THE PREPARATION OF CULTURE MEDIA 19 



and this is done by exposing the vessels containing the 

 serum to a temperature of 58-60 C. for from one to 

 two hours on five to six consecutive days. For this 

 purpose a water-bath may be used, but special serum 

 sterilisers are more convenient, and may be obtained 

 from any bacteriological apparatus maker. The solidi- 

 fication of the serum is effected at a temperature of 

 from 65-68 C., and can be conveniently carried out 

 in the same apparatus as the sterilisation. The tubes, 

 as in the case of agar-agar, may be sloped, care being 

 taken that the blood-serum is at least an inch distant 

 from the cotton-wool plug. Condensation water collects, 

 as in the case of the agar-agar, and this serves to keep 

 the air within the vessel moist, but if necessary its ac- 

 cumulation can be easily prevented by adding a small 

 quantity (1 per cent.) of gelatine, or 6 to 8 per cent, of 

 glycerin (Nocard and Eoux). The latter not only absorbs 

 a considerable proportion of the condensed water, but 

 prevents the formation of a dry scaly surface on the 

 serum. When glycerin is added to the serum it must 

 be heated to between 75 and 78 C. to produce its 

 solidification. Blood-serum prepared in this manner 

 will remain a long time unaltered, but the fresher it is 

 the better are the results obtained. It can, however, 

 be preserved for six to eight weeks without suffering 

 any material deterioration. A thin film of cholesterin 

 forms on the surface of the serum, but this must not be 

 mistaken for bacterial growth. 



Blood-serum can be adapted for plate or dish c.ultures 

 by means of Hueppe's modification. Hueppe uses a 

 mixture of blood-serum and agar-agar by taking sterilised 

 fluid serum at a temperature of 37 C., inoculating it, 

 shaking it in order to ensure the even distribution of 

 the organisms, and then pouring it into a fluid agar- 

 agar meat-peptone solution at 42 C. The mixture is 



c 2 



