Type 1 fimbriae of Salmonella enterica serovar Typhimurium bind to enterocytes and contribute to colonization of swine in vivo

Abstract

Salmonella enterica serovar Typhimurium strain 798 is a clinical isolate from a pig and is known to be able to cause persistent, asymptomatic infections. This strain also is known to exist in two phenotypes (adhesive and nonadhesive to enterocytes) and can switch between the two phenotypes at a rate consistent with phase variation. Cells in the adhesive phenotype are more readily phagocytosed by leukocytes than nonadhesive cells. Once in a leukocyte, adhesive-phase cells survive while nonadhesive-phase cells die. In the present study, nonadhesive mutants were obtained with the transposon TnphoA. A nonadhesive mutant was selected for study and was shown by electron microscopy not to produce fimbriae. The gene encoding the adhesin was cloned and sequenced. Based on its sequence, the adhesin was shown to be FimA, the major subunit of type 1 fimbriae. The nonadhesive mutant was attenuated in its ability to colonize both mouse and pig intestines, but remained capable of systemic spread in mice. The nonadhesive mutant was phagocytosed to the same extent as parental cells in the adhesive phase and then survived intracellularly. These results demonstrated that type 1 fimbriae were important for attachment to enterocytes and promoted intestinal colonization. However, they were not important in promoting phagocytosis or intracellular survival.

FIG. 1.

Electron micrographs of mutant 14 (A) and the parent strain, ι519′ (B). The cells were negatively stained with phosphotungstate. Fimbriae can be seen on ι519′ but not on mutant 14.

FIG. 2.

Colonization of pig intestines by S. enterica serovar Typhimurium 1 and 2 weeks postchallenge. Symbols show results for strain 798 in the mid-ileum (▪) and ileocecal junction (♦) and mutant 14 in the mid-ileum (•) and ileocecal junction (▴). Results are the mean of four pigs per time point.

FIG. 3.

Uptake (A) and survival (B) of S. enterica serovar Typhimurium strains ι518 (▪), ι519′ (•), and mutant 14 (▴) in porcine leukocytes. Representative results from three replicates are presented. The differences in the uptake at 30 and 60 min between ι518 and ι519 and ι518 and mutant 14 were statistically significant (P = 0.0001). Comparison of survival at 120 and 240 min between ι518 and ι519 was statistically significant (P = 0.0004), as was that between ι518 and mutant 14 after adjusting for the differences in the initial numbers of cells in the leukocytes.