Absence of proline-peptide transporter YjiY in Salmonella Typhimurium leads to secretion of factors which inhibits intra-species biofilm formation

Regulatory roles of the AraC family transcription factor yeaM in the virulence and biofilm formation of Salmonella Typhimurium

Salmonella Typhimurium (S. typhimurium) is a significant zoonotic pathogen responsible for gastroenteritis and severe systemic infections in various hosts. The AraC family transcription factors are key gene expression regulators in prokaryotes, essential for bacterial adaptation to the environment and virulence. Despite their importance, the role of yeaM, a member of this family in S. typhimurium, remains unexplored. To elucidate yeaM regulatory function in virulence and biofilm formation, we engineered mutant and complementary strains of the yeaM gene using homologous recombination. We assessed their capabilities in biofilm formation under different conditions, macrophage adherence and invasion, and virulence in mice. Additionally, we identified potential target genes regulated by yeaM through transcriptome sequencing and confirmed these findings using an electrophoretic mobility shift assay (EMSA) and a dual-luciferase reporter assay. Our results demonstrate that, compared to the parental strain SL1344 and the complemented strain CΔyeaM, the ΔyeaM strain exhibited significantly enhanced biofilm formation, increased invasion of mouse intestinal epithelial cells, enhanced intracellular proliferation within macrophages, and elevated induction of macrophage apoptosis. Furthermore, the ΔyeaM deletion strain displayed significantly increased virulence in mice and enhanced proliferation in milk. Transcriptome analysis revealed that S. typhimurium pathogenicity island 4 (SPI4) genes (siiA, siiB, siiC, siiD, siiF, and siiE) were significantly upregulated following the deletion of the yeaM gene. EMSA and dual-luciferase reporter assays further showed that the yeaM protein can bind to the promoter of the siiA gene and suppress its expression, thereby modulating the biofilm formation and virulence of S. typhimurium.

Spermidine constitutes a key determinant of motility and attachment of Salmonella Typhimurium through a novel regulatory mechanism

Spermidine is a poly-cationic molecule belonging to the family of polyamines and is ubiquitously present in all organisms. Salmonella synthesizes, and harbours specialized transporters to import spermidine. A group of polyamines have been shown to assist in Salmonella Typhimurium's virulence and regulation of Salmonella pathogenicity Inslad 1 (SPI-1) genes and stress resistance; however, the mechanism remains elusive. The virulence trait of Salmonella depends on its ability to employ multiple surface structures to attach and adhere to the surface of the target cells before invasion and colonization of the host niche. Our study discovers the mechanism by which spermidine assists in the early stages of Salmonella pathogenesis. For the first time, we report that Salmonella Typhimurium regulates spermidine transport and biosynthesis processes in a mutually inclusive manner. Using a mouse model, we show that spermidine is critical for invasion into the murine Peyer's patches, which further validated our in vitro cell line observation. We show that spermidine controls the mRNA expression of fimbrial (fimA) and non-fimbrial adhesins (siiE, pagN) in Salmonella and thereby assists in attachment to host cell surfaces. Spermidine also regulated the motility through the expression of flagellin genes by enhancing the translation of sigma-28, which features an unusual start codon and a poor Shine-Dalgarno sequence. Besides regulating the formation of the adhesive structures, spermidine tunes the expression of the two-component system BarA/SirA to regulate SPI-1 encoded genes. Thus, our study unravels a novel regulatory mechanism by which spermidine exerts critical functions during Salmonella Typhimurium pathogenesis.