Localisation of nitrate-reducing and highly abundant microbial communities in the oral cavity

Abstract

The nitrate (NO3-) reducing bacteria resident in the oral cavity have been implicated as key mediators of nitric oxide (NO) homeostasis and human health. NO3--reducing oral bacteria reduce inorganic dietary NO3- to nitrite (NO2-) via the NO3--NO2--NO pathway. Studies of oral NO3--reducing bacteria have typically sampled from either the tongue surface or saliva. The aim of this study was to assess whether other areas in the mouth could contain a physiologically relevant abundance of NO3- reducing bacteria, which may be important for sampling in clinical studies. The bacterial composition of seven oral sample types from 300 individuals were compared using a meta-analysis of the Human Microbiome Project data. This analysis revealed significant differences in the proportions of 20 well-established oral bacteria and highly abundant NO3--reducing bacteria across each oral site. The genera included Actinomyces, Brevibacillus, Campylobacter, Capnocytophaga, Corynebacterium, Eikenella, Fusobacterium, Granulicatella, Haemophilus, Leptotrichia, Microbacterium, Neisseria, Porphyromonas, Prevotella, Propionibacterium, Rothia, Selenomonas, Staphylococcus, Streptococcus and Veillonella. The highest proportion of NO3--reducing bacteria was observed in saliva, where eight of the bacterial genera were found in higher proportion than on the tongue dorsum, whilst the lowest proportions were found in the hard oral surfaces. Saliva also demonstrated higher intra-individual variability and bacterial diversity. This study provides new information on where samples should be taken in the oral cavity to assess the abundance of NO3--reducing bacteria. Taking saliva samples may benefit physiological studies, as saliva contained the highest abundance of NO3- reducing bacteria and is less invasive than other sampling methods. These results inform future studies coupling oral NO3--reducing bacteria research with physiological outcomes affecting human health.

Commentary of the SKLM to the EFSA opinion on risk assessment of N-nitrosamines in food

“…When reaching the salivary glands, nitrate is secreted by active transport from blood into saliva, achieving salivary nitrate levels up to 20-times the plasma level. In the oral cavity, salivary nitrate is partially converted into nitrite by oral and commensal microbial reductases (Eisenbrand et al 2022 ; Eisenbrand et al 1980 ; L’Heureux et al 2023 ; Liu et al 2023 ). Approximately 25% of the orally ingested nitrate is secreted through the salivary glands and up to 7–8% of the totally ingested nitrate becomes converted to nitrite in the oral cavity during entero-salivary circulation (Spiegelhalder et al 1976 ; Tannenbaum et al 1976 ; Tricker and Preussmann 1987 ).…”

The interaction between oral microbiota and gut microbiota in atherosclerosis

“…Salivary glands absorb 25% of nitrate in food, and nitrate is formed by endogenous oxidation. Symbiotic bacteria in the mouth produce nitrate reductase, metabolize NO3− in saliva to NO2−, and help the host complete the first step of the NO conversion process ( 98 ). Another study has shown that periodontal pathogens can reduce the synthesis of NO and tetrahydrobiopterin (BH4) in mice, which is a cofactor of nitric oxide synthetase (NOS), and its reduction will lead to a decrease in NOS and NO synthesis in blood vessels and colon ( 99 ).…”