Bacillus anthracis-derived nitric oxide is essential for pathogen virulence and survival in macrophages.

Phagocytes generate nitric oxide (NO) and other reactive oxygen and nitrogen species in large quantities to combat infecting bacteria. Here, we report the surprising observation that in vivo survival of a notorious pathogen-Bacillus anthracis-critically depends on its own NO-synthase (bNOS) activity. Anthrax spores (Sterne strain) deficient in bNOS lose their virulence in an A/J mouse model of systemic infection and exhibit severely compromised survival when germinating within macrophages. The mechanism underlying bNOS-dependent resistance to macrophage killing relies on NO-mediated activation of bacterial catalase and suppression of the damaging Fenton reaction. Our results demonstrate that pathogenic bacteria use their own NO as a key defense against the immune oxidative burst, thereby establishing bNOS as an essential virulence factor. Thus, bNOS represents an attractive antimicrobial target for treatment of anthrax and other infectious diseases.