84 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



After being placed in suitable containers the medium is 

 then ready for sterilization. The method used will depend 

 upon the character of the medium. Most media are steril- 

 ized by heating in the autoclave at one atmosphere pressure 

 for from ten to fifteen minutes. Care must be used in the 

 sterilization of sugar media, however, because at certain 

 hydrogen ion concentrations they are rapidly decomposed, 

 the disaccharides in particular being hydrolyzed to mono- 

 saccharides. Some media are sterilized by exposure either 

 for one long period or for short periods on successive days, 

 to streaming steam. Still other media, particularly those 

 containing blood serum, are solidified by heating at a tem- 

 perature of from 75 to 80 C., and sterilized by exposure to 

 these temperatures on several successive days. In some 

 cases it is necessary to sterilize the ingredients of the 

 medium separately arid mix them in 'the right proportions 

 after sterilization. 



PURE CULTURE METHODS 



It has previously been noted that it is exceptional in 

 nature to find a single kind of microorganism growing by 

 itself, that is, free from all other kinds. This sometimes 

 occurs with disease-producing bacteria in the blood or tis- 

 sues, and under some exceptional conditions of environment 

 it may be possible for only one kind of germ to grow. It 

 is evident, therefore, that if bacteria are to be studied they 

 usually must first be separated from each other and grown 

 in pure cultures. A mixed or impure culture is one in 

 which several kinds of organisms are growing together. 



It is possible sometimes to secure pure cultures by direct 

 transfer. For example, it is usually possible to get a pure 

 culture of a mold such as Mucor by touching a sterile needle 

 to the sporangia. A few spores will adhere and these may 

 be scattered over the surface of the culture medium upon 

 which the organism is to be grown. If the medium is suit- 

 able some at least of the young mold plants developing will 



