STERILIZATION BY HEAT 83 



possible so to retard the germination of the spores as to 

 render it impossible for them to develop into the vegetative 

 stage during the interval between the heatings. By exces- 

 sively long exposures to high temperature, but not long 

 enough to destroy the spores directly, the object aimed at 

 in the method may be defeated, and in the end the substance 

 undergoing sterilization be found still to contain living 

 bacteria. In this process the plan that has given most satis- 

 factory results is to subject the materials to be sterilized 

 to the action of steam, under the ordinary conditions of 

 atmospheric pressure, for fifteen minutes on each of three 

 successive days, and during the intervals to maintain them 

 at a temperature of about 25-30 C. At the end of this 

 time all living organisms which were present will, as a general 

 rule, have been destroyed, and, unless opportunity is given 

 for the access of new organisms from without, the substances 

 thus treated remain sterile. As an exception to this, certain 

 species of spore-forming bacteria are occasionally encountered 

 that are not readily destroyed by this mode of treatment. 

 These species are found so uniformly in the soil that the 

 customary designation for them is that of "the soil bacteria." 

 This group includes a number of species that are endowed 

 with remarkable resistance to heat. Some of them are 

 probably thermophilic by nature, which would account not 

 only for the failure to destroy their spores by the ordinary 

 exposures to steam, but also for their slow and incomplete 

 development from the spore to the less resistant vegetative 

 stage during the intervals between the heatings, for, as a 

 rule, the materials containing them are kept at a temperature 

 during these intervals that is too low to favor the rapid 

 germination of the species having thermophilic tendencies. 

 As a result of the presence of these species, media that 



