Roles of nitric oxide in inducible resistance of Escherichia coli to activated murine macrophages

Nitric oxide (NO·) is produced as a cytotoxic free radical through enzymatic oxidation of L-arginine in activated macrophages. Pure NO· gas was previously found to induce the Escherichia coli soxRS oxidative stress regulon, which is readily monitored by using 3 soxS'::lac fusion. The soxRS system includes antioxidant defenses, such as a superoxide dismutase and a DNA repair enzyme for oxidative damage, and protects E. coli from the cytotoxicity of NO·-generating macrophages. Previous experiments involved exposing E. coli to a bolus of NO· rather than the steadily generated gas expected of activated macrophages. We show here detectable induction of soxS transcription by NO· delivered at rates as low as 25 μM/h. Maximal induction was observed at 25 μM NO· per h under anaerobic conditions but at 125 μM/h aerobically. After incubation with tone macrophages, soxS expression was induced in the phagocytosed bacteria up to ~30-fold after an 8-h exposure. This in vivo induction was almost completely eliminated by the NO· synthase inhibitor N(G)-monomethyl-L-arginine. The inhibitor increased the survival of a ΔsoxRS strain but not that of wild-type E. coli after phagocytosis, which suggests that induction of the soxRS regulon by NO· can counteract most of the cytotoxic effects of NO· production by the macrophages. We show that the soxRS-regulated enzyme glucose-6-phosphate dehydrogenase is an important element of the defense against macrophages.