Attenuated Salmonella Typhimurium Lacking the Pathogenicity Island-2 Type 3 Secretion System Grow to High Bacterial Numbers inside Phagocytes in Mice

Abstract

Intracellular replication within specialized vacuoles and cell-to-cell spread in the tissue are essential for the virulence of Salmonella enterica. By observing infection dynamics at the single-cell level in vivo, we have discovered that the Salmonella pathogenicity island 2 (SPI-2) type 3 secretory system (T3SS) is dispensable for growth to high intracellular densities. This challenges the concept that intracellular replication absolutely requires proteins delivered by SPI-2 T3SS, which has been derived largely by inference from in vitro cell experiments and from unrefined measurement of net growth in mouse organs. Furthermore, we infer from our data that the SPI-2 T3SS mediates exit from infected cells, with consequent formation of new infection foci resulting in bacterial spread in the tissues. This suggests a new role for SPI-2 in vivo as a mediator of bacterial spread in the body. In addition, we demonstrate that very similar net growth rates of attenuated salmonellae in organs can be derived from very different underlying intracellular growth dynamics.

Dormant Intracellular Salmonella enterica Serovar Typhimurium Discriminates among Salmonella Pathogenicity Island 2 Effectors To Persist inside Fibroblasts

“…Our study also shows that SifA is dispensable for intracellular survival, in marked contrast to the key role of this effector in bacterial growth inside macrophages (47,69). Interestingly, a recent study by Grant et al reported a role of SPI-2 in negatively modulating the growth rate in vivo in phagocytic cells (37). Therefore, the analysis of phenotypes shown by S. Typhimurium in fibroblasts may provide new insights into how this pathogen establishes a nonproliferative intracellular state and the extent to which the lifestyles that the pathogen adopts in fibroblasts and phagocytes are comparable.…”

Replication of Salmonella enterica Serovar Typhimurium in Human Monocyte-Derived Macrophages

“…Host cell death was clearly associated with SPI2-dependent replication at Ϸ20 h p.i., since death was observed in cells containing replicating WT but not ⌬SPI2 bacteria. SPI2-dependent late apoptosis of infected macrophages in vitro has been described previously in both mouse and human macrophages (20,31) and may be an important mechanism for the spread of the bacteria to neighboring cells in vivo (58).…”

The lipopolysaccharide modification regulator PmrA limitsSalmonellavirulence by repressing the type three-secretion system Spi/Ssa

“…Inactivation of the SPI-2 T3SS enhances Salmonella's growth inside phagocytes during infection of an animal (42). These results suggest that PmrA repression of ssrB might enable Salmonella to control bacterial proliferation within a cell and to favor dissemination to neighboring cells.…”