824 



BRUCELLA 



antigenic variation of the S — > R type. If, as "Wilson and Miles (1932) showed, 

 care is taken to exclude all but absolutely smooth strains, then it is possible by means 

 of quantitative agglutinin-absorption tests to differentiate between Br. melitensis 

 on the one hand and Br. abortus and Br. suis on the other. The antigenic picture so 

 obtained is represented in Fig. 175. It will be seen that all three types contain the 

 same two antigens, but with a different quantitative distribution, the M antigen 

 being in excess in the melitensis, the A antigen in the abortus and suis types. By 

 carefully adjusting the absorbing dose to the titre of the serum, it is generally 

 possible to absorb out all the minor agglutinins without removing more than a 

 fraction of the major agglutinins. The resulting serum is therefore monospecific, 



Br abortus. Br suis. Br melileri^is. 



Fig. 175. — Schematic Representation of Antigenic Structure of Brucella Strains. 



and will agglutinate only those strains in which the corresponding antigen is pre- 

 dominant. Usually an absorbing dose standardized by opacity to match 3,000 X 10' 

 coli per. ml. is satisfactory for absorbing a serum diluted to 1/32-1/64 of its titre, 

 but preliminary adjustment may be necessary before a monospecific serum can be 

 obtained. With such a serum direct agglutination tests can be put up against 

 strains of unknown type, and their antigenic identity established. 



Why it is that a serum, from which the whole of the minor and part of the major 

 agglutinins have been absorbed, wUl agglutinate only those organisms in which the corre- 

 sponding antigen predominates, is rather puzzling. Why is it, for example, that a melitensis 

 serum, from which the minor A agglutinin has been absorbed by an abortus or suis strain, 

 will agglutinate only melitensis strains ? On general grounds, abortus and suis strains, 

 each of which contains a certain amount of M antigen, might be expected to agglutinate 

 with such a serum to at least a quarter or half titre, since there must be ample M agglu- 

 tinins in the serum to satisfy the limited number of M receptors on the organisms. The 

 fact that no agglutination does occur suggests that the quantitative and spatial distri- 

 bution of the satisfied M receptors is such that any considerable degree of adhesion between 

 adjacent organisms is improbable. This explanation is borne out by the observations 

 of Miles (1939), who finds that the antigenic A : M ratio in Br. abortus is about 20 : 1, 

 and in Br. melitensis about 1 : 20. If, therefore, the minor antigenic surface constitutes 

 only about 5 per cent, of the total, it may be supposed on the two-stage hypothesis (see 

 Chapter 7) that its sensitization is insufficient to decrease the salt stabihty of the suspension 

 beyond the critical point, or on the lattice hypothesis that the hnkages between the minor 

 antigenic groups on the heterologous bacteria are insufiicient to hold the individual bacteria 

 in clumps. 



The conclusions reached on the antigenic structure of these organisms have 

 received confirmation from the work of Miles (1933), who has shown that the 

 melitensis may be separated from the abortus-suis types by the optimal proportion 

 agglutination method, and from that of Habs (1933), Habs and Sievert (1935), 



