A comparison of cecal colonization of Salmonella enterica serotype Typhimurium in white leghorn chicks and Salmonella-resistant mice

Abstract

Background: Salmonellosis is one of the most important bacterial food borne illnesses worldwide. A major source of infection for humans is consumption of chicken or egg products that have been contaminated with Salmonella enterica serotype Typhimurium, however our knowledge regarding colonization and persistence factors in the chicken is small.

Results: We compared intestinal and systemic colonization of 1-week-old White Leghorn chicks and Salmonella-resistant CBA/J mice during infection with Salmonella enterica serotype Typhimurium ATCC14028, one of the most commonly studied isolates. We also studied the distribution of wild type serotype Typhimurium ATCC14028 and an isogenic invA mutant during competitive infection in the cecum of 1-week-old White Leghorn chicks and 8-week-old CBA/J mice. We found that although the systemic levels of serotype Typhimurium in both infected animal models are low, infected mice have significant splenomegaly beginning at 15 days post infection. In the intestinal tract itself, the cecal contents are the major site for recovery of serotype Typhimurium in the cecum of 1-week-old chicks and Salmonella-resistant mice. Additionally we show that only a small minority of Salmonellae are intracellular in the cecal epithelium of both infected animal models, and while SPI-1 is important for successful infection in the murine model, it is important for association with the cecal epithelium of 1-week-old chicks. Finally, we show that in chicks infected with serotype Typhimurium at 1 week of age, the level of fecal shedding of this organism does not reflect the level of cecal colonization as it does in murine models.

Conclusion: In our study, we highlight important differences in systemic and intestinal colonization levels between chick and murine serotype Typhimurium infections, and provide evidence that suggests that the role of SPI-1 may not be the same during colonization of both animal models.

Figure 1

Systemic colonization following serotype Typhimurium infection in chicks and mice. Recovery of serotype Typhimurium from the spleen following oral inoculation in chicks (A) and mice (B) is shown as mean log CFU/gm of organ tissue. Mean spleen weight of chicks (C) and mice (D) is shown in grams of organ tissue. Error bars indicate standard error and asterisks indicate a significant difference between control and infected groups at P < 0.05.

Figure 2

Intestinal colonization following serotype Typhimurium infection in chicks and mice. Recovery of serotype Typhimurium from the small intestine (SI), cecum (C), and large intestine (LI) following oral inoculation in chicks (A) and mice (B) is shown as mean log CFU/gm of organ tissue, and error bars denote standard error.

Figure 3

Different niches for cecal colonization of serotype Typhimurium following oral infection in chicks and mice. Recovery of wild type ATCC14028 and ΔinvA mutant from the whole cecum (tissue + contents), the cecal contents only, the cecal epithelium associated (extracellular attached+intracellular), and intracellular to the cecal epithelium of both chicks and mice. Colonization levels of these compartments by WT ATCC14028 in chicks (A) and mice (B), and ΔinvA mutant of chicks (C) and mice (D) following oral inoculation is shown as mean log CFU/gm of cecal tissue, and error bars denote standard error.

Figure 4

Colonization of the whole cecum and cecal contents by ATCC14028 and ΔinvA mutants. Recovery of wild type serotype Typhimurium ATCC14028 and ΔinvA mutant in the whole cecum (tissue + contents) of chicks (A) and mice (B) and in the isolated cecal contents of chicks (C) and mice (D) is shown as the mean log ratio of wild type versus mutant and error bars denote standard error. Asterisks indicate a significant difference relative to the ratio of wild type vs. mutant in the inoculum, at P < 0.05.

Figure 5

Cell-associated and intracellular growth of ATCC14028 and ΔinvA mutants. Recovery of cell-associated (extracellular but firmly attached + intracellular) serotype Typhimurium and ΔinvA mutant in the cecum of chicks (A) and mice (B) and intracellular wild type and ΔinvA mutant in the cecum of chicks (C) and mice (D) is shown as the mean log ratio of wild type versus mutant. Error bars denote standard error and asterisks indicate a significant difference relative to the ratio of wild type vs. mutant in the inoculum at P < 0.05.

Figure 6

Dynamics of S. Typhimurium fecal shedding in chicks and mice. Recovery of wild type ATCC14028 in the feces of chicks (A) and mice (B) is shown as mean log CFU/g of feces. Lines represent the level of cecal colonization of each animal model. Recovery of wild type and ΔinvA mutant in the feces of chicks (C) and mice (D) is shown as the mean log ratio of wild type versus mutant. Error bars indicate standard error, and asterisks indicate a significant difference relative to the ratio of wild type vs. mutant in the incoculum at P < 0.05.