VIII. 
CULTURES IN SOLID MEDIA. 
Tue 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 isa 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 “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 material containing the bac- 
teria to be planted in the gelatin medium. We may ob- 
tain our seed for a pure culture from a single colony, from Fic. 89, 
another stab 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 with the sides of the tube. It is best 
