||The genome of the typhoid fever bacterium, Salmonella enterica serovar Typhi, contains at least three large inserts ('pathogenicity islands') relative to the chromosome of Salmonella enterica serovar Typhimurium (which is normally non-invasive for humans). DNA encoding genes homologous to Escherichia coli plasmid R64 pil operon was found in the major 'pathogenicity island' of the S. typhi chromosome, suggesting that S. typhi might synthesize Type IV pili. Recent research shows that blocking the primary stages of infection, namely bacterial attachment to host cell receptors and the colonization of the mucosal surface, may be the most effective strategy to prevent bacterial infections. Bacterial attachment usually involves an interaction between a bacterial surface protein called an adhesin and the host cell receptor. In preclinical vaccine studies, it was confirmed that antibodies elicited against an adhesin can impede colonization, block infection and prevent disease. Therefore the study of the Type IV pili, surface appendages that may serve as adhesins, of S. typhi may help in the exploration of a new vaccine that is efficacious in preventing typhoid fever. The whole pil operon (~11kb) of S. typhi was sequenced and found to contain eleven genes (pilL, pilM, pilN, pilO, pilP, pilQ, pilR, pilS, pilT, pilU and pilV). Three non-coding regions of 509bp, 575bp and 308bp were found upstream of pilL, between pilM and pilN, and between pilN and pilO, respectively. Presumptive promoters were found upstream of pilL, but not upstream of pilN, by using β-galactosidase assay and mutational analysis. Among the Pil proteins, PilS protein is believed to be the major structural protein of the pili, while PilV protein is a minor component that may serve as an adhesin. As PilV proteins in R64-bearing E. coli K-12 strains appear to serve as important initiators of pilus synthesis, the expression of S. typhi Type IV pili in various pilV mutants was determined by immunoblotting.