44 
PRINCIPLES OR BACTERIOLOGY 
posure; the third exposure makes the sterilization com¬ 
plete. Remember that in noting the time of exposure 
it is necessary to calculate, not from the time of light¬ 
ing the flame, but from the time “the steam is up”; 
that is, from the time the temperature had reached 100° 
C. When substances to be sterilized can not be sub¬ 
jected to the temperature of 100° C., e. g., culture media 
containing albuminous materials (because they will 
coagulate above 60° C.) the principle of fractional 
sterilization may be adhered to by immersing the ob¬ 
jects in a water bath at a temperature of 55° to 60° C. 
for an hour on five or six consecutive days. 
Steam under pressure is the most powerful and cer¬ 
tain method of sterilization we possess. 
It is used when the objects can not be injured by mois¬ 
ture—operating room gowns, dressings, etc.; in the 
laboratory this method is used for infected glassware. 
The apparatus for steam sterilization under pressure 
is called an “autoclave (Fig. 7). Exposure to steam 
at the pressure of fifteen pounds for fifteen to twenty 
minutes will destroy all bacteria and spores. 
Two precautions to be borne in mind in connection 
with the use of autoclave, are: 1. Allow all air to es¬ 
cape from the autoclave before closing the vent. 2. Do 
not open the door for at least fifteen to twenty minutes 
after the flame has been turned off, as the sudden re¬ 
lief of pressure thus produced may burn the operator 
and the stoppers (in tubes or flasks) will “pop out.” 
II. Bacterial Destruction by Chemical Agents 
Many chemicals can either completely destroy the 
bacteria (bactericides or germicides) or merely inhibit 
their growth (antiseptics). Just how such injury is 
done to bacteria is, to a large extent, unknown; some 
