The ability of the endospore-forming, gram-positive bacterium Bacillus anthracis to survive exposure to antibacterial killing mechanisms by activated macrophages is key to its germination and survival. These antibacterial killing mechanisms include, but are not limited to the generation of free radicals such as nitric oxide (•NO) and superoxide (O2) from the upregulation of inducible nitric oxide synthase (NOS 2) along with products derived from them, e.g., peroxynitrite (ONOO), as part of microbicidal activity. However questions still remain as to how these species are involved in microbial killing, specifically with respect to B. anthracis. In a previous study, we demonstrated that exposure of primary murine macrophages to sonicated B. anthracis endospores up-regulated NOS 2 and demonstrated a •NO-dependent bactericidal response, but unanswered in that study was which of the NOS 2-derived reactive oxygen species was responsible for the observed bactericidal response. Since NOS 2 also generates O2, experiments were designed to determine whether NOS 2 formed ONOO from the reaction of •NO with O2 and if so, was ONOO microbicidal toward B. anthracis.

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