122 METHODS OF EXAMINING SERUM 



In comparing the effect of a serum on different bacteria, the 

 sedimentation method is usually employed. A series of 

 emulsions of the different bacteria to be tested is prepared by 

 scraping off the growth on an agar tube, and suspending in 

 bouillon. Each of these should contain approximately the same 

 number of bacteria per unit volume. This is attained by using 

 emulsions of equal capacity, as judged of by noting the point 

 at which transparency to some arbitrary standard such as a 

 particular type or set of parallel lines ceases. A given amount 

 of each emulsion is now mixed with different dilutions of the 

 serum to be tested, the mixtures are all made up to the same 

 volume, say 1 c.c., and the tubes placed at 37 C. for two or 

 three hours. The results are then read, the tubes are set aside 

 at room temperature for twenty-four hours, and read again ; 

 usually the two readings correspond. 



The Absorption Method of testing Agglutinins. This 

 method is applied under circumstances similar to those of the 

 last, namely, when several agglutinins acting on allied organisms 

 are present in a serum. The principle is to remove all the 

 agglutinins acting on one organism, and to study the properties 

 of those which remain. In practice, the method consists in 

 adding to the serum a mass of one of the bacteria of the group 

 under study (the organisms being scraped off an agar slope), 

 allowing the mixture to stand at 37 C. for two or three hours, 

 and then separating the bacteria with the centrifuge. The 

 supernatant clear fluid is now pipetted off, and its agglutinating 

 properties studied on the other members of the bacterial group 

 either by sedimentation or by the microscopic method. The 

 object of the method is to determine which member of a 

 bacterial group is causally related to the condition from which 

 the serum is obtained, and examples of its application for this 

 purpose will be found in the chapter on Typhoid Fever (p. 386). 

 It has also been used by Park and Collins and by Bainbridge 

 for identifying strains of organisms of the typhoid-coli group. 

 Here the principle is that, when an unknown strain belonging 

 to such a bacterial group is under investigation, if its capacities 

 for absorbing agglutinins from a serum containing a mixture 

 of such are the same as those of an already recognised strain, 

 then the two are probably identical. 



OPSONIC METHODS. 



Method of measuring the Phagocytic Capacity of the 

 Leucocytes. This was first done by Leishman by a very 



