v] METHODS OF INOCULATION 59 



The whole is then put into an incubator, the temperature of 

 which does not reach above 21° or 22° C. (or the tempera- 

 ture of the room in the warm months), in order to insure 

 the gelatine setting and remaining so. If a trace of material 

 containing various species of bacteria is thus distributed into 

 several cc. of gelatine, each microbe fixed by the gelatine on 

 setting will start a separate colony after a few daj's' growth, 

 and the individual colonies, if different, will be appparent 

 by different characters, according to shape, colour, size of 

 the colonies, and according to whether they liquefy the 

 gelatine or not during their growth. In order to insure 



Fig. 12. — Two Gelatine Plate-cultures containing growing Colonies 

 ON THE Surface of Gelatine. 



success, it is necessary to infect the original gelatine in the 

 test-tubes with only a trace of the bacterial mixture; if too 

 many bacteria are introduced, their colonies sprouting up 

 are too numerous and soon become confluent. But if the 

 experiment is successful, the colonies are well separated from 

 one another, and from the individual and separate colonies 

 it is then easy by re-inoculation of gelatine tubes, or other 

 nutritive material, to start pure subcultures of the different 

 species. It must be borne in mind that not all bacteria can 

 be isolated by this method, for some species of pathogenic 

 organisms require for their growth a higher temperature 



