The nonpathogenic Bacillus subtilis and the pathogen Staphylococcus aureus are gram-positive model organisms that have to cope with the radical nitric oxide (NO) generated by nitrite reductases of denitrifying bacteria and by the inducible NO synthases of immune cells of the host, respectively. The response of both microorganisms to NO was analyzed by using a two-dimensional gel approach. Metabolic labeling of the proteins revealed major changes in the synthesis pattern of cytosolic proteins after the addition of the NO donor MAHMA NONOate. Whereas B. subtilis induced several oxidative stress-responsive regulons controlled by Fur, PerR, OhrR, and Spx, as well as the general stress response controlled by the alternative sigma factor SigB, the more resistant S. aureus showed an increased synthesis rate of proteins involved in anaerobic metabolism. These data were confirmed by nuclear magnetic resonance analyses indicating that NO causes a drastically higher increase in the formation of lactate and butanediol in S. aureus than in B. subtilis. Monitoring the intracellular protein thiol state, we observed no increase in reversible or irreversible protein thiol modifications after NO stress in either organism. Obviously, NO itself does not cause general protein thiol oxidations. In contrast, exposure of cells to NO prior to peroxide stress diminished the irreversible thiol oxidation caused by hydrogen peroxide.Bacillus subtilis and Staphylococcus aureus are gram-positive model bacteria. Whereas B. subtilis is considered to be harmless, S. aureus is a facultative pathogen and the leading cause of nosocomial and community-acquired infections. Both microorganisms have to cope with high amounts of nitric oxide (NO) (up to the M range) generated from coexisting denitrifying bacteria or from the innate immune response of the host, respectively (17,31,55,68). Hence, the ability of B. subtilis and S. aureus to protect themselves against NO might be crucial for survival in their respective natural habitats.The cytotoxic properties of the small, lipophilic, and freely diffusible radical NO are attributed to its high reactivity. Indirect effects of NO are caused by the reaction of the radical with oxygen or superoxide, resulting in the formation of a number of additional reactive nitrogen species, including nitrogen dioxide, peroxynitrite, and dinitrogen trioxide. These nitrogen species differ in reactivity, stability, and biological activity but result in a broad spectrum of antimicrobial activity (25). In general, reactive oxygen and nitrogen species can interact with numerous targets, including thiols, metal centers, tyrosine residues in proteins, nucleotide bases, and lipids (20,69). NO directly affects the activity of enzymes by the reaction with bound free radicals or with metal centers (51, 72, 100). For example, the formation of metal-nitrosyl complexes in respiratory enzymes was shown to inhibit bacterial respiration (8,13,71,88) and the formation of a dinitrosyl-iron complex of a protein essential for branched-chain am...