11 



R form has up to the present stage of the investigation failed 

 to stimulate the production of protective substances against a 

 virulent strain, whereas in the same time the injection of the S 

 form produced a strongly protective serum. 



3. As regards the influences which cause attenuation of 

 virulence, I have found that this result can be produced invariably 

 by growth in specific immune sera. I have also noted that R 

 forms occasionally appear after prolonged residence on ordinary 

 solid media. It is possible that similar changes may arise under 

 natural conditions, as, for example, during convalescence from 

 pneumonia. The study of the various influences which cause 

 exaltation or attenuation of virulence may be facilitated by 

 observing the occurrence of S and R colonies under different 

 conditions. 



4. This alteration from an S to an R type appears to be a 

 natural tendency with many species of bacteria. It can, I think, 

 be attributed to degenerative changes and is associated with the 

 loss of certain antigenic qualities. For example, while studying 

 the serological types of meningococci,* I subcultured separately 

 40 colonies of identical appearance and tested them against an 

 agglutinating serum prepared with the whole strain. With one 

 exception, all the daughter colonies absorbed the homologous 

 agglutinin. The exceptional colony, although it agglutinated as 

 well as any of the others, failed to lower the titre of the serum 

 for the parent strain. This last result I considered to be evidence 

 of a diminution of antigenic complexity. Now the relationship 

 of the R pneumococcus to the S is in some respects similar 

 to that of the variant meningococcus to the whole culture. A 

 serum prepared with the stock culture or with the S form of 

 pneumococcus agglutinates both R and S to the same titre. 

 The S form removes the whole of the agglutinin both for R 

 and S, but absorption with the R form has no effect on the titre 

 of the serum for S. The S and R possess related antigens, 

 as is shown by the agglutination test, but the R lacks an antigen 

 which is contained in the S. Before it can be said that the R 

 is simply an S form from which part of the latter's antigen 

 has been removed, it must be shown that the R type of antigen 

 is contained within the S form. A serum, prepared with R. 

 and agglutinating that form up to 1 in 640, barely agglutinates 

 the S form ; if present in the S form the R antigen therefore 

 is not available, or its union with the R antibody does not 

 result in agglutination of S. Still, repeated absorption of R 

 serum with S reduces somewhat the agglutinin for R, thus 

 indicating that there are receptors of the R type present in the S. 



5. What are the distinguishing features of the part of the 

 antigen of the pneumococcus which is concerned in virulence ? 

 As one would naturally expect, in view of the intimate association 

 of a capsule with virulence, it appears to be the outer portion 



* Journal of Hygiene, XVII., p. 166, 1918. 



