THE PREPARATION OF CULTURE MEDIA 133 



The agar is thoroughly dissolved in 1,000 c.c. of distilled water. When 

 the agar is melted, the gelatin, meat extract, and salt are added and dis- 

 solved by further heating. Any loss in weight is then adjusted by the 

 addition of water. No titration or adjustment of reaction is necessary. 

 The medium should be cleared with the whites of two eggs, and filtered 

 through cotton. To the cleared medium is added one per cent of dex- 

 trose, and the medium tubed, about 8 c.c. to each tube, and sterilized. 

 Hiss' Tube Medium. The composition is as follows: 



Agar 5 gms. 



Gelatin 80 " 



Liebig's meat extract 5 " 



Sodium chloride . . 5 " 



Dextrose. 10 " 



Distilled water 1,000 c.c. 



The method of preparation is the same as for the plating medium. The agar 

 is thoroughly dissolved, and then the gelatin, meat extract, and salt are added 

 and dissolved. After adjusting the loss in weight, the volume should be deter- 

 mined, a careful titration made, and the reaction adjusted to one and five-tenths 

 per cent acid by the addition of y HC1 solution. The medium is cleared with 

 eggs, filtered, and one per cent dextrose added. It is then tubed and sterilized. 



Hesse's Medium. 1 The medium devised by Hesse for typhoid-colon 

 differentiation depends for its usefulness, as does the Hiss tube medium, 

 upon the great motility of the typhoid bacillus. It may be used directly 

 for the examination of feces or; as suggested by Jackson and Melia, 2 

 after preliminary enrichment of the material to be examined by the use 

 of the lactose-bile medium of Jackson. (See p. 138.) 



The Hesse medium is made up as follows: 



Agar . 5 gms. (4.5 gms. absolutely dry) 



Pepton (Witte) 10 " 



Liebig's beef extract. 5 " 



Sodium chloride. 8.5 " 



Distilled water 1,000 c.c. 



Jackson and Melia found that the use of 4.5 gms. of completely 

 dried agar give more uniform results, as the amount of moisture in 

 commercial agar varies. The preparation of the medium is as follows: 



Dissolve 4.5 gms. of dry agar in 500 c.c. of distilled water over a free flame, 

 making up for loss by evaporation. In another vessel 10 gms. of pepton, 5 gms. 

 of beef extract, and 8.5 gms. salt are dissolved in 500 c.c. distilled water. This 

 may be heated until dissolved and loss by evaporation made up. 



1 Hesse, Zeit. f . Hyg., Iviii, 1908. 2 Jackson and Melia, Jour, of Inf. Dis., vi, 1909. 



