254 INFECTION AND IMMUNITY. 



any subsequent time. On the other hand, it is impos- 

 sible to preserve a culture of living bacteria so that 

 the number of the organisms and the virulence 

 of the culture remain constant, nor will two cul- 

 tures made at different times contain the same 

 number of cells in a given volume. Hence, stand- 

 ard cultures which are necessary for the systematic 

 valuation of serums are not easily available. One 

 may use a definite volume of a bouillon culture of 

 an organism which has grown for a certain number 

 of hours, but in all likelihood no two cultures 

 would contain the same number of organisms. 

 Pfeiffer uses the normal loop which has been men- 

 tioned, i. e., one which will take up from a surface 

 of agar two milligrams of the bacterial mass. The 

 culture must have grown for a definite period, 

 eighteen to twenty-four hours. Tests having some 

 value may be made in the test-tube with the fresh 

 or complemented serum. This, however, gives one 

 only the bactericidal power as it is manifested out- 

 side the body, and it may not be a correct index of 

 the protective power of the serum when it is in- 

 jected into the living animal. For the test-tube 

 experiment various dilutions of the serum are 

 made, as 1 to 10, 1 to 100 and 1 to 1,000, and a 

 similar quantity of each dilution, properly comple- 

 mented, is mixed with a given mass of the culture ; 

 the mixtures are then placed in the thermostat for 

 a number of hours. At the end of this time plate 

 cultures are made from each of the mixtures, the 

 plates put aside for twenty-four hours, and the 

 colonies which have developed are then counted. 

 The quantity of serum required to kill all the bac- 

 teria may be taken as the basis for computing its 

 bactericidal value. 



