BACTERIAL ANTIGENS AND SPECIFICITY 351 



one could identify any bacterial .si)eeic.s. For a number of species 

 such as E. iyphosa, Vibrio comma and several others, this 

 is in a large measure true, although inagglutinable and atypically 

 agglutinable strains of E. typliom have been described by many 

 workers. Their inagglutinability is apparently due 1o a labile 

 " Vi" antigen. (Felix and Pitt, 1934.) 



Limitation of Serological Methods. — While there is an in- 

 cr-easing amount of evidence suggesting that each species possesses 

 a s])ecies-specific antigenic fraction or component, yet this is not 

 always demonstrable by agglutination or complement fixation using 

 \ui altered bacterial suspensions. In fact, the result of agglutina- 

 tion and absorption work would have one to believe that most 

 species of bacteria are not represented by a species-specific antigen 

 (in this case an agglutinogen) but on the contrary by two or 

 more antigenic types. It is only by special methods of investiga- 

 tion that one can frequently demonstrate species-specific antigenic 

 substances. It may be that ultimately immunochemical studies 

 will lead to a more accurate classification of bacteria. 



Stevens (1923) concludes that not all strains of a specific organ- 

 ism can be recognized hy agglutination. 



Species Showing Antigenic Homogeneity. — The results of im- 

 munological and biological investigations using agglutination and 

 absorption and also complement fixation techniques indicate that 

 bacterial antigens fall into three divisions. The first includes 

 those species, each of w^iich consists of one dominant antigenic 

 type. Practically all strains can be identified by means of an 

 immune serum prepared against any single strain. 



Species Showing a Few Serological Types. — The second divi- 

 sion is composed of species, each of which is represented by a small 

 number of serological types and perhaps a heterogeneous one. 

 With these it is possible to use a polyvalent immune serum for 

 species identification and supplement this with other methods 

 designed for the heterogeneous group of each. 



Species Showing Antigenic Heterogeneity. — The third divi- 

 sion consists of those species, each of which shows so much antigenic 

 heterogeneity that agglutination and absorption or even comple- 

 ment fixation methods are not practical for species identification. 



