Virulence of Salmonella enterica serovar typhimurium and innate antibacterial host responses

Abstract

The bacterial species Salmonella enterica consists of a collection of closely related enteric bacteria giving rise to diverse diseases in a wide range of hosts. In the murine infection model Salmonella enterica serovar Typhimurium (S. Typhimurium) causes an invasive disease which in many aspects resembles human typhoid fever. The ability of this pathogen to survive and replicate within macrophages of the liver and spleen is a crucial virulence determinant which largely depends on the type III secretion system coded for by the Salmonella pathogenicity islands 2 (SPI-2).

Innate immune recognition of bacterial pattern molecules such as the lipopolysaccharide (LPS) induces immediate defence responses such as production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI), synthesized by macrophages via the action of the NADPH phagocyte oxidase (phox) and inducible nitric oxide synthase (iNOS) respectively.

The newly characterized phagocyte receptor TRAPC has been shown to trigger nitric oxide (NO) production in macrophages and dendritic cells upon receptor cross-linking, suggesting a possible role for TRAPC during bacterial infection. Surprisingly we could show that in combination with bacterial infection or LPS stimulation, cross-linking of TRAPC reduces macrophage NO production. The suppression of NO associated with a slight reduction of iNOS expression possibly mediated by a dampening the TLR-4 response.

For S. Typhimurium and many other enteric pathogens, the ability to express complete LPS molecules has conventionally been regarded as a requirement for bacterial virulence, e.g. lack of the O-antigen (the outermost part of the LPS molecule) has been shown to reduce the virulence of S. Typhimurium in murine infection models. However, S. Typhimurium has also been shown to down-regulate genes for LPS-synthesis and to reduce the chain length of the O-antigen once it resides inside macrophage-like cells. This raises the question whether S. Typhimurium may benefit from expression of O-antigen-deficient LPS during intracellular stages of infection. To settle this issue the fitness of defined mutants devoid of O-antigen in macrophage-like cells was studied. O-antigen-deficient mutants inhibited iNOS activity in macrophage-like cells in an apparently SPI-2 dependent manner. Consequently the mutants displayed increased growth yields within these cells compared to wild type bacteria. Production of ROI as well as RNI is critical for control of disease proliferation in the murine salmonellosis model. In E.coli the thioredoxin and glutathione/glutaredoxin systems have been shown to mediate protection against oxidative stress.

When studying the role of these systems in Salmonella we could show that whereas thioredoxin 1 (TrxA) is dispensable for resistance to oxidative or NO stress in vitro, it is essential for bacterial growth in both epithelial and macrophage-like cells as well as for virulence in vivo in the murine infection model. Whereas the level of replication within macrophage-like cells correlates directly to the redox potential of TrxA, in vivo virulence depends on both redox dependent and independent activities of TrxA. Moreover, TrxA was shown to be required for proper function of SPI-2 and for the ability of O-antigen deficient S. Typhimurium to inhibit iNOS activity.