38 IMMUNOLOGY 



Among the pathogenic bacteria the S type is usually capsulated 

 and virulent, while the R type is avirulent. An apparent exception 

 to this occurs with B. anthracis. It should be noted, however, that 

 in this case the R or rough colony is composed of capsulated organ- 

 isms as are the virulent S forms of other bacteria and that the 

 avirulent anthrax, althougli forming smooth S colonies, is said to 

 be noncapsulated. This is another example of the correlation of 

 capsule formation with virulence. 



Labile Antigens and Virulence. — In 1934 Felix and Pitt 

 reported that virulent strains of E. tijphosa possess a surface anti- 

 gen not possessed by avirulent smooth strains. Because of its role 

 in virulence they named it the ''Vi'' antigen of E. typhosa. It is 

 destroyed by heat and most chemicals although formaldehyde is 

 only slightly injurious to it. Perhaps one of the ways in which it 

 confers virulence upon the bacteria is that it renders them more 

 or less resistant to antibody action. Numerous reports have 

 appeared in the literature stating that the .strains of E. tijphom 

 containing "Vi" antigen are more or less inagglutinable. Craigie 

 (1936) discovered a bacteriopliage specific for inagglutinable 

 strains of E. typhosa. 



The question of inagglutinability of strains of E. typhosa passess- 

 ing ''Vi" antigen has lieen investigated in this laboratory by 

 Faucett (1940). He confirmed the results of others that the 

 difference in agglutinalnlity of resistant strains depends partly 

 upon the "Vi" antigen content of the individual organisms. The 

 strains of intermediate resistance are mixtures of resistant and 

 sensitive strains. 



Some doubt as to the correlation of Vi antigens and virulence 

 has been raised by the studies of Robertson and Yu (1936) and of 

 Kauffmann (1936). The former report that its presence is not 

 primarily correlated with virulence of Bacillus typhosus while 

 Kauffmann found that it was present and not correlated with viru- 

 lence in SaJmonelln paratyphi C. 



Mudd, Pettit, Lackmann, and Morgan (1939) have discovered 

 a partially labile antigen in virulent streptococci. Its relationship 

 to other specific substances and to virulence is not definiteh' 

 determined. These findings suggest that perhaps labile antigens 

 play an important role in bacterial virulence in general. 



