TYPHOID FEVER 



191 



FIG. 15 



CC 



10 



over. As the number of red cells in 1 c.c. of normal blood is about 

 5,000,000,000, and as the red cells and bacteria must be present 

 in the mixture in the same ratio to one another as in the original 

 units of volume, the number of bacteria per cubic centimeter of the 

 vaccine is ascertained according to the equation: Number of red 

 cells counted : number of bacteria counted : : 5,000,000,000 : x. This 

 method, of course, has no claims to accuracy, but it is the one 

 which is usually employed in titrating out vaccines. A more 

 accurate method has of late been suggested by Hopkins, which 

 seems to have many points in its favor. 



Hopkins' Method. 1 This is based upon 

 the concentration of a bacterial culture 

 by centrifugation and the preparation of 

 standard emulsions from the sediment. 

 To this end the washings from slant 

 cultures, after filtration through a small 

 cotton filter to remove larger clumps of 

 bacteria and particles of agar, are placed 

 in especially constructed centrifugation 

 tubes (prepared by the International 

 Instrument Company of Cambridge, 

 Mass., see Fig. 15), covered with rubber 

 caps, and centrifugalized for half an 

 hour at a speed of approximately 2800 

 revolutions per minute. The salt solu- 

 tion and bacteria above the 0.05 mark 

 are then removed and 5 c.c. of saline 

 solution measured into the tube, so as to make a 1 per cent, 

 emulsion. If the sediment does not reach the 0.05 mark its 

 volume is read on the scale and a corresponding quantity of 

 saline added to make the emulsion 1 per cent, in strength. By 

 means of a capillary pipette armed with a nipple the organisms are 

 forced into suspension, when the vaccine is transferred to a tube, 

 to be killed in the usual manner. 



Estimations of carefully counted suspensions obtained by centrifu- 

 gation in the above manner gave the following results: 



Special centrifuge tube with 

 graduated tip. 



1 Jour. Amer. Med. Assoc., vol. xl, No. 21, p. 1615. 



