Nitric oxide (NO) plays a crucial role in the antimicrobial activity of host defense systems. We investigated the function of Pseudomonas aeruginosa NO reductase as a detoxifying enzyme in phagocytes. We found that the growth of the NO reductase-deficient mutant of P. aeruginosa under a microaerobic condition was inhibited by the exogenous NO. Furthermore, the intracellular survival assay within the NO-producing RAW 264.7 macrophages revealed that the wild-type strain survived longer than the NO reductase-deficient mutant. These results suggest that the P. aeruginosa NO reductase may contribute to the intracellular survival by acting as a counter component against the host's defense systems. (c) 2007 Elsevier Inc. All rights reserved.