76 



CULTURES IN SOLID MEDIA. 



neck of the little flask along the side of the cotton filter (see Fig. 4G) 

 and applying gentle heat to the bulb. The slender neck is first ste- 

 rilized by passing it through a flame, and the point is broken off 

 with sterile forceps. After inoculating the liquefied medium in the 

 test tubes in the usual manner we may make plates or roll tubes. 



Cultures on Cooked Potato. The method of preparing pota- 

 toes for surface cultures has already been given (page 48). It was 

 in using them that Koch first got his idea of the importance of solid 

 media, which led to his introduction of the use of gelatin and agar- 

 agar and the invention of the plate method. By means of streak 



FIG. 46. 



cultures upon potato he had succeeded in obtaining isolated colonies 

 and pure cultures. We now use the potato chiefly for the purpose 

 of differentiating species. Some bacteria grow on the surface of 

 cooked potato and some do not. Those which do present various 

 characters of growth. Thus we have differences as to color, as to 

 rapidity of growth, as to the character of the mass formed thick 

 or thin, viscid, moist or dry, restricted to line of inoculation or ex- 

 tending over the entire surface, etc. 



Instead of using a cut section of the potato in the manner here- 

 tofore described, we may make a puree by mashing the peeled and 

 cooked tubers and distributing the mass in Erlenmeyer flasks. After 



