Carbon Starvation Genes Mediate the Cross-talk Between Metabolism and Pathogenesis of Salmonella Typhimurium

Abstract

Carbon starvation genes mediate the cross-talk between metabolism and pathogenesis of Salmonella Typhimurium Salmonella enterica serovar Typhimurium (S. Typhimurium) can infect a wide range of host animals to cause diseases like gastroenteritis and typhoid fever. Therefore it serves as an ideal model organism to study Salmonella pathogenesis. The adaptability of Salmonella to diverse environmental conditions is due to its ability to utilize a plethora of nutrients. While various carbon and nitrogen sources are supplied by the host, the role of peptides as alternate nutrient source for Salmonella is not clearly defined. These peptides are presented by the host as antimicrobial peptides, but can serve as nutrients too, once taken up by peptide transporters and digested by the pathogen. The importance of peptide transporters is also reported for alternate functions such as quorum sensing, competence, chemotaxis and virulence. The ABC transporters of peptides are well studied in Salmonella, whereas little is known about the putative peptide transporter family named as carbon starvation genes which lack ATP binding site. Two carbon starvation (cst) genes, cstA and yjiY, are the only genes known to belong to this group of peptide transporters in Salmonella. cstA was previously reported to be required for virulence of Salmonella in C. elegans. To establish the role of cst genes in the metabolism and pathogenesis of S. Typhimurium, the knockout strains for the genes cstA and yjiY in S. Typhimurium, denoted as ΔcstA and ΔyjiY, were generated. The metabolic capacity of these mutants was checked by phenotype microarray revealing that cst knockout strains were compromised in peptide metabolism and ΔyjiY strain showed remarkable difference from the wild type in the ability to utilize a few peptides. Fluorescent peptide uptake assay showed reduced uptake of specific dipeptides by ΔyjiY strain. Thus, cst genes contribute to metabolism of Salmonella by transporting specific peptides. Upon infecting C. elegans, ΔcstA was unable to colonize the intestine of the worm verifying the reported role of cstA. However, in mammalian model systems, ΔyjiY, but not ΔcstA, was unable to invade various types of host cells, attributed to defective adhesion of ΔyjiY because of lack of flagella. The in vivo significance of yjiY was established when ΔyjiY showed decreased colonization of mouse gut. Transcriptome analysis showed upregulation of the virulence factor mgtC in ΔyjiY, which led to better proliferation of ΔyjiY inside macrophages. The expression of mgtC is induced in the absence of proline suggesting that yjiY might be involved in transporting proline rich peptides. Therefore, both cst genes are required for the virulence of Salmonella, but in different host systems. When biofilm forming ability of the wild type and cst mutant strains was tested in vitro, only ΔyjiY strain was unable to form biofilm. Confocal microscopy and assessment of rdar morphotype revealed that ΔyjiY strain lacked extracellular polymeric substance (EPS) production, which is an essential component of biofilm matrix. The mechanism behind was found to be the downregulation in the expression of the biofilm master regulator gene, csgD that controls EPS biosynthesis in Salmonella. Conclusively, yjiY is required for EPS biosynthesis and hence biofilm formation in Salmonella.