THE ANTIGENIC STRUCTURE OF THE HEMOLYTIC STREPTOCOCCI 675 



There is also (Avery and Heidelberger 1923) a nucleo-protein antigenic component, 

 precipitable from extracts by acetic acid. It is probably situated deeply within the 

 intact bacterial cell. It is shared by all pneumococci and by many other bacteria, 

 including all those species of streptococci that have been examined. 



The picture of the antigenic structure of the species Sir. pneumonicB that emerges 

 from these studies may be tentatively outlined as follows : There is a central 

 protoplasmic portion of the cell which, in its antigenic relationships, is neither 

 species- nor type-specific. Situated probably at the cell surface, there is another 

 component, mainly carbohydrate in nature, but containing nitrogen and phosphorus, 

 that is specific for Sir. pneumonicB as a species. External to this, in the normal 

 smooth forms, there is a capsule, composed wholly or in part of a polysaccharide 

 that is specific for each pneumococcal type. There are, we must suppose, over 

 seventy of these capsular polysaccharides within the pneumococcal species ; pro- 

 bably there are many more. The antigenic behaviour, and to some extent the 

 virulence, of the intact pneumococcal cells are, it should be noted, determined by 

 these capsular antigenic components, so that they are of particular importance to 

 the medical bacteriologist. 



The Antigenic Structure of the Haemolytic Streptococci. — ^When we come to 

 study the antigenic structure of the haemolytic streptococci we are on much more 

 difficult and debatable ground ; in part because, as we have already indicated, it 

 is by no means easy to define exactly what we mean by a haemolytic streptococcus ; 

 in part because, if we accept the usual definition — the occurrence of /3-haemolysis on 

 a blood agar plate — we shall include in our haemolytic group, not one species, but 

 several ; in part because the technical difficulties of antigenic analysis are far 

 greater than in the case of the pneumococcus. In spite of all these difficulties a 

 great advance in our knowledge has been made during recent years ; and, though 

 we cannot as yet present any clear and detailed picture, we can provide a sketch-plan 

 which, with the necessary modifications, will certainly provide the basis for any 

 future, and more complete, classification. 



To obtain a clear picture of the present position, it will be better to disregard 

 the sequence in which our knowledge has been reached. 



Group Relationships. — The observations of Hitchcock (1924) and of Lancefield 

 (1928, 1933, 1941) have revealed the presence of a number of different serologically 

 active polysaccharides in haemolytic streptococci from different sources. Instead 

 of being responsible, as in pneumococci, for type-specificity, each polysaccharide 

 is common to a group of organisms derived from a particular source. Thus, the 

 majority of strains isolated from pathological lesions in man share the same poly- 

 saccharide and are classified as Group A, Strains from mastitis in the cow possess 

 another polysaccharide and are classified as Group B. Strains from infections 

 in lower animals possess still another polysaccharide and are classified as Group C 

 and so on. Altogether 12 groups have now been recognized, each with its peculiar 

 polysaccharide antigen (Table 37). 



The association between the type of polysaccharide and the source of the strains is 

 close, but not absolute. Human beings, for example, may be infected occasionally with 

 organisms belonging to Groups B, C, D, and G, though Group A strains far outnumber 

 the rest. Moreover, when infection with these other types occurs, it is often confined 

 to one part of the body. Thus, Group B strains are seldom found except in infections 

 of the female genital tract ; Group D strains are restricted mainly to cystitis and wound 

 infections ; and Group G strains to genito-iu-inary and occasional throat infections. 



