| S. enterica serotype Typhi differs from non-typhoidal Salmonella serotypes by its strict host adaptation to humans. Since fimbriae have been implicated in host adaptation, this study investigates whether the Typhi genome contains fimbrial operons, which are unique to this pathogen or are restricted to typhoidal Salmonella serotypes. The Typhi genome, sequenced by the Typhi Sequencing Group at the Sanger Centre, contained twelve putative fimbrial operons. In addition to four operons described previously, we identified eight new putative fimbrial operons, which were termed bcf, sta, stb, stc, std, ste, stg, and sth. Hybridization analysis performed with 16 strains of Salmonella reference collection C (SARC) and 21 strains of Salmonella reference collection B (SARB) showed that all twelve putative fimbrial operons of Typhi were also present in a number of non-typhoidal Salmonella serotypes. However, the Typhi genome differed from that of all other Salmonella serotypes investigated in that it contained a unique combination of putative fimbrial operons.; Expression of fimbrial antigens in enteric pathogens is commonly regulated by phase variation. The S. typhimurium genome project reveals that this pathogen possesses twelve fimbrial operons. Single copy translational fusions between lacZYA and each operon were constructed and showed that expression of each operon was regulated by phase variation. The fraction of phase ON cells present in a S. typhimurium culture differed dramatically depending on in vitro growth conditions. Cultures passaged on LB agar plates, until phase equilibrium was reached, contained only a small fraction of bacteria carrying fimbrial operons in the phase ON expression state. In contrast, most bacterial cells (between 60% and 95%) carried fimbrial operons in the phase ON expression state after passage of S. typhimurium in LB broth culture. EM revealed that less than 1% of wild type S. typhimurium cells were fimbriate at phase equilibrium on agar plates, while 75% of cells at phase equilibrium in broth were fimbriate. The pef operon was most highly expressed at 37°C, and the phase ON content at phase equilibrium in broth was 70%. Thus, our observations of the phase ON culture were most likely a result of the elaboration of pef fimbriae. |
