CULTURE MEDIA 49 



Bouillon needs to be passed through several thicknesses of filter-paper, some- 

 times repeatedly, while it is hot and again when cool. Gelatin and agar are 

 cleared by one passage through a single thickness of filter-paper. Filter-papers 

 should be moistened with water and allowed to stand a few minutes before media 

 is poured on them. 



If the white of egg is added and the media then boiled for 5 minutes before 

 filtration, much of the suspended matter is caught in the coagulum that forms 

 and less filtration is required. At least one egg to every 1000 cc. of media 

 should be used, the whites well beaten and then thoroughly mixed with the 

 media by stirring. 



When egg has been added gelatin and agar can be cleared by filtering through 

 cotton, provided it is done carefully. A piece of gauze should be placed in a 

 funnel, upon this a piece of cotton about J^ inch thick, with all the fibers 

 running in one direction, should be placed. Upon this place a second piece of 

 cotton the same thickness, with its fibers at right angles to those of the bottom 

 layer. Moisten with water and smooth the edges to the side of the funnel; 

 then, gradually run culture media upon it so as to prevent the media from 

 escaping between the cotton and side of funnel. Filtration through cotton is 

 quicker than through paper. 



Blood serum to be used in the fluid state and other media that cannot be 

 heated are both cleared and sterilized by filtration through unglazed porcelain 

 tubes. 



CONTAINERS FOR CULTURE MEDIA 



Media kept in bulk are placed in flasks of from 100 cc. to 1000 cc. capacity; 

 otherwise they are kept in test-tubes. These containers should be made of 

 alkali-free glass, be clean and clear. They are plugged with cotton and steril- 

 ized in a hot-air sterilizer before use. The plugs are made of clean non-absorbent 

 cotton. A smooth piece of cotton is placed across the mouth of a tube and 

 pushed in with a pencil or similar instrument; the resultant plug will be tight 

 or loose, according to the size of the piece of cotton, and this must be regulated 

 so that the plug, extending into the tube from }/% to i inch, will be firm enough 

 to permit lifting of the tube by grasping the cotton and still not so tight that it 

 cannot be replaced with ease when removed. . The cotton plug is made to extend 

 about ^ inch above the top of the tube so that it can be readily removed and 

 held without touching the portion that enters the tube. 



Media in cotton-plugged containers tend to dry out, the smaller the bulk, 

 the more rapidly does it dry out. 



When necessary this is guarded against by placing rubber caps over the top 

 of the tube and plug or by dipping the top of the tube and plug in melted 

 paraffin. 



Culture media keep best when placed in a clean refrigerator. 



Should the portion of a tube that the plug comes into contact with be wet 

 with culture media, more or less cotton adheres when the plug is removed, in- 

 terfering with the introduction and removal of bacteria. This is avoided by 



