Cited by 311Gut microbiota and healthWastewater Treatment and Nitrogen RemovalProbiotics and Fermented Foods

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Nitrate Reduction to Nitrite, Nitric Oxide and Ammonia by Gut Bacteria under Physiological Conditions

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Abstract

The biological nitrogen cycle involves step-wise reduction of nitrogen oxides to ammonium salts and oxidation of ammonia back to nitrites and nitrates by plants and bacteria. Neither process has been thought to have relevance to mammalian physiology; however in recent years the salivary bacterial reduction of nitrate to nitrite has been recognized as an important metabolic conversion in humans. Several enteric bacteria have also shown the ability of catalytic reduction of nitrate to ammonia via nitrite during dissimilatory respiration; however, the importance of this pathway in bacterial species colonizing the human intestine has been little studied. We measured nitrite, nitric oxide (NO) and ammonia formation in cultures of Escherichia coli, Lactobacillus and Bifidobacterium species grown at different sodium nitrate concentrations and oxygen levels. We found that the presence of 5 mM nitrate provided a growth benefit and induced both nitrite and ammonia generation in E.coli and L.plantarum bacteria grown at oxygen concentrations compatible with the content in the gastrointestinal tract. Nitrite and ammonia accumulated in the growth medium when at least 2.5 mM nitrate was present. Time-course curves suggest that nitrate is first converted to nitrite and subsequently to ammonia. Strains of L.rhamnosus, L.acidophilus and B.longum infantis grown with nitrate produced minor changes in nitrite or ammonia levels in the cultures. However, when supplied with exogenous nitrite, NO gas was readily produced independently of added nitrate. Bacterial production of lactic acid causes medium acidification that in turn generates NO by non-enzymatic nitrite reduction. In contrast, nitrite was converted to NO by E.coli cultures even at neutral pH. We suggest that the bacterial nitrate reduction to ammonia, as well as the related NO formation in the gut, could be an important aspect of the overall mammalian nitrate/nitrite/NO metabolism and is yet another way in which the microbiome links diet and health.

Overview

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TL;DR

In vitro cultures of common gut bacteria show that nitrate can be reduced first to nitrite and then to ammonia under oxygen and nitrate conditions relevant to the gastrointestinal tract. E. coli and L. plantarum were the main nitrate reducers, while lactic acid bacteria also generated NO from nitrite largely through acid-driven non-enzymatic reduction.

Key findings

  • E. coli and L. plantarum grown with 5 mM nitrate at low oxygen showed a growth benefit and produced nitrite and ammonia; nitrite and ammonia accumulated when nitrate was at least 2.5 mM, reaching about 260 μM NO2− and 180 μM NH4+ in E. coli media at 5–20 mM nitrate.
  • Time-course experiments at 2% O2 with 5 mM nitrate suggested nitrate is first converted to nitrite and subsequently to ammonia; nitrite peaked at 30–36 h at about 0.4 mM for E. coli and 0.1 mM for L. plantarum.
  • L. rhamnosus, L. acidophilus and B. longum infantis showed minor changes in nitrite or ammonia with nitrate, but all LAB cultures produced NO from exogenous nitrite; LAB lactic acid acidified the medium and drove non-enzymatic nitrite reduction to NO, whereas E. coli converted nitrite to NO even at neutral pH.
  • E. coli NO production from nitrite was low at 2% O2 (<40 ppb/109 CFU) and much larger after anaerobic growth, roughly correlating with increasing nitrate; arginine and L-NAME did not change NO production, arguing against bacterial NOS activity.

Methods

  • In vitro cultures of E. coli (MG1655 in Results; Table 1 lists MG1566), L. acidophilus ATCC 4356, L. plantarum ATCC BAA-793 (WCFS1), L. rhamnosus ATCC 7469, and B. longum infantis ATCC 15697 were grown at 37°C in modified LMRS broth without ammonium citrate, with controlled O2 levels from 0% to 21% and nitrate from 0 to 20 mM; LAB were supplemented with heme and vitamin K2.
  • Nitrite was measured by acidic tri-iodide-based gas-phase chemiluminescence, ammonia by colorimetric kits, NO gas by ozone-based chemiluminescence, and lactic acid enzymatically.
  • Experiments were performed in triplicate, values reported as mean ± SD, nitrite and ammonia normalized to OD600 after 24 h growth, and one-way ANOVA used where applicable.

Limitations

  • The paper states that the importance of this pathway in vivo demands further studies, that how much ammonia comes from nitrate-nitrite reduction versus deamination or urease activity requires detailed animal and/or human metabolic studies, and that the biological significance of dietary nitrate conversion in the intestinal lumen remains to be established.
  • The paper notes that very little information exists on in vivo O2 concentrations in intestinal fluids, so the oxygen conditions used are approximations of gastrointestinal levels.
  • Evident from the text: findings are from in vitro single-strain cultures, not complex gut communities or host tissue; ammonia measurements in cell pellets had large errors and possible ammonia evaporation, leading the authors to measure nitrate metabolites mainly in cell-free supernatant.

