l6 LABORATORY BACTERIOLOGY 



on the surface of the agar covered by the loop. This is an 

 agar culture of B. coli communis, {h) Stick cultures. These 

 are made with a platinum needle in the uninchned agar. The 

 impregnated needle is pushed down through the center of the 

 agar. In all other respects this culture is made like the slant- 

 agar culture. 



22. Inoculating tubes of gelatin. Tube cultures in gela- 

 tin are usually made without inclining the gelatin, i.e. stick 

 cultures. The tube of gelatin is inoculated in the same man- 

 ner as the stick culture in agar. This tube is to be placed in 

 the locker, as the gelatin will melt at the incubator tempera- 

 ture. The growth will appear along the needle track in about 

 two days. This is a gelatin culture of B. coli communis. 



23. Sealing culture tubes. It is often desirable to seal 

 cultures to prevent their drying out quickly. A convenient 

 method, and one which has long been in use in some labora- 

 tories, is to boil a small quantity of paraffin in a small agate- 

 iron dish and while it is still hot carefully dip the tube end 

 of the plug into it and quickly replace it in the tube. The 

 paraffin on coohng fills the spaces between the fibers of cotton 

 and also adheres to the sides of the tube, forming a tight plug. 

 AVhen the tube is to be opened the end must be warmed 

 slightly before the plug can be withdrawn. The plugs should 

 be paraffined and the sterility of the tubes determined before 

 they are used for cultures. 



