New insights into the roles of long polar fimbriae and Stg fimbriae in Salmonella interactions with enterocytes and M cells

Salmonella enterica serovar Typhi causes the systemic disease typhoid fever. After ingestion, it adheres to and invades the host epithelium while evading the host innate immune response, causing little if any inflammation. Conversely, Salmonella enterica serovar Typhimurium causes gastroenteritis in humans and thrives in the inflamed gut. Upon entering the host, S. Typhimurium preferentially colonizes Peyer's patches, a lymphoid organ in which microfold cells (M cells) overlay an arrangement of B cells, T cells, and antigen-presenting cells. Both serovars can adhere to and invade M cells and enterocytes, and it has been assumed that S. Typhi also preferentially targets M cells. In this study, we present data supporting the alternative hypothesis that S. Typhi preferentially targets enterocytes. Using a tissue culture M cell model, we examined S. Typhi strains with a deletion in the stg fimbriae. The stg deletion resulted in increased adherence to M cells and, as expected, decreased adherence to Caco-2 cells. Adherence to M cells could be further enhanced by introduction of the long polar fimbriae (Lpf), which facilitate adherence of S. Typhimurium to M cells. Deletion of stg and/or introduction of lpf enhanced M cell invasion as well, leading to significant increases in secretion of interleukin 8. These results suggest that S. Typhi may preferentially target enterocytes in vivo.