QO BACTERIOLOGY 



to avoid burning, adding hot distilled water from time to time 

 to compensate for the loss by evaporation. Instead of boiling 

 it is convenient to cook the medium in the autoclave at 110 C. 

 for 45 minutes to an hour. In either case, the agar should be 

 very completely dissolved. The medium is then cooled to 60 C. 

 and an egg previously beaten up in water is added and thor- 

 oughly mixed with the agar. It is then boiled again for 10 minutes 

 over the free flame, with constant stirring at the bottom, or for 

 45 minutes on the water bath, or for 15 minutes in the auto- 

 clave at 110 C. Distilled water is added to restore the original 

 weight, and the medium is then filtered, usually through a layer 

 of cotton wet with hot water, although filter paper may be used. 

 Filtration is favored by keeping the funnel hot, either with the 

 hot-water funnel heater or in a steam bath, and it may be hastened 

 by the use of suction. The filtrate need not be perfectly clear, 

 and it usually clouds on cooling unless it is acid in reaction. The 

 reaction should be alkaline to litmus. After filling into tubes or 

 flasks, agar should be sterilized in the autoclave at 110 C. for 30 

 to 35 minutes. 



Modifications of the Common Media. Broth is made nearly 

 free from sugar by fermenting the meat infusion over night at 

 37 C. after inoculating it with B. coli, and then proceding with 

 the filtrate in the usual way. This medium is designated as 

 sugar-free broth. Various sugars or other substances are added 

 to such broth in order to test the ability of bacteria to ferment 

 them. Acetic acid, 0.5 per cent, is added to broth to make a 

 selective medium for acid-resisting bacteria. Glycerin, 5 to 7 

 per cent, is added to broth for the cultivation of the tubercle 

 bacillus. Naturally sterile ascitic fluid or blood is added to broth 

 to promote the growth of certain types of microbes, and to en- 

 courage anaerobes. Bits of naturally sterile tissue are added 

 to broth for similar purposes. 



Gelatin is modified by the addition of various sugars, especially ' 

 dextrose and lactose, often with the further addition of litmus. 

 The production of acid by fermentation of the sugar is at once 



