Development of a DNA probe to detect Salmonella typhi

This study was undertaken to identify a DNA sequence that could be used to facilitate the diagnostic identification of Salmonella typhi, the causative agent of typhoid fever. All virulent S. typhi strains encode a relatively unique capsular antigen termed the virulence (Vi) antigen. Two distinct genetic loci, viaA and viaB, are involved in the synthesis of this antigen. The structural genes, located at viaB, were considered as a possible specific DNA probe. The viaB locus, contained in a recombinant cosmid, was subcloned to various plasmid vectors for this purpose. Selected viaB-region DNA fragments were then analyzed for specificity in DNA colony hybridization reactions with more than 170 strains representing a variety of enteric bacteria. An 8.6-kilobase EcoRI fragment was highly specific for the viaB gene region and was considered a good hybridization probe. This DNA probe should prove useful in rapid diagnostic assays set up to detect S. typhi in mixed bacterial samples (e.g., stools) within a few hours of specimen collection.