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Integrity signals

Registry

Crossref · 10.1371/journal.pone.0119712

Cites

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Figures

Fig 1 Nitrate and oxygen effect on E.coli bacterial cultures growth and formation of nitrite and ammonia. (A) Growth curves for E.coli MG1655 grown in the absence (black closed symbols) or in the presence of 5 mM nitrate (red open symbols) at 37°C in LMRS broth at 21%, 2%, and 0% O2 concentrations (respectively square, circle and diamond symbols). (B) Concentration of nitrite and ammonia (blue and red solid lines) in E.coli pellets after 24 h growth at different oxygen levels with 5 mM nitrate. (C) and (D) Respectively dependence on nitrate (at 0% O2) and oxygen (at 5 mM nitrate) of nitrite and ammonia concentrations in the cell-free culture media after 24h growth. The ammonia content of LMRS alone is indicated by the dashed lines. Values are means ± SD (n = 3). The average SD resulted smaller than the symbols dimensions (0.04 OD) and it is not shown for clarity.
Fig 1 Nitrate and oxygen effect on E.coli bacterial cultures growth and formation of nitrite and ammonia. (A) Growth curves for E.coli MG1655 grown in the absence (black closed symbols) or in the presence of 5 mM nitrate (red open symbols) at 37°C in LMRS broth at 21%, 2%, and 0% O2 concentrations (respectively square, circle and diamond symbols). (B) Concentration of nitrite and ammonia (blue and red solid lines) in E.coli pellets after 24 h growth at different oxygen levels with 5 mM nitrate. (C) and (D) Respectively dependence on nitrate (at 0% O2) and oxygen (at 5 mM nitrate) of nitrite and ammonia concentrations in the cell-free culture media after 24h growth. The ammonia content of LMRS alone is indicated by the dashed lines. Values are means ± SD (n = 3). The average SD resulted smaller than the symbols dimensions (0.04 OD) and it is not shown for clarity.CC0 · PubMed Central
Fig 2 The effect of nitrate and oxygen gradients on the generation of nitrite and ammonia in different LAB cultures. Nitrite (A) and ammonia (B) concentrations were measured in LMRS media after 24 h growth at 2% O2 with supplementation of different nitrate concentrations (0 to 10 mM). Similarly in (C) and (D) nitrate was fixed at 5 mM and we measured nitrite and ammonia dependence on oxygen concentrations (0, 2, 4, 6, 10 and 21%). Each point represents the mean ± SD (n = 3).
Fig 2 The effect of nitrate and oxygen gradients on the generation of nitrite and ammonia in different LAB cultures. Nitrite (A) and ammonia (B) concentrations were measured in LMRS media after 24 h growth at 2% O2 with supplementation of different nitrate concentrations (0 to 10 mM). Similarly in (C) and (D) nitrate was fixed at 5 mM and we measured nitrite and ammonia dependence on oxygen concentrations (0, 2, 4, 6, 10 and 21%). Each point represents the mean ± SD (n = 3).CC0 · PubMed Central
Fig 3 Time-course curves suggest that nitrate is first converted to nitrite and subsequently to ammonia. Samples from E.coli (red line with circles) and L. plantarum (blue line with triangles) cultures grown at 2% O2 with 5 mM nitrate were collected at regular intervals and analyzed for nitrite (panel A) and ammonia (panel B). (C). Ratio between nitrite and ammonia concentrations measured at each time point. Black lines represent E.coli cultures containing no added nitrate.
Fig 3 Time-course curves suggest that nitrate is first converted to nitrite and subsequently to ammonia. Samples from E.coli (red line with circles) and L. plantarum (blue line with triangles) cultures grown at 2% O2 with 5 mM nitrate were collected at regular intervals and analyzed for nitrite (panel A) and ammonia (panel B). (C). Ratio between nitrite and ammonia concentrations measured at each time point. Black lines represent E.coli cultures containing no added nitrate.CC0 · PubMed Central
Fig 4 Bacterial NO generation and correlation with acidity of the growth medium. (A) Chemiluminescence detection of NO emission after injection of 100 μM nitrite in the vessel containing E.coli grown at different nitrate and oxygen conditions in modified LMRS broth for 24h. (B) Comparison of NO emission at 2% O2 as in panel A, but using LAB cultures. Diagonal patter and solid bars indicate respectively the values detected before and after bacteria were re-suspended in fresh media at pH = 6.5 (as described in text). (C) Quantification of the amount of NO detected after injection of 100 μM or 250 μM nitrite in fresh LMRS media at different pH obtained by acidification with concentrated L-lactic acid. All data represent mean ± SD in duplicates.
Fig 4 Bacterial NO generation and correlation with acidity of the growth medium. (A) Chemiluminescence detection of NO emission after injection of 100 μM nitrite in the vessel containing E.coli grown at different nitrate and oxygen conditions in modified LMRS broth for 24h. (B) Comparison of NO emission at 2% O2 as in panel A, but using LAB cultures. Diagonal patter and solid bars indicate respectively the values detected before and after bacteria were re-suspended in fresh media at pH = 6.5 (as described in text). (C) Quantification of the amount of NO detected after injection of 100 μM or 250 μM nitrite in fresh LMRS media at different pH obtained by acidification with concentrated L-lactic acid. All data represent mean ± SD in duplicates.CC0 · PubMed Central
Fig 5 Schematic representation of the link between different pathways of nitrogen oxides reduction in the human gut and the fate of ammonia. Each colored box represents a distinct pathway: Bacterial respiratory denitrification to dinitrogen in red box: the dissimilatory nitrate reduction to ammonia (DNRA) in blue box and the non-enzymatic conversion of nitrite to NO in green box (this route become significant only at pH<5.5). The endogenous L-arginine/NO synthase pathway from epithelial cells of the intestinal mucosa lining is also noted.
Fig 5 Schematic representation of the link between different pathways of nitrogen oxides reduction in the human gut and the fate of ammonia. Each colored box represents a distinct pathway: Bacterial respiratory denitrification to dinitrogen in red box: the dissimilatory nitrate reduction to ammonia (DNRA) in blue box and the non-enzymatic conversion of nitrite to NO in green box (this route become significant only at pH<5.5). The endogenous L-arginine/NO synthase pathway from epithelial cells of the intestinal mucosa lining is also noted.CC0 · PubMed Central

