Characterization of an outer membrane protein of Salmonella enterica serovar Typhimurium that confers protection against typhoid

Typhoid caused by Salmonella enterica serovar Typhi remains a major health concern worldwide. The emergence of multidrug-resistant strains of Salmonella with increased virulence, communicability, and survivability leading to increased morbidity and mortality has further complicated its management. Currently available vaccines for typhoid have less-than-desired efficacy and certain unacceptable side effects, making it pertinent to search for new immunogens suitable for vaccine formulation. The outer membrane proteins (OMPs) of Salmonella have been considered possible candidates for conferring protection against typhoid. OMPs interface the cell with the environment, thus representing important virulence factors with a significant role in the pathobiology of gram-negative bacteria and bacterial adaptation. An OMP of Salmonella enterica serovar Typhimurium with an apparent molecular mass of 49 kDa that is highly immunogenic, evokes humoral and cell-mediated immune responses, and confers 100% protection to immunized rats against challenge with very high doses (up to 100 times the 50% lethal dose) of Salmonella enterica serovar Typhimurium has been identified. Further, very efficient clearance of bacteria from the reticuloendothelial systems of immunized animals was seen. This protein is recognized by the antibodies present in serum of typhoid patients. When sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel-eluted protein was further analyzed by high-performance liquid chromatography (HPLC) and two-dimensional electrophoresis, two polypeptides with the same molecular weight were resolved. These have different isoelectric points and gave two peaks with different retention times in reverse-phase HPLC. However, only one of the two bands interacted with patient serum. The immunogenicity studies (enzyme-linked immunosorbent assay and delayed-type hypersensitivity [DTH]) indicated that the immunoreactive protein evoked a strong immune response in rats. The N-terminal sequencing and analysis of the homology of this protein with sequences in the protein database of Salmonella resulted in a match with the N-terminal sequences of a protein in Salmonella enterica serovar Typhi (CT18 and Ty2 strains). The homology search further revealed it to be a hypothetical protein, whose gene had unidentified open reading frames in Salmonella serovar Typhi encoding 447 amino acid residues, corresponding to a molecular mass of 49 kDa. The nucleotide sequence of the encoding gene was deduced, and the gene was amplified by PCR using appropriate primers. An amplified 1.3-kb band was purified and sequenced to confirm its identity. These OMPs provide promising targets for the development of a candidate vaccine against typhoid.