• DNA microarrays and global gene expression patterns
• Genomic indexing of Salmonella and E.coli
• Salmonella virulence gene expression during infectionLag Phase
• Salmonella Infection of Chicken oviduct (SafeHouse)
• Influence of Commensal flora, immune response and behaviour on Salmonella infection (VTRI104)
• Environmental regulation of the invasion and intracellular virulence gene programmes of Salmonella: a ppGpp dependent switch

• Regulation of Salmonella SPI1 & SPI2
• Bacterial nitrate metabolism in colonisation of the mammalian gastrointestinal tract
• Analysis of factors affecting Salmonella contamination of meat during the slaughter process (Biotracer)

• "Non genetic variation" gene expression within bacterial populations (Finished)

• Nucleoid associated proteins: DNA topology and gene expression.
• Acambis project on enterotoxigenic E.coli (Finished)

Environmental regulation of the invasion and intracellular virulence gene expression of Salmonella: a ppGpp dependent switch

Main Researcher: Arthur Thompson

Responding to changes in the bacterial growth environment requires the rapid modification of gene expression programmes. The bacterial signal molecule, guanosine tetraphosphate (ppGpp), mediates a system that enables bacteria to grow under conditions of nutrient deprivation and environmental stress. ppGpp is synthesised by two enzymes, RelA and SpoT. RelA synthesises ppGpp only under conditions of amino acid starvation, whereas SpoT produces ppGpp throughout growth and responds to different environmental stressors including carbon, phosphate and nitrogen starvation. The ppGpp dependent modulation of gene expression works as ppGpp binds to the β subunit of RNA polymerase (RNAP). This binding reallocates RNAP from genes involved in growth on rich nutrients to those enabling bacteria to grow under starvation conditions.

Fig. 1. Comparison of the intracellular growth characteristics of the wild type and ΔrelAΔspoT strains in intestinal epithelial (A) or murine macrophage cell lines (B) Fig.2. Expression levels of genes encoded within SPI1 and SPI2 in the wild type and ΔrelAΔspoT strains. The data shown are the logs of the signal intensity ratio of Cy5-labelled cDNA to Cy3-labelled genomic DNA for the wild type and ΔrelAΔspoT strains.	For Salmonella to invade the host cell it must upregulate SPI1 genes, but once inside the cell SPI2 gene expression is needed for survival and growth. The mechanism for the alternate regulation of invasion genes and genes required for intracellular growth of Salmonella within the host has puzzled scientists for many years. We have shown that ppGpp is required for the survival and intracellular growth of S. Typhimurium in macrophages and epithelial cells respectively (Fig 1). We have also demonstrated that ppGpp is required for the expression of nearly all known Salmonella virulence genes under growth conditions relevant to host infection (1). For example the low-oxygen activation of SPI1 requires ppGpp (Fig. 2), as does the growth phase-dependent activation of SPI2; but under aerated growth conditions (Fig. 2). ppGpp therefore plays an important role in mediating the environmental regulation of virulence genes required for both invasion and intracellular survival of Salmonella in host cells   1. Thompson, A., Rolfe, M.D., Lucchini, S., Schwerk, P., Hinton, J.C., & Tedin, K. (2006) J Biol Chem 281: 30112-3

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