperature. The effects of very low temperatures on resistance and 

 survival should be studied. 



The selection of appropriate test organisms should be made with care. Ideally, 

 one should employ organisms that are easy to handle and which are well known 

 with respect to their genetics, nutrition, and physiology. In addition, organisms 

 exhibiting greater than usual resistance should be considered. The following two 

 organisms are possible prototypes: 



1. Escherichia coli, a gram-negative heterotroph with about average powers 

 of resistance and capable of reasonable survival in nature 



2. Bacillus subtilis, a gram-positive heterotroph, the vegetative cells of which 

 are no more resistant than those of E. coli, but which produces a structure, the 

 endospore, capable of surviving extremes of temperature, desiccation, and 

 nutritional deprivation 



Throughout the 3.5 to 4 billion years of life history on Earth, adaptation to 

 conditions comparable to outer space seems never to have been required. In view 

 of the lack of perfectly suitable test systems, one could turn to the use of organ- 

 isms adapted to growth or to survival, at least in extreme regions of the bio- 

 sphere such as in soil or rock from deserts or Antarctica, or in the upper layers 

 of the atmosphere. Airborne microbes commonly are in a resting or temporarily 

 inactive state, either as a spore with built-in resistance mechanisms against envi- 

 ronmental extremes or modified by desiccation and starvation. They are repre- 

 sented by endospores of bacteria and spores of actinomycetes, fungi, ferns, 

 mosses, pollen of flowering plants, and cysts of protozoa. Some of them are 



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