Dietary Iron Supplementation Increases Severity of Salmonella Typhimurium Infection

Background: Less than 20% of non-heme iron is absorbed in the small intestine and the remaining iron travels to the lower intestine for excretion. Salmonella enterica serovar Typhimurium (S. Typhimurium) and other enteric pathogens may use unabsorbed iron in the lower intestine as a nutrient source, potentially leading to a more severe infection. Objective: The objective of this study was to determine whether unabsorbed iron in the lower intestine of mice increases the severity of S. Typhimurium infection. Method: Twenty-one-day-old female C57BL/6 mice were fed diets containing 300 ppm iron (iron supplemented, FeS) or <6 ppm iron (iron deficient, FeD) for 6 weeks (n=30/diet). After 6 weeks, mice were given an oral gavage of streptomycin 24 h prior to oral gavage of 108 CFU S. Typhimurium (FeSS.Tm or FeDS.Tm, n=15/diet/treatment). The remaining mice in each group acted as control animals (FeSCtrl or FeDCtrl, n=15/diet/treatment). Mice were monitored for up to 1- week post-infection prior to euthanasia and tissue collection. Severity of infection was assessed by cecal weight, spleen index, and change in bodyweight post-infection. Data are presented as means ± SD. Results: Hematocrits were greater in FeSCtrl (51.3 ± 0.8%) compared to FeDCtrl (43.8 ± 2.7%), FeSS.Tm (40.3 ± 3.8%) and FeDS.Tm (40.2 ± 7.3%, P<0.01 for all comparisons). FeSCtrl had higher cecal iron (331.3 ± 59.3 μg Fe/mg tissue) compared to FeDCtrl (5.7 ± 2.4 μg Fe/mg tissue), FeSS.Tm (64.6 ± 26.4 μg Fe/mg tissue) and FeDS.Tm (6.7 ± 3.3 μg Fe/mg tissue, P<0.0001 for all comparisons). Cecal weight was 40% lower in FeSS.Tm (0.3 ± 0.1 g) compared to FeDCtrl (0.5 ± 0.1 g), FeSCtrl (0.5 ± 0.2 g) and FeDS.Tm (0.5 ± 0.1 g, P<0.05 for all comparisons). Spleen index was greater in FeSS.Tm (15.3 ± 7.2 A.U.) compared to FeSCtrl (4.3 ± 0.5 A.U.), FeD (4.0 ± 0.5 2 A.U.) and FeDS.Tm (8.8 ± 5.2 A.U., P<0.01 for all comparisons). FeSS.Tm lost more bodyweight (- 21.7 ± 5.4%) compared to FeDS.Tm (-14.5 ± 6.3%, P<0.01) post-infection. Conclusions: Findings suggest that unabsorbed iron in the lower intestine of mice promotes a more severe S. Typhimurium infection. Alternative iron supplementation strategies should be considered in areas where S. Typhimurium infections are common.