Salmonella infection in the rat : the role of type 1 fimbriae
Infection by Salmonella enteritidis and S. typhimurium has been studied in the rat (Hooded Lister) model in vivo. Salmonellosis in the Hooded Lister rat has many similarities with the disease in humans. Salmonellae associated with the small intestinal epithelium as early as 4 h after intragastric intubation. Therefore, reinfection from the large intestine initially may not play a significant role in the infection process. Both Salmonella serotypes colonised, persisted and proliferated in the gastrointestinal tract and invaded sub-epithelial tissues, mainly via the ileum, leading to the systemic distribution of these pathogens. Coincidental with the infection, polyamine levels and crypt cell proliferation rates in the small intestine increased resulting in substantial growth of the tissue. This growth was particularly dramatic in the ileum where there was also some disruption of the villous epithelium. It is possible that these effects of the infection on the metabolism and morphology of the small bowel, which strongly resembles the changes induced by some plant lectins, may facilitate the colonisation and invasion of the gut by Salmonella sp. It was shown for the first time, that S. enteritidis and S. typhimurium strains expressing type 1 fimbriae closely associated with the rat ileal epithelium in vivo. Moreover, orally administered purified type 1 fimbriae also associated with the ileal surface. Thus, type 1 fimbriae alone or in combination with other fimbriae may play an important role in the early stages of infection with these pathogenic bacteria. This was further supported by the finding that deletion of the type 1 fimbriae from a strain of S. enteritidis led to a reduction in the numbers of Salmonella present in the distal ileum 6 h post-dosing. The deletion of type 1 fimbriae did not however affect the long-term progression of infection, suggesting that type 1 fimbriae do not play a prominent role in persistence.