Flavohemoglobin Hmp protects Salmonella enterica serovar typhimurium from nitric oxide-related killing by human macrophages

Survival of macrophage microbicidal activity is a prerequisite for invasive disease caused by the enteric pathogen Salmonella enterica serovar Typhimurium. Flavohemoglobins, such as those of Escherichia coli, Salmonella, and yeast, play vital roles in protection of these microorganisms in vitro from nitric oxide (NO) and nitrosative stress. A Salmonella hmp mutant defective in flavohemoglobin (Hmp) synthesis exhibits growth that is hypersensitive to nitrosating agents. We found that respiration of this mutant exhibited increased inhibition by NO, whereas wild-type cells pregrown with sodium nitroprusside or S-nitrosoglutathione showed enhanced tolerance of NO. Most significantly, hmp mutants internalized by primary human peripheral monocyte-derived macrophages survived phagocytosis relatively poorly compared with similarly bound and internalized wild-type cells. That the enhanced sensitivity to macrophage microbicidal activity is due primarily to the failure of Salmonella to detoxify NO was suggested by the ability Of L-N-G-monomethyl arginine-an inhibitor of NO synthase-to eliminate the difference in killing between wild-type and hmp mutant Salmonella cells. These observations suggest that Salmonella Hmp contributes to protection from NO-mediated inhibition by human macrophages.