Peroxynitrite stress is exacerbated by flavohaemoglobin-derived oxidative stress in Salmonella Typhimurium and is relieved by nitric oxide.

Oxidative and nitrosative stresses including nitric oxide (NO), superoxide (O2⁻) and peroxynitrite play key roles in determining the outcome of bacterial infections. In order to survive within the host and allow proliferation within immune cells such as macrophages, Salmonella isolates have a number of inducible proteins that are able to detoxify these highly reactive species, notably the anoxically functioning NO reductase NorVW, and the aerobically functioning flavohaemoglobin, Hmp, which catalyses the reaction between oxygen and NO to produce relatively inert nitrate. However, in the absence of NO but in the presence of reducing substrates and oxygen, O2⁻ is generated from Hmp-mediated electron transfer to bound oxygen and may form a variety of further oxidative species. Hence, Hmp expression is under tight negative regulation by the transcription factor NsrR, abolition of which causes an increase in the production of Hmp. In a previous study, this increase in Hmp levels conferred resistance to the nitrosating agent S-nitrosoglutathione but, perhaps surprisingly, the organism became more sensitive to killing by macrophages. Here, we report that an nsrR mutant that constitutively overexpresses Hmp is also hypersensitive to peroxynitrite in vitro. This sensitivity is alleviated by deletion of the hmp gene or pre-incubation of growing bacteria with NO-releasing agents. We hypothesize that Hmp-expressing cells, in the absence of NO, generate reactive oxygen species, the toxicity of which is exacerbated by peroxynitrite in vitro and in macrophages. RT-PCR confirmed that peroxynitrite causes oxidative stress and upregulation of katG and ahpC, whilst hmp and norV expression are affected very little. The katG gene upregulated by peroxynitrite encodes a catalase peroxidase enzyme with well-established roles in detoxifying peroxides. Here, we report that KatG is also able to enhance the breakdown of peroxynitrite, suggesting that the protective role of this enzyme may be wider than previously thought. These data suggest that spatial and temporal fluctuations in the levels of NO and reactive oxygen species will have important consequences for bacterial survival in the macrophage.