We used metabolomics to characterize the effects of
S. Typhimurium infection and streptomycin administration on the liver due to systemic infection, and importantly, to determine the role that mCRAMP plays in these changes. Results showed that the liver metabolome of SA− mice did not differ. In contrast, the liver metabolome differed significantly in SA+ mice. When the microbiota is not altered by a broad spectrum antibiotic, naïve mice inoculated with
S. Typhimurium develop a typhoid-like disease [
58], characterized by parenchymal necrosis of the liver and spleen [
59]. To address if SA+/ST−/mCRAMP
−/− and SA+/ST−/mCRAMP
+/+ mice differed in the development of typhoid-like fever, we compared their liver metabolic profiles. We observed that the liver metabolome of SA+/ST−/mCRAMP
−/− mice was conspicuously more affected than SA+/ST−/mCRAMP
+/+ mice at 24 hpi, but the difference between the treatments abated by 48 hpi. SA+/ST+/mCRAMP
−/− mice also showed a higher degree of change in the liver metabolome relative to SA+/ST+/mCRAMP
+/+ at 24 and 48 hpi. Moreover, an increase in valine, leucine, taurine, and cadaverine was observed in the livers of SA+/ST−/mCRAMP
−/− relative to SA+/ST−/mCRAMP
+/+ mice at 24 hpi. While lower plasma levels of branched-chain amino-acids (BCCA) (e.g. valine, leucine) have previously been reported in mice with septicemia, such as typhoid fever [
60,
61], the same amino acids tend to increase in liver under infection [
62]. Since BCCA have been observed to induce an immune response enhancing neutrophil function [
63], their higher concentration in SA+/ST−/mCRAMP
−/− mice could be due to an enhanced immune response as a result of the higher degree of colonization by the pathogen. An elevated concentration of taurine in the intestine is associated with higher levels of oxidants, and this may aid in preventing tissue injury under conditions of inflammation [
64‐
66]. A study conducted in intestinal epithelial cells showed a correlation between the production of the polyamine cadaverine and inhibition of polymorphonuclear transmigration induced by
Shigella infection via avoidance of the immune response reaction triggered by epithelial cells [
67]. Therefore, the higher concentration of taurine and cadaverine observed in SA+/ST−/mCRAMP
−/− mice is likely the result of an exacerbated immune response triggered against the pathogen.
Salmonella dissemination and liver damage triggered by the bacterium progress over time [
57], and the significant time effect that we observed in the liver metabolome is consistent with this conclusion. More specifically, we observed that SA+/ST−/mCRAMP
+/+ and SA+/ST+/mCRAMP
−/− mice exhibited higher levels of phenylalanine, taurine, cadaverine, and carnitine at 48 hpi relative to the same treatments at 24 hpi. The concentration of phenylalanine in livers has previously been described to increase during periods of bacterial infection [
61,
68]. This could be explained by an increase in the uptake of phenylalanine from the liver to produce acute phase proteins [
61,
69]. Carnitine is an essential component of cellular metabolism, in charge of transporting activated long-chain fatty acids across the mitochondrial membrane for β-oxidation [
70]. However, this metabolite has also been observed to modulate polymorphonuclear activities and the production of radical oxygen species [
71]. Thus, all the alterations to the liver metabolome that we observed in the current study could be the result of higher colonization and damage induced by the pathogen. Moreover, as these changes were more prominent in mCRAMP deficient mice, the results are consistent with our hypothesis that the absence of mCRAMP increases susceptibility to
Salmonella infection.