1. Loss and variation of antigenic substance (especially blood group substance). There is a close relation between somatic antigen and blood group substance. For examples, antigen 5 of Salmonella B group is nearly the same substance with Forssman antigen (F, FA), antigen 13 of Salmonella G group and Paracolon with O (H) substance, antigen 6 of Escherichia coli O group and antigen 40 of Salmonella with A substance. Serial subculture of these organisms in media respectively containing anti-5, anti- 13 and anti-40 sera produces strains which have lost antigen 5 (F, FA), 13 (O) and 40 (A), respectively. In S→R variation of these organisms, group substances which are present in S form strains are lost with the advance of the process, and replaced by the ones specific to R form strains, which are also lost in the end.
2. Antigenic transformation. As the factor of antigenic transformation, bacteriophage is known besides DNA such as found in Diplococcus pneumoniae. In Salmonella E group, subgroup E2 which have antigen 3, 15 are lysogenic strains, and E1 which have antigen 3, 10 are sensitive strains. E1 strains, when infected with phage derived from R2 strains, gain antigen 3, 15 by transformation; and E2 strains, when cultured in a medium containing phage antiserum, lose the prophage and gain antigen 3, 10 of E1. Subsequently, this epsilon phage as a prophage is considered to take a definite site of the chromosome in bacterial cells of E2 subgroup and to form a gene-like unit related to the production of antigen 15. When subgroup E4 strains which have antigen 1, 3, 19 are infected with this phage and become lysogenic, they turn into strains having antigen 1, 3, 15, 19. A similar fact can be seen in the production of antigen 1 of Salmonella B, A and D groups, namely, lysogenic strains infected with iota phage produce antigen 1.
3. Transformation and transduction. The epsilon phage derived from subgroup E2 of Salmonella E group not only acts as an antigen transforming factor against sensitive E1 strains but also transduces genetic characters such as drug resistance, suger fermentation, nutritional factor, flagellar antigen etc. Subsequently, one of these characters is transduced into some of strains which have their antigens transformed. But when S. macallen of E1 subgroup as a sensitive strain is acted upon by the phage, the transduction takes place but not the antigenic transformation. This indicates that, different from the case of the antigenic transformation, it is not necessary for a sensitive strain to turn lysogenic in order that the transduction takes place into this strain.
4. Inheritance of antigen in microorganisms. When genetic recombination takes place between biochemical mutant W-1177 of Escherichia coli K-12 by Lederberg and Escherichia coli C2 whose antigen is slightly different from that of W-1177, some of the daughter strains have antigen from either one of the parents, and the other have antigen from both parents.
