140 BACTERIOLOGY. 
As pie ger cer to cultivating bacteria which method is employed, 
and why ? 
Sterilization by heat is the only method that is of value in this class 
of work. For, if chemical agents are used to sterilize and rid different 
nutrient media of all bacteria, these agents render the media so acted 
upon useless for the purpose of growing other forms of bacteria. 
What form of heat is most applicable to this form of work, and why ? 
Moist heat in the form of steam is best, for the bacteria and their 
spores are killed by a less degree of moist than of dry heat. 
Why is dry heat not used to any great extent for sterilizing pur- 
poses ? 
A temperature of 150° C. dry heat is necessary. to kill absolutely all 
bacteria and their spores. This high temperature is destructive to most 
substances that are to be sterilized in bacteriological work, and is there- 
fore only used in sterilizing glass and metal apparatus. 
In what way is moist heat used for sterilization ? 
It may be used under pressure as superheated steam and applied 
only once, or as live steam, which is of the same temperature as boiling 
water, and applied discontinuously. Lastly, moist heat may also be 
applied discontinuously, a little while at a time, at a temperature of 
60° C. 
When anise heat is used under pressure what temperatures are 
use 
It is found that temperatures of from 125° to 150° C. are sufficient 
to kill all forms of bacteria. 
What is the principle of discontinuous sterilization ? 
Most bacteria in their growing forms are killed by a short exposure 
to moist heat at from 60° to 100° C. The spores are not affected by 
these temperatures. A few hours after the first exposure some of 
these spores may develop into growing organisms and then may be 
killed by a second exposure. This process is kept up a number of 
days until all the spores have developed and have been killed. Most 
substances are sterilized by using live steam at a temperature of 100° 
C. Some kinds of nutrient media are injured by this temperature, 
therefore a temperature of about 60° C. is employed, and the process 
is repeated a greater number of times. 
What form of apparatus is used for sterilizing materials under 
pressure ? 
This apparatus is constructed of metal, and is of sufficient strength 
