226 MECHANISM OF AGGLUTINATION 



quantity of normal serum (human or animal) required to produce agglutina- 

 tion must also be determined. 



For example, it can be shown that while normal serum frequently aggluti- 

 nates the typhoid bacillus in a dilution of 1 in 10 a typhoid serum will 

 agglutinate it in dilutions of 1 in 200, 1 in 500, and even in 1 in 5,000. 



(ii) It becomes even more imperative to dilute the serum when it is 

 recognized that a specific serum will agglutinate not only its corresponding 

 organism but also, not infrequently, closely related species, provided that 

 the quantity of serum used be sufficient [Griinbaum]. It is obvious therefore 

 that unless a serum be adequately diluted its specific characters will escape 

 recognition. 



Take an example : a patient is suffering from a para-typhoid infection. 

 His serum agglutinates both the typhoid and the para-typhoid bacillus in 

 dilutions of 1 in 20 and 1 in 50 : so far there is nothing specific about the 

 serum. Dilute the serum further, say to 1 in 100, 1 in 200, and 1 in 500. In 

 these higher dilutions it has entirely lost all its agglutinating property for 

 the typhoid bacillus but still agglutinates the para-typhoid bacillus. In 

 this case the specific nature of the agglutination is determined by the titre 

 of agglutination and not by the mere fact of agglutination itself. 



(iii) It is also of the highest importance in studying the phenomena of 

 agglutination that only homogeneous emulsions or cultures be used in which 

 the organisms are as far as possible lying separately, for if they be clumped 

 or massed together the results of the experiments will obviously be misleading. 

 This spontaneous clumping is a source of great difficulty when working at 

 agglutination with organisms which naturally grow in clumps. The difficulty 

 may be overcome either by using very young cultures in broth (typhoid 

 bacillus) or by having resort to one or other of the various methods which 

 have been devised for obtaining homogeneous cultures (of the tubercle 

 bacillus, etc.). 



(iv) Finally, in performing agglutination tests with serums care must be 

 taken to add the serum to the culture and never to add the culture to the 

 serum. It can be easily understood that in the latter case the first drops of 

 culture would be mixed with an undiluted serum and that agglutinated 

 masses of organisms might form even though there were no specific relation- 

 ship between the organism and the serum. 



The technique of serum diagnosis will be described in detail in the chapter 

 on the typhoid bacillus, and under the head of each micro-organism data 

 with regard to agglutination will be given. 



The mechanism of agglutination. 



It would appear that the phenomena of agglutination are not dependent 

 upon any vital activity of the organisms since they can be observed with 

 dead cultures. 



The substances in serums producing agglutination are known as agglutinins. 

 Agglutinins are distinguishable from bactericidal substances in that unlike 

 the latter they withstand heating at 55 C. for half an hour and are only 

 destroyed at about 60 C. in serum and 70 or 80 C. in milk. They are pre- 

 cipitated by alcohol and do not pass through a Chamberland or Berkefeld 

 bougie. But since they can be demonstrated in the milk, urine, etc., of 

 infected or immunized animals it would appear that they can pass through 

 certain living animal membranes. 



The phenomena of agglutination may be explained on the assumption 

 that the agglutinin acts on some agglutinatible substance present in the bodies 

 of the organisms agglutinated. Organisms which have been separated from 



