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 mecha nism involving the type III secretion machinery encoded by the Salmonella p athogenicity island 1 (SPI-1), Here, we show that bacteria in the transitio n from logarithmic to stationary phase cause 90% of the macrophages to unde rgo phagocytosis independent, caspase-mediated apoptosis,within 30 to 60 mi n of infection, The ability of Salmonella to induce this rapid apoptosis wa s growth phase regulated and cell type restricted, with epithelial cells be ing 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 t ransiently 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 mann er. The expression of SPI-1 genes and the secretion of invasion-associated proteins correlated temporally with the induction of apoptosis and are like ly to represent its molecular basis. Thus, growth phase transition regulate s the expression and secretion of virulence determinants and represents the most efficient environmental cue for apoptosis induction reported to date.