GELATINE PLATE CULTUEES 67 



reappear. In order, therefore, to continue the propagation 

 of bacteria in cultures, it is necessary to reinoculate them 

 from time to time into fresh media. 



We have already described the preparation and sterilisa- 

 tion of the various solid and liquid culture media, and will 

 now give the manner in which they are used for the 

 isolation and study of organisms, and the special advan- 

 tages which they afford in particular cases. 



Gelatine Plate Cultures. — Three test-tubes, containing 

 nutrient gelatine, are placed in warm water at about 40° C. 

 ( = 104° F.) until the contents are liquid. This temperature 

 is sufficient to keep the gelatine liquid, but is not high 

 enough to destroy the vitality of the bacteria which are to be 

 experimented upon. The tubes are then numbered 1, 2, and 3. 



We next, by means of a platinum-wire loop, which has 

 been previously sterilised at red heat, introduce into tube 1, 

 after carefully withdrawing the plug, a small amount of the 

 pure culture or mixture of organisms which it is desired to 

 examine. Care must be taken not to introduce too much 

 of this material, as it must be remembered that the smallest 

 trace may contain millions of organisms. If the material 

 added is rather too coherent, attempts must be made to 

 separate the organisms by rubbing them with the point of 

 the platinum wire against the side of the tube below the 

 surface of the gelatine. The wire is again sterilised by pass- 

 ing it through the flame, and when cool it is again intro- 

 duced into tube 1, and a loopful of the gelatine transferred 

 to tube 2, and the contents well mixed ; after which a loop- 

 ful from tube 2 is transferred to tube 3. The reason for 

 using three tubes will now be apparent. ^J It is usually 

 impossible to introduce a few organisms into the first tube, 

 so we effect our object by dilution ; by the above proce- 

 dure we commonly succeed in so reducing the number of 

 organisms that only a few will develop upon the plate we 



5—2 



