70 BACTERIOLOGY. 



into the less resistant vegetative cells. A second short 

 exposure to the steam kills these forms in turn, and 

 by a repetition of this process all bacteria that were 

 present may be destroyed without the application of 

 the steam having been of long duration at any time. 

 It should be remembered that while spores which 

 may be present are not directly killed by such an 

 exposure to heat as they experience in the intermit- 

 tent method of sterilization, still their power of ger- 

 mination is somewhat inhibited by this treatment. In 

 this method, therefore, if the temperature of 100 C. 

 be employed for too long a time, it is 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 con- 

 tain living bacteria. In this process the plan that has 

 given most satisfactory results is to subject the materials 

 to be sterilized to the action of steam, under the ordi- 

 nary conditions of atmospheric pressure, for fifteen min- 

 utes on each of three successive days, and during the 

 intervals to maintain them at a temperature of about 

 25-30 0. At the end of this time all living organ- 

 isms 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 sub- 

 stances 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 



