BACTERIOLOGICAL TECHNIC. 



57 



B. Isolating Bacteria by the Plate Method. In order to separate or isolate 

 the several species and varieties of bacteria in any contaminated substance, 

 it is only necessary to dilute the inoculating material sufficiently. For this 

 purpose there is necessary, sterilized Petri dishes containing heat-sterilized 

 gelatin or other solid media through which the bacteria from the contami- 

 nated substance are disseminated in numbers so small that the colonies from 

 each and every microbe present may be visible to the naked eye (or aided 

 by a simple lens). This is done as follows: 



FIG. 34. Streak culture on agar in a Petri dish. (Delafteld and Prudden.) 



To obtain isolation cultures of air bacteria it is only necessary to expose 

 the Petri dish (with a layer of gelatin or agar-gelatin medium, sterilized) for 

 about two minutes, immediately closing the dish and setting it aside to await 

 developments. Making isolation cultures from contaminated solids or 

 liquids is not quite so simple. Proceed as follows: Liquefy the gelatin in 

 four or five test-tubes and keep them at a temperature of not more than 30 C., 