Verification

Claims the paper makes

41 Own result · 21 Not checked yet · 10 Own interpretation · 3 No basis given

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  • It may state two separate claims as one. 32
  • The quote only partly states this claim. 2
  • Probably not a substantial claim. 6
38 of 38
  • The paper relies on this without support.

    The O2 level of 2% used in some experiments is similar to the partial pressure found on the luminal surface of the intestinal mucosa.

    “with the O2 level fixed at 2%, a partial pressure similar to the one found on the luminal surface of the intestinal mucosa”
  • The paper relies on this without support.

    Ammonia generated by gut microbiota from nitrate would likely be carried to the liver via the portal vein and enter the urea cycle to be converted into urea and amino acids.

    “The ammonia thus generated would likely be carried to the liver via the portal vein, where it can enter the urea cycle and be converted into urea and amino acids.”
  • The paper relies on this without support.

    The in vitro conditions used are compatible with nitrate-rich diets and oxygen levels found on the mucosal surfaces of the GI tract.

    “using in vitro conditions compatible with nitrate-rich diets and oxygen levels found on the mucosal surfaces of the GI tract”
  • The paper relies on this without support.

    The intestinal pH ranges between 5.7 and 7.5.

    “Measurements of the intestine pH ranges between 5.7 and 7.5”
  • The paper relies on this without support.

    Approximately 60% of an ingested nitrate dose in humans is not accounted for in excreted nitrogen-containing compounds.

    “the biological fate of exogenous nitrate could not be accounted for in the excreted nitrogen-containing compounds which amount to approximately 60% of an ingested nitrate dose in humans”
  • The paper relies on this without support.

    The bacterial species and strains studied in vitro are representative of those performing nitrate reduction in the human gut.

    “selected common bacterial species colonizing the human intestine”
6 of 6

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Citations

Cites

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Research works printed but not linked: 1

  1. XB Ji, 1989, Nitrate Reductase of Escherichia-Coli as a No-Producing Nitrite Reductase, Biochemical Archives, 5, 61

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Metadata

Authors
Tiso, M., & Schechter, A. N.
Affiliations
National Institutes of Health (US)
Corresponding author
Schechter, Alan N.
Venue
PLOS ONE
Published
2015-03-24
Volume
10
Issue
3
Pages
e0119712
ISSN
1932-6203 (ISSN-L)
OpenAlex
W2016792399
Cited by
311
References
55
Citations by year
2026: 8 · 2025: 30 · 2024: 41 · 2023: 39 · 2022: 27 · 2021: 32 · 2020: 43 · 2019: 22 · 2018: 32 · 2017: 21 · 2016: 11 · 2015: 4
FWCI
9.85
Type
journal article
Language
en
Open access
Locations
  • PLoS ONE (publishedVersion, public-domain)
  • Europe PMC (PubMed Central) (publishedVersion, public-domain)
  • PubMed Central (publishedVersion, public-domain)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
National Institutes of Health
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Gut microbiota and health
  • Physical Sciences › Environmental Science › Pollution › Wastewater Treatment and Nitrogen Removal
  • Life Sciences › Agricultural and Biological Sciences › Food Science › Probiotics and Fermented Foods
Keywords
nitrate reduction, nitrite, nitric oxide, ammonia synthesis, gut microbiota, Escherichia coli, Lactobacillus, Bifidobacterium, lactic acid
MeSH terms
Gastrointestinal Microbiome, Ammonia, Analysis of Variance, Bifidobacterium, Escherichia coli, Humans, Lactobacillus, Nitrates, Nitric Oxide, Nitrites, Oxygen, Lactic Acid, Nitrogen Cycle

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