PREPARATION OF MEDIA n^o 



for the sake of uniformity; for purposes of pure culture study, how- 

 ever, any brand known to give best results for the purpose at hand 

 may be employed. 



Beef-exiract agar may be of the same composition plus the addition 

 of 12 grams of oven-dried agar or 15 grams of commercial agar. The 

 agar is to be dissolved by heat (e.g. autoclaving) and the sediment 

 removed either by decantation or by filtration through cotton. 



Beef-extract gelatin may be of the same composition as the broth 

 but with the addition of 100 g. of "Bact5-gelatin" (or some other 

 gelatin of the same jellying strength; i.e., 175-200 by Bloom test). 

 Carefully adjust reaction (see below) after dissolving gelatin and 

 heat for 5 or 10 min. at about 100" C. Filter through cotton. 



Meat infusion broth. This is usually prepared as follows : Pour 1 litre 

 of water over 400-600 g. of lean beef or veal, ground through a meat 

 chopper. Allow to stand in a refrigerator overnight and then skim 

 off the scam of fat with a piece of absorbent cotton. Squeeze out the 

 infusion through a strong muslin cloth and make the amount up to 

 1000 ml. Dissolve 5 g. of peptone in this by adding the infusion 

 (without heating) little by little to the peptone in a mortar and 

 rubbing up with the pestle until the solution is complete. (When 

 making this bouillon as a basis of blood agar or for serological work, 

 one should also dissolve in it 0.5 g. sodium chloride.) Adjust reaction 

 (see below). Heat for about 20 minutes at about 100°C without 

 stirring; filter through wet filter paper and make up to 1000 ml. 



Meat infusion agar. In 1000 ml. meat infusion broth prepared 

 as above, dissolve 12 g. of oven dried agar (or 15 g. commercial agar) 

 by heating at about 100°C; filter off any sediment formed. 



Yeast-extract broth and agar. These may be made the same as beef- 

 extract broth and agar except for replacing the beef-extract with 2.5 g. 

 yeast-extract per litre. The latter should be used in powdered form, 

 as for example the product of the Difco Laboratories. 



Semi-solid agar. With some organisms, especially microaerophiles, 

 more successful cultivation can be obtained by means of semi-solid 

 media, containing only 0.2 to 0.5% agar. For such purposes any of 

 the above agar formulae may be followed, merely decreasing the 

 quantity of agar. The exact quantity of agar recommended varies. 

 Thus Hitchens' semi-solid medium (see p. 1I44-II) calls for 0.2%, while 

 Tittsler and Sandholzer (1936) employ a 0.5% agar for the macro- 

 scopic determination of motility: the latter is almost solid in con- 

 sistency. 



