Salmonella enterica Serovar Gallinarum Requires ppGpp for Internalization and Survival in Animal Cells

To elucidate the pathogenic mechanism of Salmonella enterica serovar Gallinarum, we examined the expression of the genes encoded primarily in Salmonella pathogenicity island 1 (SPI-1) and SPI-2. These genes were found to be induced as cultures entered stationary phase under high- and low-oxygen growth conditions, as also observed for Salmonella serovar Typhimurium. In contrast, Salmonella serovar Gallinarum in the exponential growth phase most efficiently internalized cultured animal cells. Analysis of mutants defective in SPI-1 genes, SPI-2 genes, and others implicated in early stages of infection revealed that SPI-1 genes were not involved in the internalization of animal cells by Salmonella serovar Gallinarum. Following entry, however, Salmonella serovar Gallinarum was found to reside in LAMP1-positive vacuoles in both phagocytic and nonphagocytic cells, although internalization was independent of SPI-1. A mutation that conferred defects in ppGpp synthesis was the only one found to affect animal cell internalization by Salmonella serovar Gallinarum. It was concluded that Salmonella serovar Gallinarum internalizes animal cells by a mechanism independent of SPI-1 genes but dependent on ppGpp. Intracellular growth also required ppGpp for the transcription of genes encoded in SPI-2.