432 A MANUAL OF BACTERIOLOGY 



In Tube 1 of each row the serum acts in a dilution of 1 in 25. 

 ,,2 . linSO. 



3 1 in 125. 



4 1 in 250. 

 Tube 5 containing no serum is control against spontaneous 



agglutination. 



If the limit of agglutination is not reached within this series 

 higher dilutions are followed out in a similar manner. 



The tubes are examined after two hours at 50-55 C. followed 

 by fifteen minutes' standing at room temperature. The reading 

 is taken by comparing each tube in succession with the control 

 tube, and is preferably made by means of artificial light against a 

 black background. If daylight is used, the tubes inspected should 

 be partly shadowed by passing a finger up and down behind them. 



The highest dilution in which marked agglutination (without 

 sedimentation) can be detected by the naked eye is Standard 

 Agglutination. But owing to the rate at which the dilution 

 increases in the series of tubes employed it will commonly happen 

 that no tube in the series exhibits Standard Agglutination. If 

 this be so it will be found in looking along the series that while 

 one tube shows strong agglutination with sedimentation the next 

 succeeding tube shows no agglutination at all or only a trace. In 

 such cases Standard Agglutination lies approximately midway 

 between the two dilutions. 



(If the stand is left at the room temperature, sixteen to twenty - 

 four hours must be allowed before the reading is taken, but the 

 reaction is not then so sharply defined. In this case the highest 

 dilution in which a definite flocculent sedimentation appears 

 corresponds approximately to Standard Agglutination.) 



When the standard degree of agglutination (" Standard Agglu- 

 tination ") occurs with Standard Agglutinable Culture in a serum 

 dilution of 1 in x, then x divided by the figure given on the label 

 of the Standard Agglutinable Culture employed gives the number 

 of " Standard Agglutinin Units " * contained in 1 c.c. of the serum 

 examined. 



1 Note. The Standard Agglutinin Unit is that amount of agglutinating 

 serum which when made up to 1 c.c. volume with normal saline solution 

 causes Standard Agglutination on being mixed with 1-5 c.c. of a particular 

 Standard Agglutinable Culture and maintained at 55 C. for two hours in a 

 water-bath followed by fifteen minutes at the room temperature. 



