CHAPTER XVI. 



THE NATURE OF THE SUBSTANCES CONCERNED IN 

 AGGLUTINATION. 



THE agglutinating substances, which develop in animals because of 

 bacterial infection, have proven of such value in the identifying of 

 bacteria and the detection of bacterial infection that a knowledge of 

 them is of great practical as well as theoretical importance. (See pages 

 81-83 for technique of investigation.) 



The agglutinins were discovered by Gruber and Durham. Their 

 effect on bacteria can be observed either macroscopically or microscopi- 

 cally. For example, if a serum from an animal which has passed 

 through a typhoid infection is added to a twenty-four-hour culture of 

 typhoid bacilli, and the mixture placed in a thermostat, the following 

 phenomenon will be noticed: The bacteria, which previously clouded 

 the bouillon uniformly, clump together into little masses, settle to the 

 sides of the test-tube, and gradually fall to the bottom until the fluid 

 is almost entirely clear. In a control test, on the contrary, to which 

 no active serum is added the fluid remains uniformly cloudy. The 

 reaction is completed in from one to twelve hours. If the reaction is 

 observed in a hanging drop, it is seen that the addition of the active 

 serum first produces an increased motility of the bacteria which lasts 

 a short time and is followed by a gradual formation of clumps. Fre- 

 quently one sees bacteria which have recently joined a group make 

 violent motions as though they were attempting to tear themselves away; 

 then they gradually lose their motility completely. Even the larger 

 groups of bacteria may exhibit movement as a whole. After not more 

 than one or two hours the reaction is completed ; in place of the bacteria 

 moving quickly across the field, one sees one or several groups of abso- 

 lutely immobile bacilli. Now and then in a number of preparations one 

 sees a few separate bacteria still moving about among the groups. If the 

 reaction is feeble, either because the immune serum has been highly 

 diluted or because it contains very little agglutinin, the groups are small 

 and one finds comparatively many isolated and perhaps also moving 

 bacteria. It is essential each time to make a control test of the same 

 bacterial culture without the addition of serum. Under some circu in- 

 stances the reaction proceeds with extraordinary rapidity, so that the 

 bacilli are clumped almost immediately. By the time the microscopic 

 slide has been prepared and brought into view, nothing is to be seen 

 of any moving or isolated bacteria, and only by means of the control 

 test is it possible to tell whether the culture possessed normal motility. 

 As to the nature of these phenomena a number of theories have been 



