CHEMICAL AND IMMUNOLOGICAL CHARACTERIZATION OF A LOW-MOLECULAR-WEIGHTOUTER-MEMBRANE PROTEIN OF SALMONELLA-TYPHI

A new immunogenic outer membrane protein, Omp-28 (MW 28,000 and pi 4.6 ), was isolated from smooth Salmonella typhi cells by the use of an ex tracting medium containing 6 M urea, 1% deoxycholate and 5 mM EDTA, Th e purification of Omp-28 was performed by gel filtration and fast ion exchange chromatography, This protein showed to be the prevalent compo nent isolated by the latter methodology, Omp-28 is formed by three ide ntical subunits (MW 9,000), not linked by disulfide bonds. The partial N-terminal amino acid sequence of Omp-28 presented great homology wit h part of the sequence of an Escherichia coli protein found in a precu rsor whose sequence was predicted by c-DNA, ELISA and Western blotting identified Omp-28 as the major antigenic protein present in the outer membrane protein fraction, isolated by gel filtration. Antibodies aga inst Omp-28 were detected by ELISA in 43% of 28 sera from typhoid feve r convalescent patients, The antisera from mice immunized with Omp-28 and the highest positive typhoid fever convalescent serum gave a posit ive bactericidal test, killing 50% of Salmonella typhi cells in serum dilutions of 1/80 and 1/320, respectively. These results indicate the immunogenic importance of Omp-28 isolated from Salmonella typhi outer membrane and strongly suggest it should be used in further studies of animal protection against the disease caused by this pathogenic bacter ia.