Periplasmic determinants of virulence in Salmonella enterica

Abstract

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen that is capable of causing systemic infection in mammals. Its normal course of infection brings this organism into the diverse environments of the stomach, the small intestine, the large intestine, and in the case of susceptible hosts, the phagosomal compartments of macrophages. Not only does S. typhimurium induce the expression of myriad virulence factors in order to successfully establish an infection, this organism must adjust its metabolism to the changing conditions present in the host in order to grow. S. typhimurium has a flexible respiratory chain that is capable of utilizing numerous terminal electron acceptors. I present evidence that although S. typhimurium passes through the microaerobic, shifting to anaerobic, conditions of the intestine, and it is capable of respiring anaerobically, it only grows in the presence of oxygen in the mouse host. When Salmonella passes from the gut to the systemic environment, it encounters oxidative stress brought on by the respiratory burst of phagocytes. Superoxide is the reactive oxygen species generated in macrophages in response to phagocytosis. S. typhimurium encodes a superoxide dismutase, SodCI, that is important for resistance to phagocytic superoxide. Currently it is not known how phagocytic superoxide kills or damages microorganisms taken up by macrophages. However, I have shown that phagocytic superoxide does not damage the DNA of S. typhimurium, and that SodCI protects an extracytoplasmic target from this exogenous superoxide. Superoxide can only directly damage a class of enzymes with a solvent exposed [4Fe-4S] cluster, and this type of enzyme is not known to be transported out of the cytoplasm. However, if an enzyme of this type were to be exported, it would be transported via the Twin Arginine Transport (Tat) system, which translocates folded proteins into the periplasm. This transport system is important for virulence in mice, but it is not needed for most growth conditions in the laboratory. I found that SodCI does not protect a Tat substrate, but I did find that strains with mutations that inactivate this transport system are attenuated because of mislocalization of three proteins involved in cell septation: AmiA, AmiC, and SufI.