[Protective role of Salmonella R mutants in Salmonella infection in mice (author's transl)].

Abstract

NMRI mice were immunized with acetone-killed bacteria of 6 salmonella R mutants, 5 homologous and 6 heterologous Salmonella S forms and 3 E. coli R mutants. The animals were then challenged with graded amounts of live S. typhimurium. The results show that the protection obtained was dependent on the number of immunizing injections and on the time interval between them. Thus in the case of Salmonella R-mutants two immunizations increased the LD50 of challenge by an index of two (log 10) compaired to one immunization. A third immunization led to only a small further increase, the protection however, was longer lasting. A 3 fold immunization with two Salmonella typhimurium mutants, one SR- and one Ra form, led to a protection comparable to that obtained with S form bacteria. In contrast to the R-mutants, with Salmonella typhimurium S form a high degree of long-lasting protection was achieved already after a single immunization, and was not increased significantly by repeated injections. In animals immunized with Salmonella typhimurium S form the difference between non-lethal and 100% lethal challenge dose varied by a factor of 10 (one injection dose). In contrast, in animals immunized with Salmonella R mutants the above differences were more gradual extending over 3, 4 or more infection doses. This was also true for animals immunized with lower doses of S. typhimurium S form and for the non-immunized control animals. For comparison the protective effect of heterologous Salmonella S forms and of E. coli R-mutants was studied. These were found to be less effective in affording protection to Salmonella typhimurium than the above Salmonella R forms. The various strains used for immunization may be placed in the following sequence in order of decreasing protection: Salmonella typhimurium S form, Salmonella R-mutants, heterologous Salmonella S forms, E. coli R mutants. In a parallel investigation the antibody inducing properties of Salmonella R mutants and heterologous Salmonella S forms were studied. In all cases homologous hemaglutinating antibodies to all the strains used for immunization were detectable. In immunization with Salmonella R mutants in addition to homologous titres, agglutinating antibodies to Salmonella typhimurium S form were also produced in significant amounts. There was, however, no correlation between the time of appearance of protection and that of appearance of antibodies nor between the hight of antibody titres and degree of protection. The detection of agglutinins to the infecting microorganisms represents therefore no valid criterium for the effectiveness of R mutants and heterologous Salmonella S forms as protective vaccines. From the present results it is concluded that in addition to the O antigen one or more further cell components exist which are involved in rendering animals immune to Salmonella typhimurium and probably also to other Salmonella S form bacteria.