108 MOLECULES, VffiUSES, AND BACTERIA 



is a major constituent of spores. I want to discuss each of these 

 observations. 



From early studies it was known that spores suspended in a growth 

 medium would lose their resistance to heat in a relatively short time. 

 In looking into this further, Hills ( 1949 ) found that yeast extract had 

 the same effect upon spores. He then proceeded to fractionate this to 

 determine what nutrients were responsible for this eflFect, and he dis- 

 covered that spores would germinate in a few minutes in the presence 

 of a few amino acids, with or without nucleosides and glucose. L- 

 alanine and adenosine were found to be sufficient for some species. 

 This was studied in more detail by the late Joan Powell ( 1956 ) and 

 co-workers. They found that a large percentage of the spores in a sus- 

 pension ( in the presence of the proper nutrilites ) would simultaneously 

 lose their heat resistance and refractility, and at the same time would 

 become stainable. Many investigators have confimied these findings 

 since they were made early in 1950. In addition, the minimal germina- 

 tion requirements for other species have been determined. In most 

 cases germination can be initiated by a few amino acids along with 

 nucleosides, although some species will germinate with glucose only. 



As a result of the studies on the germination requirements, we 

 have also gained a fairly good insight into the eflFect of environmental 

 factors upon germination. It is necessary, at this point, to indicate that 

 "germination" is used here in a somewhat diflFerent sense from that 

 commonly meant in the past. In the older literature the word was used 

 to describe all of the changes taking place during the development 

 from the spore to the fully grown vegetative cell. But because the 

 changes observed by Hills and later by Powell and other workers occur 

 very early in the process, bacteriologists have now come to use the 

 term germination to encompass these early changes only, and to use the 

 word "outgrowth" to represent all the other changes. 



A number of investigators have shown that many species of spores 

 require a heat shock as a sensitizing mechanism before they will re- 

 spond to germination nutrients. The length of the time of heating and 

 the temperature required vary with diflFerent species, with the age of 

 the spores, and with the conditions of storage. Freshly grown spores of 

 Bacillus cereus can be adequately sensitized by heating to 65° C. for 

 about 15 minutes. On the other hand, spores of Bacillus stearofhermo- 

 philus have to be heated to 100° C. or more for an equivalent length of 

 time (Ordal, personal communication). Most workers in the field be- 

 lieve that this heat sensitization initiates biochemical reactions which 

 either release compounds to stimulate germination or alter the perme- 

 ability of the spore wall. In any event, precise information as to what 

 actually does happen is not available. 



The rate of germination is notably influenced by the temperature. 



