64 ESSENTIALS OF BACTERIOLOGY 



It is well to pass the mouth of the tube and the cotton plugs 

 through a flame, scorching the latter before reinserting. 



After the needle has been withdrawn, the plugs are reinserted 

 and the tubes labeled with the kind and date of culture. 



Plate Cultures. This method once common is now seldom 

 or ever used, and has been superseded by the Petri dishes; -as 

 a matter of history the description is retained. Several tubes 

 of the* culture-medium are made liquid by heating in water- 

 bath, and then inoculated with the material as follows. A 

 looped platinum needle is dipped into the material and then 

 shaken in the tube of liquid media (gelatin, agar, etc.). 



This first tube is called original. From this three drops 

 (taken with the looped platinum rod) are placed in a second 



Fig. 30. Manner of holding plugs. 



tube, the rod being shaken somewhat in the gelatin or 

 agar; this is labeled first dilution (a colored pencil is useful 

 for such markings) . From the first dilution three drops are 

 taken into a third tube, which becomes the second dilution 

 (Fig. 29). 



The plugs of cotton must be replaced after each inoculation, 

 and during the same must be carefully protected from contami- 

 nation. 



Glass Plates. The larger the surface over which the nutrient 

 medium is spread, the .more isolated will the colonies be; 

 window glass cut in rectangular plates 6x4 inches in size is 

 used; about ten such plates are cleaned with dry towel and 

 placed in a small iron box or wrapped in paper, and steril- 



