Nitrate reductase activity of bacteria in saliva of term and preterm infants

► We measure the nitrate and nitrite concentrations in the saliva of newborn infants. ► Oral bacterial nitrate reductase activity is lower in newborns than in adults. ► The most prevalent nitrate reducing bacteria in adults are also found in newborns. ► Eliminating nitrate reducing bacteria in the mouth lowers plasma nitrite levels. ► Infants’ low salivary nitrite levels and saliva production result in low nitrite ingestion. The salivary glands of adults concentrate nitrate from plasma into saliva where it is converted to nitrite by bacterial nitrate reductases. Nitrite can play a beneficial role in adult gastrointestinal and cardiovascular physiology. When nitrite is swallowed, some of it is converted to nitric oxide (NO) in the stomach and may then exert protective effects in the gastrointestinal tract and throughout the body. It has yet to be determined either when newborn infants acquire oral nitrate reducing bacteria or what the effects of antimicrobial therapy or premature birth may be on the bacterial processing of nitrate to nitrite. We measured nitrate and nitrite levels in the saliva of adults and both preterm and term human infants in the early weeks of life. We also measured oral bacterial reductase activity in the saliva of both infants and adults, and characterized the species of nitrate reducing bacteria present. Oral bacterial conversion of nitrate to nitrite in infants was either undetectable or markedly lower than the conversion rates of adults. No measurable reductase activity was found in infants within the first two weeks of life, despite the presence of oral nitrate reducing bacteria such as Actinomyces odontolyticus, Veillonella atypica, and Rothia mucilaginosa. We conclude that relatively little nitrite reaches the infant gastrointestinal tract due to the lack of oral bacterial nitrate reductase activity. Given the importance of the nitrate–nitrite–NO axis in adults, the lack of oral nitrate-reducing bacteria in infants may be relevant to the vulnerability of newborns to hypoxic stress and gastrointestinal tract pathologies.