VI. 
STERILIZATION OF CULTURE MEDIA. 
A MOST important part of bacteriological technology consists in 
the sterilization of the various culture media employed. A sterile 
medium is essential for maintaining a pure culture, and we can only 
obtain an exact knowledge of the biological characters of a species 
by studying its growth in various media, its physiological reactions, 
its pathogenic power, etc., independently of all other microérgan- 
isms—v?.e., in pure cultures. 
We may sterilize a culture medium either by heat or by filtration 
through a substance which does not permit bacteria to pass. The 
last-mentioned method is useful for certain special purposes ; but, in 
general, sterilization of culture media, and of the vessels in which 
they are preserved, is effected by heat. 
The scientific use of heat as an agent for sterilizing our culture 
media depends upon a knowledge of the thermal death-point of the 
various microorganisms which are liable to be present in them, and 
upon various facts relating to the manner in which heat is applied. 
All this has been determined by experiment, and before giving 
practical directions for sterilization it will be well to consider the 
experimental data upon which our methods are based. 
As arule, bacteria which do not form spores are killed at a com- 
paratively low temperature. Thus, in a series of experiments made 
by the writer upon the thermal death-point of various pathogenic 
organisms, the pus cocci were found to be the most resistant, and all 
of these were killed by exposure for ten minutes to a temperature 
of 62° C, (143.6° F.). There are several species of bacteria known, 
however, which not only are not killed by this temperature, but are 
able to grow and multiply at a temperature of 65° to 70° C. (Miquel, 
Van Tieghem, Globig). But it is safe to say that exposure to a 
boiling temperature for a minute or two will infallibly destroy all 
microérganisms in the absence of spores, when they are in a moist 
condition or moist heat ts used—i.e., when they are directly ex- 
posed to the action of boiling water or of steam. The power of dry 
heat to destroy microérganisms in a desiccated condition is a differ- 
ent matter and will require special consideration. 
