Investigating the possible role of foodborne nitrite-producing bacteria during digestion in infant methemoglobinemia

Background: Nitrite is a major oxidant involved in dietary-induced infant methemoglobinemia. However, in foods linked to methemoglobinemia, nitrite concentrations are significantly lower than nitrate concentrations. In this study, we focused on the potential role of foodborne nitrite-producing bacteria (NPBs) in the nitrate-to-nitrite conversion after food ingestion. Scope and approach: Literature on following themes were reviewed: (1) dietary-induced infant methemoglobinemia cases, (2) nitrite and nitrate content in foods, (3) prevalence and taxonomical distribution of NPBs in foods, (4) nitrate metabolism by commensal lingual or gut microbiomes, and (5) survivability of exogenous NPBs in the infant gastrointestinal system. Key findings and conclusions: Vegetables, which typically contribute 60–80 % of total nitrate intake, were identified as the primary foods linked to dietary-induced infant methemoglobinemia cases. Since nitrite concentrations in vegetables are approximately 100 times lower than nitrate concentrations, it has been widely accepted that nitrate-to-nitrite conversion occurs either during post-harvest storage or through commensal microbiome metabolism. However, it has also been suggested that NPBs may play a crucial role in this transformation, based on two key factors: (i) the infant commensal microbiome exhibits significantly lower nitrate reduction activity (∼10 % of adult levels in the oral microbiome) and (ii) the infant gastrointestinal defense system is weaker than that of adults, as indicated by differences in gastric acid levels (postprandial pH: adults, pH 2; infants, pH 4–6) and bile salt concentrations (postprandial state: adults, 12–16 mM; infants, <1–8 mM). The substantiated plausibility of NPBs introduction, subsequent survival, and nitrate metabolism supports their potential involvement in infant methemoglobinemia.