Importance of Nitric Oxide Synthase in the Control of Infection by Bacillus anthracis

The spore-forming, gram-positive bacterium Bacillus anthracis, the causative agent of anthrax, has achieved notoriety due to its use as a bioterror agent. In the environment, B. anthracis exists as a dormant endospore. Upon infection, germination of endospores occurs during their internalization within the phagocyte, and the ability to survive exposure to antibacterial killing mechanisms, such as O₂[superscript ·-], NO[superscript ·], and H₂O₂, is a key initial event in the infective process. Macrophages generate NO[superscript ·] from the oxidative metabolism of L-arginine, using an isoform of nitric oxide synthase (NOS 2). Exposure of murine macrophages (RAW264.7 cells) to B. anthracis endospores up-regulated the expression of NOS 2 12 h after exposure, and production of NO[superscript ·] was comparable to that achieved following other bacterial infections. Spore-killing assays demonstrated a NO[superscript ·]-dependent bactericidal response that was significantly decreased in the presence of the NOS 2 inhibitor L-N⁶-(1-iminoethyl)lysine and in L-arginine-depleted media. Interestingly, we also found that B. anthracis bacilli and endospores exhibited arginase activity, possibly competing with host NOS 2 for its substrate, L-arginine. As macrophage-generated NO[superscript ·] is an important pathway in microbial killing, the ability of endospores of B. anthracis to regulate production of this free radical has important implications in the control of B. anthracis-mediated infection.