NITRIC-OXIDE POTENTIATES HYDROGEN PEROXIDE-INDUCED KILLING OF ESCHERICHIA-COLI

Previously, we reported that nitric oxide (NO) provides significant pr otection to mammalian cells from the cytotoxic effects of hydrogen per oxide (H2O2). Murine neutrophils and activated macrophages, however, p roduce NO, H2O2, and other reactive oxygen species to kill microorgani sms, which suggests a paradox. In this study, we treated bacteria (Esc herichia coli) with NO and H2O2 for 30 min and found that exposure to NO resulted in minimal toxicity, but greatly potentiated (up to 1,000- fold) H2O2-mediated killing, as evaluated by a clonogenic assay. The c ombination of NO/H2O2 induced DNA double strand breaks in the bacteria l genome, as shown by field-inverted gel electrophoresis, and this inc reased DNA damage may correlate with cell killing. NO was also shown t o alter cellular respiration and decrease the concentration of the ant ioxidant glutathione to a residual level of 15-20% in bacterial cells. The iron chelator desferrioxamine did not stop the action of NO on re spiration and glutathione decrease, yet it prevented the NO/H2O2 syner gistic cytotoxicity, implicating metal ions as critical participants i n the NO/H2O2 cytocidal mechanism. Our results suggest a possible mech anism of modulation H2O2-mediated toxicity, and we propose a new key r ole in the antimicrobial macrophagic response for NO.