THE ENVIEONMENT OF BACTEEIA. 73 



teria have been especially isolated from garden soil. 

 Strange to say, some have been found in the soil, even at a 

 depth of 12 feet. They are present in the dust of rooms 

 and a few have been obtained from water, especially in the 

 case of hot springs. 



In the latter instance the necessary temperature is fur- 

 nished by the hot water It is more difficult, however, to 

 understand how the organisms of this group can obtain 

 their required high temperature when present in the soil. 

 The heat of the sun will not warm up the soil to this extent. 

 A considerable amount of heat may be generated by the 

 fermentative changes that occur on the surface of the 

 earth. The heat from this source may at times favor the 

 development of these organisms. It has been shown that 

 these bacteria, under anaerobic conditions, will grow at 

 considerably lower temperatures than when cultivated in 

 the presence of air. Thus, certain species which required 

 a temperature of 50 — 70° under aerobic conditions could be 

 grown as anaerobes at 34 — 44°. The anaerobic conditions, 

 as shown on p. 70, are readily supplied in the soil as a 

 result of microbic associations. This explains very satis- 

 factorily how the thermophilous bacteria can live in nature. 

 The growth of these organisms at temperatures which are 

 rapidly fatal to ordinary bacteria can only be due to the 

 presence of difficultly coagulable proteins. 



As shown above, a temperature of 0°, or less, stops the 

 multiplication of bacteria. These organisms, however, 

 may be exposed to extreme cold without loss of vitality. 

 In the case of bacteria, cold arrests the functions of pro- 

 toplasm but does not destroy its vitality. This is true not 

 only of the spore condition bui also of the vegetating form. 

 The typhoid bacillus may remain frozen in ice for months 

 and yet be, apparently, uninjured. The weakest individu- 

 als, of course, will die out first. Moreover, some species 

 are more susceptible to cold than others. Alternate freez- 



