Difference in the protection against infection with different challenge strains of Salmonella enteritidis by killed vaccine.

Differences in protection against the challenge of different strains of formalin-killed cells obtained from Salmonella enteritidis were investigated. When strains 2547, 116M, 116-54, SR-98G, and 3775 were used as the challenge strain, protective effects were apparent in groups of mice immunized with formalin-killed cells from S. enteritidis strains (protective strains). Conversely, no protective effects were observed with the challenge of strains 2822, 3975, and IID-604 (nonprotective strains). Electrophoretic banding patterns of lipopolysaccharides in SDS-PAGE were similar between the LPS obtained from the various strains used in this study. Additionally, no differences in sensitivity to macrophage intracellular killing were observed between the protective and nonprotective strains. Phagocytic experiments by macrophages in vitro indicated that immune serum used as the opsonin promotes phagocytosis of various strains by macrophages as compared to using normal serum as the opsonin, but the rate of phagocytosis enhanced by immune serum is higher in protective strains than nonprotective strains. In studies of passive transfer of immune serum, it was found that mice passively immunized with immune serum could only protect against infection by challenging with protective strains with the exception of strain SR-98G. These results suggest that the protective effect observed with the challenge of protective strains may be due to macrophage phagocytosis enhanced by opsonization with immune serum.