Differential regulatory control of curli (csg) gene expression in Salmonella enterica serovar Typhi requires more than a functional CsgD regulator

The human-specific Salmonella enterica serovar Typhi (S. Typhi) causes typhoid fever, a systemic disease with no known reservoir. Curli fimbriae are major components of biofilm produced by Salmonella and are encoded by the csg gene cluster (csgBAC and csgDEFG). The role of curli in S. Typhi is unknown, although detection of anti-curli antibodies suggests they are produced during host infection. In this study, we investigated curli gene expression in S. Typhi. We demonstrated that the CsgD regulatory protein binds weakly to the csgB promoter. Yet, replacing S. Typhi csgD with the csgD allele from S. Typhimurium did not modify the curli negative phenotype on Congo Red medium suggesting that differential regulation of curli gene expression in S. Typhi is not dependent on modification of the CsgD regulator. The entire csg gene cluster from S. Typhimurium was also cloned into S. Typhi, but again, despite introduction of a fully functional csg gene cluster from S. Typhimurium, curli were still not detected in S. Typhi. Thus, in addition to intrinsic genomic differences in the csg gene cluster that have resulted in production of a modified CsgD protein, S. Typhi has likely undergone other changes independent of the csg gene cluster that have led to distinctive regulation of csg genes compared to other Salmonella serovars.