Salmonella enterica: a surprisingly well-adapted intracellular lifestyle

Abstract

The infectious intracellular lifestyle of Salmonella enterica relies on the adaptation to nutritional conditions within the Salmonella-containing vacuole (SCV) in host cells. We summarize latest results on metabolic requirements for Salmonella during infection. This includes intracellular phenotypes of mutant strains based on metabolic modeling and experimental tests, isotopolog profiling using (13)C-compounds in intracellular Salmonella, and complementation of metabolic defects for attenuated mutant strains towards a comprehensive understanding of the metabolic requirements of the intracellular lifestyle of Salmonella. Helpful for this are also genomic comparisons. We outline further recent studies and which analyses of intracellular phenotypes and improved metabolic simulations were done and comment on technical required steps as well as progress involved in the iterative refinement of metabolic flux models, analyses of mutant phenotypes, and isotopolog analyses. Salmonella lifestyle is well-adapted to the SCV and its specific metabolic requirements. Salmonella metabolism adapts rapidly to SCV conditions, the metabolic generalist Salmonella is quite successful in host infection.

Keywords: Salmonella-containing vacuole; metabolism; regulation; virulence.

FIGURE 1

Intracellular lifestyle of Salmonella. Salmonella is taken up by host cells either by Salmonella-induced invasion (T3SS-1 triggered macropinocytosis) or via phagocytosis. By translocating effector proteins via the T3SS-2 into the host cell, the SCV undergoes an altered maturation process characterized by specific endosomal markers. The onset of Salmonella-induced filaments (SIFs) formation coincides with the start of Salmonella replication between 4 and 6 h after infection. SIFs develop on a microtubule scaffold.

FIGURE 2

Principles of isotopolog profiling with labeled [U-¹³C₆] glucose. The fate of labeled glucose via different metabolic routes can be followed by isotopolog profiling in analyzing resulting metabolites or products (in this case amino acids). An example is given for the biosynthetic routes of the two aspartate isotopologs ¹³C₂-aspartate and ¹³C₃-aspartate (represented by orange and blue arrows, respectively) and the origin of a ¹³C₃-alanine. Isotopolog studies are described in Eisenreich et al. (2006), Eylert et al. (2008), and Eylert et al. (2010). ¹³C-atoms are marked in red and indicated by an asterisk.

FIGURE 3

Activity of nutrient transport systems in various host habitats colonized by Salmonella. Transporter gene expression and carbon source usage is exemplified according to Hautefort et al. (2008), Eisenreich et al. (2010), and Harvey et al. (2011). During the colonization of the intestine, Salmonella first resides in the cecal lumen (bottom left), then attaches to the mucosal layer (bottom right) until it invades macrophages and epithelial cells forming the SCV (upper part). Transporter proteins differentially expressed in the specific niches are represented as small ovals with transported metabolites at the outside of the Salmonella cell (central oval) and gene names on the inside. Carbon sources suggested according to literature are illustrated as gray boxes. Salmonella genome information is according to McClelland et al. (2001).

FIGURE 4

Salmonella central carbon metabolism. Shown are genes and enzymes of the central carbon metabolism covering glycolysis, PPP, KDPG/Entner–Doudoroff pathway, oxidative carboxylation, TCA cycle, and anaplerotic reactions. Boxes represent metabolites, arrows indicate enzyme reactions. Metabolites and enzymes are colored according to the color of their pathway name. Arrows are directed according flux direction under glucose metabolism but may vary under different conditions. Detailed information on importance of the illustrated pathways in intracellular infection is outlined in the text including behavior of an array of different enzyme mutations. 1,3pg, 1,3-bisphospho-D-glycerate; 2pg, 2-phospho-D-glycerate; 3pg, 3-phospho-D-glycerate; 6pg, 6-phospho-D-gluconate; αkg, α-ketoglutarate; ac-coa, acetyl-CoA; cit, citrate; dhap, dihydroxyacetone phosphate; e4p, D-erythrose-4-phosphate; f1,6bp, fructose-1,6-bisphosphate; f6p, D-fructose-6-phosphate; fum, fumarate; gap, D-glyceraldehyde-3-phosphate; glyox, glyoxylate; icit, isocitrate; kdpg, 2-dehydro-3-deoxy-D-gluconate-6-phosphate; mal, (S)-malate; mg, methylglyoxal; oaa, oxaloacetate; pep, phosphoenolpyruvate; pyr, pyruvate; r5p, D-ribose-5-phosphate; ru5p, D-ribulose-5-phosphate; s7p, D-sedoheptulose-7-phosphate; suc, succinate; suc-coa, succinyl-CoA; x5p, D-xylulose-5-phosphate.