148 ANTIGENS AND THE TECHNIC OF SERUM REACTIONS 



to a suspension of an unknown organism, and characteristic clumping 

 takes place, a specific diagnosis of the serum or of the organism can 

 be arrived at. In the first instance, a diagnosis of disease may be 

 made; in the second instance the identity of an organism may be 

 established. The laboratory diagnosis of typhoid and paratyphoid 

 fever, of the various types of bacillary dysentery and of other bacterial 

 infections is frequently made by testing the serum of the patient 

 for agglutination with a known culture of the organism. 1 The labora- 

 tory identification of specific bacteria, conversely, is frequently estab- 

 lished or corroborated through their agglutination with known specific 

 agglutinating sera. 



The reaction of agglutination may be made either microscopically 

 or macroscopically. 



1. Microscopic Method. A drop of serum from a patient, diluted to 

 the proper degree, is mixed with an equal amount of a broth culture 

 of the desired organism on a clear cover-glass, 2 and then suspended 

 over the cavity of a hollow ground slide, ringed with vaseline to pre- 

 vent evaporation, and examined under the microscope. Motile bac- 

 teria, as for instance B. typhosus, soon lose their motility (immo- 

 bilization) and gradually collect in small groups which tend to coalesce 

 into larger and larger clumps, leaving the field between them practically 

 free from organisms. The bacteria are not necessarily killed by agglu- 

 tination. The reaction ordinarily is complete within two hours. 

 Killed cultures of bacteria may be used in place of living cultures 

 but the reaction is usually less clear-cut. 



The advantage of the microscopic method lies chiefly in the small 

 amount of serum required to perform the test. One of its chief disad- 

 vantages lies in the relative inaccuracy of the dilution of the serum. 

 (See chapter on B. typhosus for full discussion of technic.) 



2. Macroscopic Method. Various dilutions of serum, accurately 

 measured by volumetric pipettes, are brought into small, sterile test- 

 tubes, together with suspensions or broth cultures of the bacteria. 

 Agglutination is manifested by the gradual accumulation of a floccu- 

 lent sediment of bacteria, leaving the supernatant liquid perfectly 

 clear. Control tubes without serum remain uniformly clouded. 



The part played by agglutinins in immunity is unknown; the 



1 The technic and precautions to be observed are discussed individually in the chap- 

 ters upon specific pathogenic bacteria. 



2 For a majority of bacteria, eighteen-hour cultures in 0.1 per cent, dextrose broth 

 are particularly advantageous. Cultures grown in plain broth are usually much less 

 actively motile and agglutinate less readily. 



