Background. There are no reported data to explain how Salmonella suppress nitrite ion production in macrophages or whether this phenomenon is unique to typhoidal or non-typhoidal serovars. The aims of this study were, therefore, to investigate these phenomena. Methods. We measured survival of S. typhimurium 14028 and its phoP mutant in murine J774 macrophages, cultured with or without interferon gamma. We compared expression of inducible nitric oxide synthase (iNOS) mRNA and protein, and nitrite ion production and also examined binding of nuclear factor B (NF B) and activator protein 1 (AP-1) to macrophage DNA. Results. S. typhimurium 14028 inhibited binding of NF B and AP-1 to DNA in murine J774. A macrophages via an intact phoP regulon. This correlated with increased survival and reduced iNOS expression. Suppression of NF B activity was ameliorated in macrophages cultured with IFN-γ and this correlated with increased expression of iNOS mRNA and nitrite ion production, although IFN-γ had no effect on AP-1/DNA interaction. We show, that with one exception, suppression of iNOS is unique to typhoidal serovars. Conclusion. S. typhimurium inhibit NF B and AP-1 interaction with macrophage DNA via the PhoP regulon, this reduces nitrite ion production and is principally associated with typhoidal serovars. 1. Introduction S. typhimurium infection in mice is a standard laboratory model for human typhoid, and previous studies have shown that S. typhimurium mutants which are unable to survive in murine macrophages are avirulent [1]. Thus, survival of Salmonella in macrophages appears to be a critical step in the induction of typhoid. The Salmonella phoP/phoQ regulon regulates genes located on Salmonella Pathogenicity Island 2 (SPI-2) which encode proteins needed for survival of Salmonella inside of macrophages [2] and Salmonellae which have mutations in their phoP/phoQ regulon are avirulent in mice [3]. The affect of phoP on salmonella survival is multifaceted but studies by Svensson et al. [4] have shown that phoP mutation induces increased nitrite ion production by macrophages compared with nitrite ion production induced by the parent strain but this study did not investigate the mechanisms behind this phenomenon. Studies using and mice indicate that reactive nitrogen species (RNS) are important in controlling Salmonella later in the infection and this is preceded by a reactive oxygen species (ROS)-dependent control phase [5, 6] and it is also known that nitric oxide increases the sensitivity to cellular acid by phoP mutants [7]. Taken together these studies
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