SALMONELLA ANTIGENS 105 



Although the Hpid A component of the endotoxins of the gram- 

 negative bacteria is essential for many of the endotoxic manifesta- 

 tions and can act as a potent adjuvant in the production of antibodies 

 (Westphal, 1960), the portion which determines the specificity of 

 protective antibodies is the polysaccharide component. Such anti- 

 polysaccharide antibodies do not protect the organism producing them 

 against the pyrogenic effects of endotoxin if it is experimentally in- 

 jected, but they do account for the species-specific immunity which 

 generally follows recovery from an infection with one of the micro- 

 organisms. Consequently it is the anti-polysaccharide antibodies 

 which are of greatest interest to immunologists. Considerable progress 

 has recently been made in the study of the chemical basis for the 

 immunological differences which are observed, especially in the group 

 of gram-negative bacteria known as the Salmonella. Before we can 

 discuss them we must pause to recall a few salient facts about this 

 important group of microorganisms. 



The Salmonella 



The Salmonella are Gram-negative, non-spore-forming, motile 

 bacteria which are generally pathogenic for both man and animal. 

 S. typJwsa, causative agent of typhoid fever, .S". paratyphi A, and 

 possibly ^. sendai, cause disease only in man. 



The Salmonella are mostly flagellated. The flagella as well as the 

 body of the organism contain antigens. The flagellar antigens are 

 called H antigens, the somatic antigens O antigens. The letters origi- 

 nated with German writers who observed that colonies of the motile 

 (i.e., flagellated) Salmonella on agar medium were surrounded by a 

 "Hauch" (breath or emanation), while colonies of the nonmotile 

 organisms were "Ohne Hauch" (without emanation). 



The H antigens are of two kinds : those shared by a number of 

 species or types, and those peculiar to a particular species or type, 

 or shared by only a few species or types. Many of the species or 

 types are diphasic ; that is, at one stage of a culture the specific 

 flagellar antigens may occur (specific phase), whereas at another 

 the group antigens may be present (group phase). Any given cul- 

 ture of such an organism may consist entirely of one or the other of 

 the phases or may contain both. A bacillus in one phase usually keeps 



