Targeting of the actin cytoskeleton during infection by Salmonella strains.

Many bacterial pathogens produce virulence factors that alter the host cell cytoskeleton to promote infection. Salmonella strains target cellular actin in a carefully orchestrated series of interactions that promote bacterial uptake into host cells and the subsequent proliferation and intercellular spread of the organisms. The Salmonella Pathogenicity Island 1 (SPI1) locus encodes a type III protein secretion system (TTSS) that translocates effector proteins into epithelial cells to promote bacterial invasion through actin cytoskeletal rearrangements. SPI1 effectors interact directly with actin and also alter the cytoskeleton through activation of the regulatory proteins, Cdc42 and Rac, to produce membrane ruffles that engulf the bacteria. SPI1 also restores normal cellular actin dynamics through the action of another effector, SptP. A second TTSS, Salmonella Pathogenecity Island 2 (SPI2), translocates effectors that promote intracellular survival and growth, accompanied by focal actin polymerization around the Salmonella-containing vacuole (SCV). A number of Salmonella strains also carry the spv virulence locus, encoding an ADP-ribosyl transferase, the SpvB protein, which acts later during intracellular infection to depolymerize the actin cytoskeleton. SpvB produces a cytotoxic effect on infected host cells leading to apoptosis. The SpvB effect appears to promote intracellular infection and may facilitate cell-to-cell spread of the organism, thereby enhancing virulence.

Genetic requirements for salmonella-induced cytopathology in human monocyte-derived macrophages.

Infection of human macrophages with Salmonella enterica serovar Typhimurium or Salmonella enterica serovar Dublin produces delayed cytotoxicity characterized by cell detachment and associated apoptosis. Using a site-specific mutant in the SpvB active site, we verify that the ADP-ribosylation activity of SpvB is required for delayed cytotoxicity in human macrophages infected with Salmonella: SipB and the type III protein secretion system (TTSS) encoded by Salmonella pathogenicity island 1 (SPI1) are not involved, whereas the SPI2 TTSS is absolutely required for SpvB-dependent cytotoxicity. Furthermore, we show that infection of macrophage cultures with wild-type or sipB mutant bacteria led to a complete loss of polymerized actin in over half of the cells after 24 h. In contrast, macrophages infected with the spvB or SPI2 (ssaV or ssaJ) mutant strain retained normal F-actin filaments, despite similar numbers of intracellular bacteria. We conclude that SpvB and a functional SPI2 TTSS are essential for Salmonella-induced delayed cytotoxicity of human macrophages.

Characterization of the spv locus in Salmonella enterica serovar Arizona.

Salmonella enterica serovar Arizona (S. enterica subspecies IIIa) is a common Salmonella isolate from reptiles and can cause serious systemic disease in humans. The spv virulence locus, found on large plasmids in Salmonella subspecies I serovars associated with severe infections, was confirmed to be located on the chromosome of serovar Arizona. Sequence analysis revealed that the serovar Arizona spv locus contains homologues of spvRABC but lacks the spvD gene and contains a frameshift in spvA, resulting in a different C terminus. The SpvR protein functions as a transcriptional activator for the spvA promoter, and SpvB and SpvC are highly conserved. The analysis supports the proposal that the chromosomal spv sequence more closely corresponds to the ancestral locus acquired during evolution of S. enterica, with plasmid acquisition of spv genes in the subspecies I strains involving addition of spvD and polymorphisms in spvA.