[Distribution of Salmonella paratyphi A outer membrane protein X gene and immune-protective effect related to its...

To determine the distribution and sequence conservation of outer membrane protein X(ompX)gene in Salmonella paratyphi A isolates as well as the immunogenicity and immuno-protection of ompX gene products.

OmpX gene in Salmonella paratyphi A isolates was detected by PCR and the amplification products were sequenced after the T-A cloning process. OmpX gene product was expressed with E. coli expression system and the expressed rOmpX was extracted by Ni-NTA affinity chromatography. SDS-PAGE and Bio-Rad Gel Image Analyzer were applied to examine the expression and yield of rOmpX. Both antigenicity and immune-reactivity of rOmpX were detected by immune-diffusion test, ELISA and Western blot assay. The immunoprotective effect of rOmpX against infection of Salmonella paratyphi in mice was determined and the agglutinative titers of sera from rOmpX-immunized mice was measured by micro-Widal's test.

All the tested Salmonella paratyphi A isolates had ompX gene with high nucleotide or amino acid sequence identity (99.2%-100.0% or 98.4%-100.0%). When rOmpX was induced to rabbits to produce high level antibody and combined with antiserum against whole cell of Salmonella paratyphi A, the results displayed a positive Western hybridization signal. Results from ELISA demonstrated that 95.6% (65/68) of the serum samples from paratyphoid-A patients were positive on rOmpX antibody. Mice that were immunized with 100 µg or 200 µg rOmpX displayed an immune-protective rate of 93.3% (14/15) or 100.0% (15/15). Sera from those rOmpX-immunized mice provided 1 : 10-1 : 40 agglutination titers in both H antigens of Salmonella paratyphi A and Salmonella typhi.

The recombinant expression product of ompX gene could be used as a candidate antigen for developing genetic engineering vaccines against Salmonella paratyphi A infection.