Significance Denitrification, a form of microbial anaerobic respiration where nitrate is sequentially reduced (NO 3 − → NO 2 − → NO → N 2 O → N 2 ) is environmentally, biologically, and chemically interesting, as well as being medically significant. Some pathogenic bacteria, including the major opportunistic pathogen Pseudomonas aeruginosa , can survive in oxygen-limited environments such as biofilms and the lungs of cystic fibrosis patients, owing to denitrification. The current proposal of a complex formation of NO-generating nitrite reductase and NO-decomposing nitric oxide reductase for rapid elimination of NO, a cytotoxic intermediate, in denitrification contributes to further understanding of denitrification and to the design of antimicrobial drugs. This paper also provides an idea of how biological systems control the dynamics of cytotoxic diffusible compounds such as NO in cells.