ISOLATION AND CULTIVATION 101 



liquefied. Lay the tubes on a tray, the top resting on the side 

 of the tray so that the surface of the agar will be about 5 cm. 

 long, and allow it to cool. In placing the tubes the label 

 should be up. When the agar has set it is ready for use. It 

 is inoculated precisely as the bouillon, excepting that the 

 loopful of culture is drawn over the inclined surface instead 

 of being thrust into the medium as in the bouillon. Label and 

 place it in the incubator. On the following day there should 

 be a growth of the inoculated organism on the surface of the 

 agar. (b) Stick or stab cultures. These are made with a 

 platinum needle in agar that has not been slanted. The im- 

 pregnated platinum needle is pushed down through the cen- 

 ter of the agar. In all other respects this culture is made 

 like the slant-agar culture. 



Gelatin cultures. Tube cultures in gelatin are usually 

 made without inclining the gelatin, i. e., stick cultures. The 

 tube of gelatin is inoculated in the same manner as the stick 

 culture in agar. Gelatin cultures are not placed in the incu- 

 bator, but must be kept at room temperature. The growth 

 will appear along the needle track usually in from one to 

 four days. 



Inoculation of other media. When media in fermenta- 

 tion tubes, or in other retainers, are inoculated, the same 

 precautions to avoid contamination should be observed. 



Apparatus and methods for isolating bacteria. The more 

 common methods for isolating bacteria and procuring pure 

 cultures of each species present or of the species desired are 

 plate cultures and animal inoculation. Formerly, dilutions 

 in liquid cultures were employed. Certain species can be 

 obtained in pure culture by adding antiseptics to liquid 

 cultures in sufficient quantity to prevent the growth of all 

 organisms excepting the species in question. Sometimes it 

 is possible to isolate certain species by inoculating media or 

 material containing them with other bacteria and growing 

 the cultures at a high temperatui-e (40 to 42 C.), or by 

 heating the cultures sufficiently to destroy the vegetative 

 forms but not to injure the spores of the species in question. 



