THE AGGLUTINATION REACTION 295 



Two methods may be employed: 



1. The microscopic method which is generally employed where the 

 Gruber-Widal reaction for typhoid fever has been employed, as the re- 

 action is quickly done and requires but a small amount of blood. 



This test is usually performed with serum separated from the clot 

 and in various dilutions (wet method) . The test may also be performed 

 with dried blood (dry method), the agglutinins being preserved and re- 

 dissolved with a diluent. The technic of the latter method is very 

 simple. The blood is easily collected and may be sent for long dis- 

 tances, and for these reasons the method has been adopted by many 

 boards of health. 



2. The macroscopic method is that generally preferred if sufficient 

 blood is on hand, and is the method of choice in scientific research. Ab- 

 sorption tests must be performed with the macroscopic technic. 



Requisites for Conducting Agglutination Reactions. (a) Bacterial 

 Emulsion. This may be prepared by growing the microorganism in 

 broth. Solid media may be used, and the culture washed off with sterile 

 salt solution and emulsified. 



(1) The bacterial emulsion should be prepared of young cultures, 

 should be homogeneous and free from clumps, and of such density as to 

 furnish a sufficient number of microorganisms to give the reaction (Fig. 

 81). 



(2) For the ordinary microscopic Widal test, eighteen to twenty- 

 four hour bouillon cultures of Bacillus typhosus, Bacillus coli, and Bacil- 

 lus paratyphosus yield uniform and satisfactory results. An old lab- 

 oratory culture one that is known to be agglutinable should be used. 

 Broth cultures should be cultivated at a temperature lower than body 

 heat, in order that long motile forms may be secured. During the sum- 

 mer and early autumn months the culture can be grown at room tem- 

 perature ; during the winter, on top of the incubator. 



Thick cultures are unsatisfactory for making the microscopic test, 

 as there is always some false clumping and motility is not well marked 

 (Fig. 82). 



When these tests are done routinely, it is good practice to subculture 

 in broth every day in order that a satisfactory culture may always be 

 on hand. When performed at irregular intervals, a broth culture can 

 be prepared from a stock agar culture and the test performed twenty- 

 four hours later. 



(3) Emulsions may be prepared of young cultures, on solid media by 

 removing portions of the growth with a platinum loop and emulsifying 



