VIII. 

 CULTURES IN SOLID MEDIA. 



THE introduction of solid culture media in 1881 by the famous 

 German bacteriologist, Robert Koch, inaugurated a new era in the 

 progress of our knowledge relating to the bacteria. His methods 

 enable us to obtain pure cultures with ease and certainty, and to 

 study the morphological and biological characters of each species 

 free from the complications which led to so much error and confusion 

 before these methods were introduced. We have already given an 

 account of the method of preparing and sterilizing the various solid 

 culture media, and are here concerned with the manner 

 in which they are used and the special advantages which 

 they afford. 



Koch's flesh-peptone-gelatin, which contains ten per 

 cent of gelatin, is a transparent jelly which liquefies at 

 from 22 to 24 C. It is a favorable culture medium for 

 a great number of bacteria, and many species show de- 

 finite characters of growth in this medium which serve to 

 differentiate them. One of the most prominent of these 

 characters depends upon the fact that some bacteria liquefy 

 gelatin and others do not. This is made apparent when 

 we make stick cultures also called "stab cultures." 

 This is the usual manner of inoculating a solid culture 

 medium, and is illustrated in Fig. 39. A platinum needle, 

 consisting of a piece of platinum wire inserted into a glass 

 rod which serves as a handle, is passed through the flame 

 of an alcohol lamp to sterilize it. When cooled, which 

 occurs very quickly, the point is introduced into the ma- 

 terial containing the bacteria to be planted in the gelatin 

 medium. We may obtain our seed for a pure culture FlG ^ 

 from a single colony, from another stick culture, from the 

 blood of an infected animal, etc. The point of the needle is then 

 carried into the sterilized jelly, as shown in the figure, care being 

 taken to introduce it in the central line and in a direction parallel 



