THE TECHNIQUE OF SERUM REACTIONS 259 



After incubation for two or three, hours similar quantities are again 

 measured into tubes of melted agar with the capillary pipette. With a 

 little practice, great accuracy in these measurements can be acquired. 

 The inoculated agar tubes are very thoroughly mixed, and plates are 

 poured. At the end of twenty-four hours' incubation, an enumeration 

 of the colonies in the various plates is made and the results are compared. 



The in vitro bactericidal tests have been employed, practically, chiefly 

 in the diagnosis of typhoid fever by Stern and Korte. 1 While the serum 

 of normal individuals shows practically no bactericidal power for 

 typhoid bacilli, the sera of typhoid patients may be actively bacteri- 

 cidal in dilutions as high as 1 : 50,000. 



Hemolytic Tests. Determination of the hemolytic action of blood 

 serum, bacterial nitrates, and of a variety of other substances, such as 

 tissue extracts and animal and plant poisons, is frequently made in 

 bacteriological laboratories. Familiarity with the methods of carrying 

 out such tests is especially essential since hemolytic tests are also em- 

 ployed in determining other serum reactions, such as the "complement- 

 fixation tests " discussed in another section. 



For these tests it is necessary to prepare washed red corpuscles 

 of the species of animal against which the hemolysins are to be 

 tested, and to obtain these, blood may be taken in one of the 

 following ways: 



A. If small quantities of blood corpuscles are desired, the blood may 

 be received into a sterile test tube into which a copper or other wire bent 

 into a loop at the lower end has been introduced. This is used to 

 prevent clotting and to remove the fibrin. Immediately after receiving 

 the blood into this tube, the wire is twirled between the fingers so that 

 the blood is beaten by the wire as by an egg-beater. At the end of five 

 minutes of continuous agitation, the fibrin adhering in a mass to the wire 

 may be lifted out. The corpuscles are then washed and centrifugalized 

 in several changes of salt solution to remove all traces of serum, and 

 are finally emulsified in salt solution. 



B. The blood may be taken into a centrifuge tube and immediately 

 centrifugalized before clotting has taken place. The plasma is then 

 poured off and the corpuscles are washed with salt solution, as before, 

 to remove the serum. 



C. The blood may be taken directly into a solution containing 

 five-tenths per cent sodium chlorid and one per cent sodium citrate. 



1 Stern und Korte, Berl. klin. Woch., 1904. 



