Pathways Linking Oral Bacteria, Nitric Oxide Metabolism, and Health.

Nitrate-reducing oral bacteria have gained a lot of interest due to their involvement in nitric oxide (NO) synthesis and its important cardiometabolic outcomes. Consortia of nitrate-metabolizing oral bacteria associated with cardiometabolic health and cognitive function have been recently identified. Longitudinal studies and clinical trials have shown that chronic mouthwash use is associated with increased blood pressure and increased risk for prediabetes/diabetes and hypertension. Concurrently, recent studies are beginning to shed some light on the complexity of nitrate reduction pathways of oral bacteria, such as dissimilatory nitrate reduction to ammonium (DNRA), which converts nitrite into ammonium, and denitrification, which converts nitrite to NO, nitrous oxide, and dinitrogen. These pathways can affect the composition and metabolism of the oral microbiome; consequently, salivary nitrate and nitrite metabolism have been proposed as targets for probiotics and oral health. These pathways could also affect systemic NO levels because NO generated through denitrification can be oxidized back to nitrite in the saliva, thus facilitating flux along the NO3--NO2--NO pathway, while DNRA converts nitrite to ammonium, leading to reduced NO. It is, therefore, important to understand which pathway predominates under different oral environmental conditions, since the clinical consequences could be different for oral and systemic health. Recent studies show that oral hygiene measures such as tongue cleaning and dietary nitrate are likely to favor denitrifying bacteria such as Neisseria, which are linked with better cardiometabolic health. A vast body of literature demonstrates that redox potential, carbon-to-nitrate ratio, and nitrate-to-nitrite ratio are key environmental drivers of the competing denitrification and DNRA pathways in various natural and artificial ecosystems. Based on this information, a novel behavioral and microbial model for nitric oxide metabolism and health is proposed, which links lifestyle factors with oral and systemic health through NO metabolism.

Urease activity in dental plaque and saliva of children during a three-year study period and its relationship with other caries risk factors.

UNLABELLED: Bacterial urease activity in dental plaque and in saliva generates ammonia, which can increase the plaque pH and can protect acid-sensitive oral bacteria. Recent cross-sectional studies suggest that reduced ability to generate ammonia from urea in dental plaque can be an important caries risk factor. In spite of this proposed important clinical role, there is currently no information available regarding important clinical aspects of oral ureolysis in children. OBJECTIVE: The objective of this study was to evaluate the distribution and pattern of urease activity in the dental plaque and in the saliva of children during a three-year period, and to examine the relationship of urease with some important caries risk factors. METHODS: A longitudinal study was conducted with repeated measures over a three-year period on a panel of 80 children, aged 3-6 years at recruitment. The dynamics of change in urease activity were described and associated with clinical, biological, and behavioural caries risk factors. RESULTS: Urease activity in plaque showed a trend to remain stable during the study period and was negatively associated with sugar consumption (P<0.05). Urease activity in unstimulated saliva increased with age, and it was positively associated with the levels of mutans streptococci in saliva and with the educational level of the parents (P<0.05). CONCLUSIONS: The results of this study reveal interesting and complex interactions between oral urease activity and some important caries risk factors. Urease activity in saliva could be an indicator of mutans infection in children.

The effect of sucrose on plaque and saliva urease levels in vivo.

OBJECTIVE: Dietary sugar exposures induce an immediate drop of the plaque pH. Based on in vitro observations, it was hypothesized that oral bacteria may rapidly respond to this environmental change by increasing the activity or expression of alkali-generating pathways, such as the urease pathway. The objective of this exploratory in vivo study was to determine the short-term effect of a brief sucrose exposure on plaque and saliva urease activity and expression, and to relate this effect to caries experience. METHODS: Urease activity levels were measured in plaque and saliva samples collected from 20 children during fasting conditions and 30 min after rinsing with a sucrose solution. Streptococcus salivarius ureC-specific mRNA in saliva was quantified using real-time RT-PCR. The impact of host-related factors, such as age, gender, sugar consumption, salivary mutans streptococci levels and caries status on urease activity was evaluated. RESULTS: Plaque urease activity under fasting conditions was higher in subjects with low caries and mutans streptococci levels. This difference was not observed after the sucrose exposure. The response of urease to sucrose in vivo did not depend on caries experience or salivary mutans levels. Significant increase in urease activity of plaque and saliva after exposure to sucrose was observed only in the subjects who had low urease levels at baseline. CONCLUSIONS: The findings of this exploratory study suggest that plaque urease activity may have an important long-term influence in caries development but not during a cariogenic challenge.