70 APPLIED BACTERIOLOGY 



precaution a great deal of labour may be rendered useless, 

 and the work has to be done entirely afresh. 



In the culture of the bacteria the store of nutrient 

 material necessary for their growth becomes gradually used 

 up by the vital activity of the organisms, and their gradual 

 development and reproduction comes to a standstill. Some 

 of the bacteria die from want of nourishment ; while 

 others, as has already been shown, develop permanent 

 forms, or ' spores,' which are able to remain quiescent for 

 long periods of time until favourable conditions of growth 

 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 in 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 



