186 A MANUAL OF BACTERIOLOGY 



be observed microscopically in a hanging- drop preparation ; 

 the organisms first lose their motility and soon become 

 aggregated into large masses or clumps. Macroscopically, 

 the reaction may be followed in a narrow test-tube into 

 which the mixture of culture and serum has been intro- 

 duced ; after some hours the micro-organisms become 

 aggregated into masses so large as to form visible flocculi. 

 The substances which bring about this agglutination are 

 known as agglutinins. Agglutinins seem to be present 

 in small amount in normal serum ; for instance, most 

 normal human sera up to a dilution of 1 in 2 or 1 in 4 will 

 agglutinate the typhoid bacillus and still more powerfully 

 the glanders bacillus. They are also present in bacterial 

 cultures ; if an old broth culture of typhoid be filtered, 

 the filtrate agglutinates the bacilli in a fresh broth culture ; 

 hence young cultures should always be used for agglutina- 

 tion tests. Agglutinin is formed by the action of antigen 

 derived from the bacterial cell, but may also be naturally 

 present. Agglutination is brought about by the action 

 of the agglutinin on the antigen ; the agglutinin first 

 unites with the antigen, and this may occur at C., and 

 afterwards exerts its specific action, which takes place 

 only at higher temperatures and in the presence of certain 

 salts. The agglutinable substance is known as aggluti- 

 nogen. Agglutinin is converted into agglutinoid at 70- 

 75 C. ; the latter does not agglutinate, though it unites 

 with bacteria and then prevents the subsequent action of 

 agglutinin. 



The agglutination of organisms by anti-sera, though 

 hardly specific, is usually very special ; given proper 

 precautions as to dilution, time-limit, condition of test 

 culture, etc., an anti-serum will generally only agglutinate 

 the homologous organism or closely allied species that is, 

 it is a group reaction. The anti- serum may agglutinate 

 both the organism with which it has been prepared, and 



