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The Salmonella enterica serovar Typhi genome contains 14 putative fimbrial systems. The Std fimbriae belong to the chaperone-usher family and its regulation has not been investigated in S . Typhi. Several regulators of Std were previously identified in the closely related serovar Typhimurium. We hypothesize that regulators of S . Typhimurium may be shared with S . Typhi, but that several other regulators remain to be discovered. Here, we describe the role of more than 50 different candidate regulators on std expression. Three types of regulators were investigated: known regulators in S . Typhimurium, in silico predicted regulators and virulence/metabolic regulators. Expression of std was determined in the regulator mutants and compared with the wild-type strain. Overall, 21 regulator mutations affect std promoter expression. The role of Crp, a newly identified factor for std expression, was further investigated. Crp acted as an activator of std expression on a distal region of the std promoter region. Together, our results demonstrate the major influence of Crp as a novel transcriptional factor on std promoter expression and later production of Std fimbriae in Salmonella .

The human-specific pathogen Salmonella enterica serovar Typhi causes typhoid, a major public health issue in developing countries. Several aspects of its pathogenesis are still poorly understood. S . Typhi possesses 14 fimbrial gene clusters including 12 chaperone-usher fimbriae ( stg, sth, bcf , fim, saf , sef , sta, stb, stc, std, ste , and tcf ). These fimbriae are weakly expressed in laboratory conditions and only a few are actually characterized. In this study, expression of all S . Typhi chaperone-usher fimbriae and their potential roles in pathogenesis such as interaction with host cells, motility, or biofilm formation were assessed. All S . Typhi fimbriae were better expressed in minimal broth. Each system was overexpressed and only the fimbrial gene clusters without pseudogenes demonstrated a putative major subunits of about 17 kDa on SDS-PAGE. Six of these (Fim, Saf, Sta, Stb, Std, and Tcf) also show extracellular structure by electron microscopy. The impact of fimbrial deletion in a wild-type strain or addition of each individual fimbrial system to an S . Typhi afimbrial strain were tested for interactions with host cells, biofilm formation and motility. Several fimbriae modified bacterial interactions with human cells (THP-1 and INT-407) and biofilm formation. However, only Fim fimbriae had a deleterious effect on motility when overexpressed. Overall, chaperone-usher fimbriae seem to be an important part of the balance between the different steps (motility, adhesion, host invasion and persistence) of S . Typhi pathogenesis.