352 COLLECTED STUDIES IX IMMUNITY. 



should, and if a reduction of colonies from an infinite number or 

 many thousands to or very few has occurred. Furthermore the 

 test can only then be regarded as having a good result if the lower 

 limits of the amount of active serum have been reached, i.e., when 

 the last plates again show an increase in the number of colonies. 



A certain degree of control on the plate experiments is obtained 

 in suitable cases by placing the tubes (from which a few drops were 

 taken for sowing into plates) into a thermostat and observing them 

 the next day. In this case the culture controls show a luxuriant 

 growth, while in the other test-tubes, depending on the amount of 

 serum, either a growth will occur or not. This test-tube experi- 

 ment, of course, will only then show a result if the bactericidal 

 power of the serum was large enough to kill even the last germ 

 in the corresponding specimens. But if even only a few germs 

 remain alive (in consequence, for example, of a special resistance), 

 it will be found that these few, after the bactericidal substances 

 are used up, will again multiply enormously. Hence the test-tube 

 method cannot give reliable results in spore-bearing bacteria. 

 For the same reason it is important, in making plate tests, to 

 keep the tubes in the thermostat for a certain particular time, 

 which must be determined separately for each bacterium; for it 

 must be borne in mind that the killing of the bacteria can be 

 represented by a curve whose lowest point (lowest number of 

 living germs) must be approximately attained if marked results are 

 desired. Either side of this point, unless this point be 0, the results 

 will be correspondingly less. Smaller results, however, are worth- 

 less for all these experiments, as is seen when we consider that agglu- 

 tination, although it has so little directly to do with bactericidal 

 action, is also able to cause a decrease in the number of colonies on 

 a plate and thus simulate a decrease in the number of germs. This 

 is one of the reasons why the control described above with inactivated 

 serum, in which, of course, the agglutinin is still present, is so im- 

 portant. 



After the fresh active immune serum has been tested as to its 

 bactericidal power one proceeds with the examination of the inactive 

 immune serum plus complement. Inactivation is accomplished in 

 accordance with the principles laid down in the preceding chapter. 

 For complement one chooses first the normal serum of the species 

 from which the immune serum is derived. A preliminary trial will 

 then be necessary to show what dose of this normal serum can be 



