STUDIES OF SPORE-PLANTS 281 



usually made up of the descendants of one bacterium which happened 

 to fall on the gelatin. Of course, two kinds of bacteria might 

 happen to be on the same dust particle which falls on the plate of 

 gelatin exposed to the air ; but microscopic examination of some of 

 the bacteria removed by a sterile needle would disclose that fact, 

 and it is easy to select spots which are pure cultures ; that is, made 

 up of one kind of bacteria. 



Now, if a sterilized inoculating needle be touched to one of these 

 spots and then stroked across the gelatin on a new sterile plate, or 



FIG. 87. A flat bottle may be used in place of a Petri dish for gelatin 

 plates. The mouth is stoppered with cotton. (From Osterhout.) 



in test-tubes, the bacteria transferred to the new plate will probably 

 be all of one kind, and when they multiply a pure culture will be 

 obtained. 



When once a pure culture is obtained, sub-cultures to new plates, 

 or to bouillon or gelatin in tubes, are easily made with a sterile in- 

 oculating needle ( 245). Great care must be taken when opening 

 plates and tubes, in order to avoid dust which may contaminate the 

 cultures with other bacteria. In research work with bacteria, special 

 glass cases are often used for keeping dust from tubes while transfers 

 of bacteria in pure cultures are being made. 



In case it is desired to separate the bacteria in a liquid, as in water 

 analysis, one or more drops of the liquid are poured on a sterile 

 gelatin plate, and the liquid is allowed to run across the plate. The 

 contained bacteria will start colonies along the path of the drop, and 

 after selecting colonies to be cultivated, some bacteria from them 

 are transferred to new sterile plates. Allow a drop of water from 

 a hydrant to flow across sterile gelatin in a Petri dish, cover 

 quickly and examine next day. Sometimes the number of colonies 

 of bacteria will be so great that it is impossible to select a colony 

 that is pure. In such a ease the drop of water to be examined must 

 be mixed with many drops of absolutely sterile water (distilled or 

 boiled) in order to scatter the contained bacteria, and a drop from 

 this run on a gelatin plate will give fewer colonies. 



