Antibodies to lipopolysaccharide and outer membrane proteins of Salmonella enteritidis PT4 are not involved in protection from experimental infection.

BALB/c and Schofield mice were inoculated with formalin-killed bacteria prepared from strains of Salmonella enteritidis belonging to phage type (PT) 4 and carrying a 38 MDa plasmid and expressing long-chain lipopolysaccharide, or strains without a 38 MDa plasmid or lacking the ability to express lipopolysaccharide. Vaccinated mice were challenged with viable bacteria belonging to a virulent strain of S. enteritidis (PT4). Mice surviving this viable challenge were examined for a humoral antibody response to membrane antigens of S. enteritidis (PT4) that might relate to the possession of a given virulence property. BALB/c mice immunized with any of the test antigens were found to be immune to S. enteritidis (PT4), and this immunity was protective. Serum antibodies, of the IgG class, were detected to OmpA and a minor outer membrane protein (OMP) of 31 kDa. Schofield mice also raised IgG antibodies to these outer membrane proteins; however, non-immunized mice of this strain were resistant to infection. The virulence of S. enteritidis (PT4) was also tested using mice belonging to strains B10D2 (new), Biozzi (high), Biozzi (low), C3HeJ, B10ITYR and C57/L.