CULTURE-MEDIA. 65 



boil thoroughly. Avoid breaking the coagulum of egg which 

 is designed to entangle the solid particles that make the medium 

 cloudy; stir at the bottom, however, to prevent burning. Filter 

 while hot, using filter-paper or absorbent cotton covered with 

 cheese-cloth. The hot water funnel originally devised for the 

 filtration of agar is not necessary. If filtration is slow, the fun- 

 nel and flask may be placed inside of the steam sterilizer and 

 kept heated during filtration. The medium is collected in suit- 

 able flasks or tubes plugged with cotton, and steriUzed once in 

 the autoclave or in the ordinary steam sterilizer for fifteen min- 

 utes on each of three consecutive days. As agar is frequently 

 used for smear-cultures where a slanted medium is desired, some 

 of the tubes may be allowed to cool in a slanting position. It 

 is not well to keep on hand many tubes which have been slanted, 

 as the medium dries more rapidly. Agar is seldom liquefied 

 by bacteria, though a few bacteria possess the power of doing 

 this. Its soKdifying qualities are impaired somewhat if the 

 reaction be acid. 



The remarks on pages 60 to 62 with regard to the use of 

 beef and the titration method for the preparation of bouiUon 

 apply equally to agar-agar. 



Glycerin-agar is used extensively. It is agar, made as 

 above directed, to which 6 per cent, of glycerin is added before 

 sterilization. It is very useful in cultivating the bacilli of 

 tuberculosis and diphtheria. 



Sugar-agar. — Before sterilizing, i per cent, of either dextrose, 

 lactose, saccharose or other sugars may be added to agar. 

 With media containing sugar, litmus forms a useful indicator 

 of the production of acid. Enough tincture of Htmus is used 

 to give the medium a blue color before sterilization ; the litmus 

 is somewhat unstable and prone to change its color during 

 sterilization. Neutral red may also be added in the same 

 manner; its color is said to be changed by certain bacteria and 

 not by others (see bacillus of typhoid fever and bacillus coli 



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