Salivary nitrite production is elevated in individuals with a higher abundance of oral nitrate-reducing bacteria

Mar 19, 2018Free radical biology & medicine

Higher levels of mouth bacteria that convert nitrate are linked to more nitrite in saliva

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Abstract

The abundance of nitrate-reducing bacteria in the oral cavity was assessed in 25 healthy humans.

Seven species of bacteria known for nitrate reduction were identified, with Prevotella melaninogenica and Veillonella dispar being the most prevalent.
Dietary nitrate supplementation led to a significant reduction in blood pressure and increased levels of nitrate and nitrite in saliva and plasma.
The abundance of nitrate-reducing bacteria was associated with increased production of salivary nitrite but not with other physiological measurements.
Participants with higher bacterial abundance (> 50%) produced more salivary nitrite and exhibited faster salivary nitrite pharmacokinetics compared to those with lower abundance (< 50%).
Higher abundance of nitrate-reducing bacteria did not lead to a greater plasma nitrite response, indicating it may not enhance nitric oxide generation from dietary sources.

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Nitric oxide (NO) can be generated endogenously via NO synthases or via the diet following the action of symbiotic nitrate-reducing bacteria in the oral cavity. Given the important role of NO in smooth muscle control there is an intriguing suggestion that cardiovascular homeostasis may be intertwined with the presence of these bacteria. Here, we measured the abundance of nitrate-reducing bacteria in the oral cavity of 25 healthy humans using 16S rRNA sequencing and observed, for 3.5 h, the physiological responses to dietary nitrate ingestion via measurement of blood pressure, and salivary and plasma NO metabolites. We identified 7 species of bacteria previously known to contribute to nitrate-reduction, the most prevalent of which were Prevotella melaninogenica and Veillonella dispar. Following dietary nitrate supplementation, blood pressure was reduced and salivary and plasma nitrate and nitrite increased substantially. We found that the abundance of nitrate-reducing bacteria was associated with the generation of salivary nitrite but not with any other measured variable. To examine the impact of bacterial abundance on pharmacokinetics we also categorised our participants into two groups; those with a higher abundance of nitrate reducing bacteria (> 50%), and those with a lower abundance (< 50%). Salivary nitrite production was lower in participants with lower abundance of bacteria and these individuals also exhibited slower salivary nitrite pharmacokinetics. We therefore show that the rate of nitrate to nitrite reduction in the oral cavity is associated with the abundance of nitrate-reducing bacteria. Nevertheless, higher abundance of these bacteria did not result in an exaggerated plasma nitrite response, the best known marker of NO bioavailability. These data from healthy young adults suggest that the abundance of oral nitrate-reducing bacteria does not influence the generation of NO through the diet, at least when the host has a functional minimum threshold of these microorganisms.

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Salivary nitrite production is elevated in individuals with a higher abundance of oral nitrate-reducing bacteria

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