Remodelling of the actin cytoskeleton is essential for replication of intravacuolar Salmonella

Maturation and maintenance of the intracellular vacuole in which Salmonella replicates is controlled by virulence proteins including the type III secr etion system encoded by Salmonella pathogenicity island 2 (SPI-2). Here, we show that, several hours after bacterial uptake into different host cell t ypes, Salmonella induces the formation of an F-actin meshwork around bacter ial vacuoles. This structure is assembled de novo from the cellular G-actin pool in close proximity to the Salmonella vacuolar membrane. We demonstrat e that the phenomenon does not require the Inv/Spa type III secretion syste m or cognate effector proteins, which induce actin polymerization during ba cterial invasion, but does require a functional SPI-2 type III secretion sy stem, which plays an important role in intracellular replication and system ic infection in mice. Treatment with actin-depolymerizing agents significan tly inhibited intramacrophage replication of wild-type Salmonella typhimuri um. Furthermore, after this treatment, wildtype bacteria were released into the host cell cytoplasm, whereas SPI-2 mutant bacteria remained within vac uoles. We conclude that actin assembly plays an important role in the estab lishment of an intracellular niche that sustains bacterial growth.