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 microorgan- 

 isms i. 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 a rule, 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 

 microorganisms in the absence of spores, when they are in a moist 

 condition or moist heat is 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 microorganisms in a desiccated condition is a differ- 

 ent matter and will require special consideration. 



