38 AGRICULTURAL BACTERIOLOGY 



tainer. Such a method is impossible in the laboratory, 

 where glass containers must be used which must be easily 

 opened and closed, and yet be protected from contamina- 

 tion. This is accomplished by plugging the tubes and 

 flasks with cotton wool. The air will pass freely into the 

 container, but all definite bodies such as microorganisms 

 will be removed. The air is thus effectively sterilized. If 

 the cotton becomes damp, molds may grow on it and will 

 soon penetrate it, thus contaminating the contents of the 

 vessel. 



Water and many other liquids can be sterilized by passage 

 through filters of unglazed porcelain. Such a process is 

 sometimes used for the sterilization of culture media that 

 would be so altered by heating as to be rendered worthless. 



Sterilization by chemicals is possible, but can not be used 

 in the laboratory ; for if chemicals are added to the culture 

 media, they can not be removed or rendered non-effective 

 so as to permit the growth of bacteria in the media. 



Isolation of pure cultures of bacteria. As previously 

 stated, little can be learned concerning the relation of spe- 

 cific kinds of organisms to a particular type of decompo- 

 sition or to disease production until the organism in ques- 

 tion has been separated from all other kinds, and thus a 

 pure culture obtained. 



The most common way of securing such pure cultures 

 is by the use of gelatin or agar. The medium is melted and 

 cooled to 45 C. (113 F.), at which temperature agar 

 remains liquid, and which is not injurious to any of the 

 bacteria. A small amount of the material from which one 

 desires to isolate the pure culture is intimately mixed with 

 the liquid medium. This is then poured into a shallow flat 

 glass dish, which is allowed to cool. The agar quickly solid- 

 ifies, holding the contained organisms in .place. In the 

 favorable nutrient medium, growth occurs with the forma- 



