574 STREPTOCOCCUS 



A great advance was made in the study of the antigenic structure of the pneumococci. 

 and, indeed, of bacteria in general, when Avery, Heidelberger and their colleagues attacked 

 the problem from the chemical side (Heidelberger and Avery 1923, 1924, Avery and 

 Heidelberger 1923, 1925, Avery et al. 1925, Heidelberger et al. 1925, Avery and Morgan 

 1925, Heidelberger 1927, Heidelberger and Goebel 1927). By suitable methods of extrac- 

 tion, followed by fractional precipitation, it was found possible to separate the capsular 

 components that determine type-specificity in a state of chemical purity. These com- 

 ponents were found to be complex polysaccharides ; and some of the main chemical and 

 physical characters of the capsular polysaccharides of the three classical types have been 

 determined (see Table 36). Solutions of these polysaccharides, it will be noted, give 

 specific precipitation, in high dilution, when mixed with the corresponding antisera. 



TABLE 36 



Characters of thk Type-specific Antigens of Pneumococci, after Avery and 



Heidelberger. 



The chemical constitution of the capsular antigens of the new types of pneumococci 

 differentiated by Cooper and her colleagues has yet to be determined, though a start has 

 been made with this work (see Heidelberger and Kendall 1931, Brown and Robinson 1943). 

 We may, however, safely assume that each pneumococcal type is characterized by a specific 

 capsular polysaccharide that determines its antigenic behaviour. We may identify these 

 types either by agglutination, or by the capsule -swelling reaction, or by precipitin tests 

 carried out with an autolysate, or extract, of the pneumococcal cells. 



Some, at least, of these pneumococcal polysaccharides may exist in immunologically 

 different forms, or may be altered during the process of chemical extraction and purification. 

 Thus, the studies of Enders ( 1930) , and of Wadsworth and Brown (1931), showed the presence 

 in Type I pneumococci of a specific antigenic component that differed in its immunological 

 reactions from the specific capsular polysaccharide as ordinarily prepared. Avery and 

 Goebel (1933) were able to show that this component is an ace ty la ted form of the Type I 

 capsular polysaccharide, and that it is apparently in this form that the polysaccharide 

 exists in the normal bacterial cell. The acetyl groups are removed by the methods of 

 extraction and purification that had been commonly employed, leaving a deacetylated 

 polysaccharide that is still specific for the Type I pneumococcus, but has lost certain anti- 

 genic activities possessed by the normal acetylated form. It is important to note that the 

 type-specificity of the capsular antigen is not destroyed by this particular chemical change ; 

 and it seems quite likely that similar minor alterations in chemical structure may be induced 

 during the extraction and purification of many other antigenic components. 



This polysaccharide capsular component is not, of course, the only antigenic constituent 

 of the pneumococcal cell. Tillett, Goebel and Avery (1930) have isolated another com- 

 ponent that gives all the usual reactions of a polysaccharide, is not inactivated by peptic 

 or tryptic digestion, and yields about 30 per cent, of reducing sugar on hydrolysis. It 

 contains about 6-1 per cent, of nitrogen, and differs from the capsular polysaccharides in 

 containing phosphoric acid. It is not type-specific, but appears to characterize the 

 pneumococcus as a species. 



