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

Abstract

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, whilesipB, 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.

Salmonella employs multiple mechanisms to subvert the TLR‐inducible zinc‐mediated antimicrobial response of human macrophages

“…Bacterial infection assays were performed as previously described (41). Bacteria grown to stationary phase were used for all infection assays to limit SPI-1-dependent cell death of S. Typhimurium-infected macrophages (42,43). In brief, bacteria were grown overnight in Luria-Bertani (LB) medium, washed twice, resuspended in antibiotic-free cell culture medium and then used to infect cultured human macrophages at the desired multiplicity of infection (MOI).…”

Salmonella pathogenicity island 1 differentially modulates bacterial entry to dendritic and non‐phagocytic cells

“…These results are consistent with previous observations relative to the involvement of the TTSS-1 in the apoptosis induced by Salmonella on host cells. 54,55 DInvC Salmonella fails to enter non-phagocytic cells but shows increased DCs invasion…”

Unravelling the mysteries of virulence gene regulation in Salmonella typhimurium

“…Regulation of several other virulence genes seems to be even more complex. For example, SPI1 invasion genes and spv genes are thought to be regulated by a host of environmental conditions and regulatory pathways (Table 1) (Guiney et al, 1995;Bajaj et al, 1996;Slauch et al, 1997;Lundberg et al, 1999) making it difficult to know which signals and regulators modulate the genes at any given site in vivo.…”

“…1B). Lundberg et al (1999) reported that they observed transient activation of invF under specific conditions in vitro even when hilA was disrupted. Still, no one has reported a condition in which wild-type S. typhimurium represses hilA expression while simultaneously activating expression of invF and invF-dependent genes through HilC or HilD.…”