The protein‐lipopolysaccharide complex extracted with trichloracetic acid from Salmonella typhimurium effective in protection of mice against S. typhimurium infection

A protein-lipopolysaccharide complex has previously been postulated as necessary to protect susceptible mice against Salmonella typhimurium infection. Lipopolysaccharide attached to non-specific proteins, bovine serum albumin or methylated BSA, gave a specific delayed-type hypersensitivity (DTH) reaction when injected into the footpad of mice sensitized with sublethal doses of S. typhimurium. However, immunization of BALB/c mice with the complex gave no survivors after challenge with 100 LD50 S. typhimurium. Trichloracetic acid extraction of bacterial cultures produced lipopolysaccharide with attached protein. This method gave simple and convenient production of an active factor, demonstrating few major protein bands after electrophoresis. The complex elicited specific DTH reactions in sensitized mice and protected 37% of BALB/c mice against 100 LD50 S. typhimurium. Combinations of protein:lipopolysaccharide were used in DTH experiments to determine the relative importance of the components. A minimum requirement for both was demonstrated, although the ratio was not critical. Use of O-antigenic mutant strains of Salmonella indicated a role for protein in the specificity of activity of the complex.