22 CLINICAL BACTERIOLOGY AND 



When we desire to obtain pure cultures, bacteria grown 

 upon test-tube media offer great difficulties. To overcome 

 these, plate cultivation is adopted. This consists of several 

 shallow glass plates, each provided with a lid. The principle 

 adopted is as follows : When suitable, gelatin is usually 

 employed, but agar also may be used. Take three tubes 

 of gelatin medium and melt in a hot-water bath. Gelatin 

 cools to 25° C, and agar to 45° C. Mark the tubes 1, 

 2, 3. No. 1 is inoculated with a trace of the material 

 from which it is desired to make the pure culture, and 

 completely mixed by rolling the tube between the hands, so 

 that it may permeate thoroughly the melted gelatin. Should 

 this mixture be plated out now, however, it will be found 

 to be too rich in bacteria, necessitating their adhering 

 together when grown, and obscuring their characteristics. 



Fig. 12. — Petri Dish, or Plate. 



To obviate this, a loopful of No. 1 tube is then taken 



to inoculate tube No. 2. A further dilution should be 



made by inoculating in a similar manner No. 3 from No. 2, 



taking up, say, a half-dozen loopfuls. The Petri dishes 



having been previously sterilized in a dry-air sterilizer, 



they are ready for use. The cotton-wool is now removed 



from the tube, and the mouth sterilized by passing 



through the flame of the spirit lamp, and the contents of 



each poured into separate culture plates, the lid being 



slightly raised for the purpose, and put aside to incubate. 



They should be examined daily with a hand lens or a low 



power of the microscope. The latter can be done by tilting 



the plate and placing it on the stage of the microscope. 



