The serological investigations of the O, K and H antigens of the coli group have led to the establishment of a diagnostic antigenic schema by means of which the coli strains can be classified in O groups and types.
Besides the O and H antigens, the K antigens (either envelope or capsular antigens) play a very important rôle. The K antigens include thermolabile antigens (L and B antigens) as well as thermostable antigens (A antigens). While the L and B antigens mostly are envelope antigens (corresponding to the Vi antigen), the A antigens appear mostly as visible capsules (corresponding to the pneumococcus capsule). Thus the term “K antigens” is a symbol for a group of different antigens, including envelope antigens and capsular antigens.
A few O groups are particularly frequent, and this applies also to certain types within these groups. A very high percentage of the strains belonging to these O groups and types are O-inagglutinable—on account of the K antigens. The O group distribution differs in normal and pathologic materials. Therefore, also the type distribution must differ, but the present material is not sufficiently large to establish the percental distribution.
Strains containing K antigens are more toxic than strains without K antigens—and particularly toxic when the strains are isolated from pathologic material.
The toxicity is a constant and characteristic type quality, differing in the individual types of an O group.
Hemolytic and necrotizing strains are particularly frequent in O groups 2, 4 and 6. O inagglutinability of these groups is due to L antigens, that of groups 8 and 9 usually to A antigens.
There is a connection between the type of a strain, its origin, O inagglutinability, hemolytic power, necrotizing capacity and toxicity.
Strains with K antigens are particularly resistant to the defensive forces of the organism and to bacteriophages.
It is suggested as a working hypothesis, that there are certain serological coli types which possess a particular pathogenicity and play an important rôle in appendicitis, peritonitis, cystitis, pyelitis and other diseases.
In the coli group, cultural tests play but a minor rôle, so that the type division has to rest on a serological basis. The prevailing classification of the coli strains, based on cultural criteria (especially fermentation tests) should therefore be abandoned.