Metabolic Pathways of Nitrate in the Eye Microbiota

Nitric oxide (NO) is a small free radical molecule with critical signaling roles. It is an endogenous vasodilator and an important mediator of vascular tone and vascular remodeling. Nitrate and nitrite are known as stable products of NO oxidation and are physiologically recycled in blood and tissues to reform NO by enzymatic and non-enzymatic pathways. Bacteria species in the human microbiome have been shown to reduce nitrate and nitrite and to produce NO under acidic conditions. In this study, we investigate the generation of nitrite and NO by bacterial species regularly identified in the eye microbiome of healthy humans, specifically Staphylococcus epidermidis and Bacillus subtilis. We measured nitrite and NO concentrations in cultures grown at oxygen and nitrate levels that approximate the conditions found in the eye. We found that Bacillus subtilis produced very low levels of NO even in the presence of millimolar nitrate concentrations. Instead, nitrite accumulated in the Staphylococcus epidermidis media after growth for 24 hours at low oxygen levels (2-6%) in a manner roughly directly proportional to the nitrate concentrations added in the media. These data indicate that during hypoxic conditions, Staphylococcus epidermidis express enzymes that efficiently reduce nitrate to nitrite and suggests that the related bacterial nitrite reduction to NO could be an important aspect of the overall nitrate/nitrite/NO metabolism. Therefore, the human microbiome can and may be a link between a nitrate rich diet and human health and disease.