Fis, a DNA nucleoid-associated protein, is involved in Salmonella typhimurium SPI-1 invasion gene expression

The ability of Salmonella enterica serovar Typhimurium to cause disease depends upon the co-ordinated expression of many genes located around the Salmonella chromosome. Specific pathogenicity loci, termed Salmonella pathogenicity islands, have been shown to be crucial for the invasion and survival of Salmonella within host cells. Salmonella pathogenicity island 1 (SPI-1) harbours the genes required for the stimulation of Salmonella uptake across the intestinal epithelia of the infected host. Regulation of SPI-1 genes is complex, as invasion gene expression responds to a number of different signals, presumably signals similar to those found within the environment of the intestinal tract. As a result of our continued studies of SPI-1 gene regulation, we have discovered that the nucleoid-binding protein Fis plays a pivotal role in the expression of HilA and InvF, two activators of SPI-1 genes. A S. typhimurium fis mutant demonstrates a two- to threefold reduction in hilA:Tn5lacZY and a 10-fold reduction in invF:Tn5lacZY expression, as well as a 50-fold decreased ability to invade HEp-2 tissue culture cells. This decreased expression of hilA and invF resulted in an altered secreted invasion protein profile in the fis mutant. Furthermore, the virulence of a S. typhimurium fis mutant is attenuated 100-fold when administered orally, but has wild-type virulence when administered intraperitoneally. Expression of hilA:Tn5lacZY and invF:Tn5lacZY in the fis mutant could be restored by introducing a plasmid containing the S. typhimurium fis gene or a plasmid containing hilD, a gene encoding an AraC-like regulator of Salmonella invasion genes.     

Growth Phase-Regulated Induction of Salmonella-Induced Macrophage Apoptosis Correlates with Transient Expression of SPI-1 Genes

Invasive Salmonella has been reported to induce apoptosis in a fraction of infected macrophages within 2 to 14 h from the time of infection by a mechanism involving the type III secretion machinery encoded by the Salmonella pathogenicity island 1 (SPI-1). Here, we show that bacteria in the transition from logarithmic to stationary phase cause 90% of the macrophages to undergo phagocytosis-independent, caspase-mediated apoptosis within 30 to 60 min of infection. The ability of Salmonella to induce this rapid apoptosis was growth phase regulated and cell type restricted, with epithelial cells being resistant. Apoptosis induction was also abrogated by disruption of the hilA gene (encoding a regulator of SPI-1 genes) and by the expression of a constitutively active PhoPQ. hilA itself and a subset of SPI-1 genes were transiently expressed during aerobic growth in liquid medium. Interestingly, however, hilA was found to be required only for the expression of the prgH gene, while sipB, invA, and invF were expressed in a hilA-independent manner. The expression of SPI-1 genes and the secretion of invasion-associated proteins correlated temporally with the induction of apoptosis and are likely to represent its molecular basis. Thus, growth phase transition regulates the expression and secretion of virulence determinants and represents the most efficient environmental cue for apoptosis induction reported to date.     

Cognate gene clusters govern invasion of host epithelial cells by Salmonella typhimurium and Shigella flexneri.

The enteric pathogens Salmonella typhimurium and Shigella flexneri differ in most virulence attributes including infectivity, pathology and host range. We have identified a new assemblage of genes responsible for invasion properties of Salmonella which is remarkably similar in order, arrangement and sequence to the gene cluster controlling the presentation of surface antigens (spa) on the virulence plasmid of Shigella. In Salmonella, this chromosomally encoded complex consists of over 12 genes, mutations in which abolish bacterial entry into epithelial cells. Although these genera use distinct invasion antigens, a non-invasive spa mutant of Salmonella could be rescued by the corresponding Shigella homolog. While spa promotes equivalent functions in Shigella and Salmonella, this constellation of genes has been acquired independently by each genus and displays motifs used by diverse antigen export systems including those required for flagellar assembly and protein secretion.