Callum Duncan

Chemical generation of nitric oxide in the mouth from the enterosalivary circulation of dietary nitrate.

High concentrations of nitrite present in saliva (derived from dietary nitrate) may, upon acidification, generate nitrogen oxides in the stomach in sufficient amounts to provide protection from swallowed pathogens. We now show that, in the rat, reduction of nitrate to nitrite is confined to a specialized area on the posterior surface of the tongue, which is heavily colonized by bacteria, and that nitrate reduction is absent in germ-free rats. We also show that in humans increased salivary nitrite production resulting from nitrate intake enhances oral nitric oxide production. We propose that the salivary generation of nitrite is accomplished by a symbiotic relationship involving nitrate-reducing bacteria on the tongue surface, which is designed to provide host defence against microbial pathogens in the mouth and lower gut. These results provide further evidence for beneficial effects of dietary nitrate.

Identification of nitrate reducing bacteria from the oral cavity of rats and pigs.

Abstract Within the oral cavity most nitrate reduction was detected on the dorsal surface of the tongue. Nitrite producing bacteria (NPB) were isolated from tongues of pigs obtained from a local abattoir and compared with previous results from laboratory rats (10). The most commonly isolated genus was Staphylococcus in both pig and rat with Staphylococcus sciuri being the most common species. Other nitrate reducing bacteria included Enterobacteriaciae, Micrococcus, Streptococcus and Pasteurella spp. in the pig and Pasteurella and Streptococcus spp. in the rat. Nitrate reduction and numbers of nitrate reducing bacteria were very similar over the surface of tongues from pigs. Conversely, in the rat the enumeration of culturable bacteria (cfu) showed an increase in the density of bacteria towards the posterior tongue. The proportion of culturable NPB in the total bacterial population increased from 6% on the anterior tongue to 65% on the posterior tongue. Nitrite production was sensitive to oxygen and significant nitrite generation was only detected on the posterior tongue of rats where the majority of bacteria are situated in deep clefts in the tongue surface. In the pig, deep clefts were found over the entire tongue surface.

Bacterial Nitrate Reductase Activity is Induced in the Oral Cavity by Dietary Nitrate

Dietary nitrate forms the basis for a non-immune defence mechanism against gastrointestinal and oral pathogens in animals and man. Facultatively anaerobic bacteria residing in specific low oxygen niches in the oral cavity produce nitrite, from salivary nitrate, which then forms antimicrobial compounds in the stomach acid. The expression of nitrate reductase has mainly been studied in soil bacteria where, in most bacteria, it is controlled by oxygen and nitrate concentrations at the transcriptional level. Since the concentration of nitrite secreted into the oral cavity via the saliva is heavily influenced by dietary nitrate intake, an increased dietary nitrate intake is likely to have a significant effect on nitrite production in the oral cavity. Nitrite production on the dorsal tongue was significantly greater in rats fed a nitrate supplemented diet for 14 days than that of rats fed a non-supplemented diet (p<0.001), while no statistical difference in bacterial density was found. These results suggest that a high nitrate diet increases the expression of bacterial nitrate reductase by oral bacteria, augmenting the rate of reduction of nitrate to nitrite and, thus, the enterosalivary circulation of nitrate.