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microaerotolerant

METPO:1000610 · CLASS · REVIEWED

An oxygen preference that tolerates low levels of molecular oxygen (O₂) without requiring it.

Microaerotolerant low-oxygen survival mechanism

Evidence-backed causal sketch linking microaerotolerance to limited oxygen exposure and oxidative stress defenses.

Microaerotolerant low-oxygen survival mechanism Interactive directed graph showing evidence-backed causal relationships for microaerotolerant.

Edge evidence

  • limited oxygen exposure tolerated by microaerotolerant

    Microaerotolerant organisms survive limited oxygen exposure.

    • DOI:10.1016/j.biortech.2011.02.011 microaerotolerant or aerotolerant anaerobes can survive Supports low-oxygen survival as the defining trait context.
  • microaerotolerant does not require limited oxygen exposure

    Microaerotolerance differs from microaerophily because oxygen is tolerated rather than required.

    • DOI:10.1038/nrmicro2970 Many obligate anaerobes tolerate transient or low levels of O2 Supports low oxygen tolerance without implying oxygen requirement.
  • limited oxygen exposure can generate reactive oxygen species

    Oxygen exposure can create ROS stress that tolerant organisms must manage.

    • DOI:10.1038/s41579-021-00583-y molecular oxygen and ROS Review links oxygen exposure with ROS stress.
  • oxidative stress defense enzymes mitigates reactive oxygen species METPO:2007407

    ROS detoxification enzymes contribute to oxygen tolerance.

    • DOI:10.1128/iai.16.1.20-25.1977 oxygen tolerance of anaerobes is usually related to their level of SOD Supports antioxidant defense as a mechanism for oxygen tolerance.
  • microaerotolerant compatible with anaerobic growth

    Microaerotolerant organisms can remain anaerobic while surviving limited oxygen exposure.

    • DOI:10.1016/j.biortech.2011.02.011 microaerotolerant or aerotolerant anaerobes Supports microaerotolerance as an anaerobe oxygen-tolerance subtype.
  • intermediate oxygen level maximizes growth of microaerotolerant

    Maximal growth of microaerotolerant cells often occurs at intermediate oxygen levels.

    • DOI:10.1128/CMR.00110-14 Growth occurs in the presence or absence of oxygen; however, maximal growth occurs at intermediate oxygen levels.
  • superoxide reductase reduces superoxide METPO:2007802

    Superoxide reductase reduces superoxide (to hydrogen peroxide) using reduced electron donors.

    • DOI:10.1128/IAI.00502-24 Review notes SOR reduces superoxide using electrons from donors like NADH.
  • superoxide reductase produces hydrogen peroxide METPO:2007800

    Reduction of superoxide by SOR yields hydrogen peroxide.

    • DOI:10.1128/IAI.00502-24 SOR reduces superoxide to H2O2 as part of the anaerobe oxygen-defense backbone.
  • rubrerythrin/peroxidase reduces hydrogen peroxide METPO:2007802

    Rubrerythrin/peroxidases reduce hydrogen peroxide to water.

    • DOI:10.1128/IAI.00502-24 Review states Rbr/peroxidases reduce H2O2 to water.
  • reduced electron donors (NADH) enables function of oxidative stress defense enzymes

    Protection by SOR/Rbr defenses depends on available reduced electron donors.

    • DOI:10.1128/IAI.00502-24 The protection conferred by SOR/Rbr depends on available reduced electron donors.
  • gut lumen oxygen gradient creates selection for low-oxygen defense systems

    Host gut oxygen gradients expose anaerobes to low O2, selecting for low-O2 defense systems.

    • DOI:10.1128/IAI.00502-24 Gut O2 gradients (colon lumen 0.1-0.4% to ~5% in tissues) expose anaerobes to low O2 during colonization/inflammation.

Provenance

Source
METPO (2025-11-25)
Definition source
DOI:10.1016/j.biortech.2011.02.011

Parent traits (1)

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1000610 [-1.136, -2.135, -2.147, +0.656, …]

512-dim DeepWalkSkipGramEnsmallen embedding from kg-microbe (2026-04-25).

Nearest neighbors in embedding space

Top-8 cosine-similar METPO traits from the 2026-04-25 deepwalk (512-D).

Deep research

Generated by just research-trait; source: research/traits/environment/microaerotolerant-deep-research-falcon.md

Unreviewed literature output — not curated TraitMech content Ontology identifiers suggested below have not been resolved against their ontologies, and some are known to be wrong. Check any CURIE against the source before using it. 
# Curation report: microbial trait **microaerotolerant**

## Executive assessment

**Trait:** microaerotolerant  
**Identifier:** `METPO:1000610`  
**Category/kind/status:** ENVIRONMENT / CLASS / REVIEWED  
**Parent:** `METPO:1000601`

Microaerotolerance is best treated as an **oxygen-survival phenotype**, not as an aerobic metabolic mode: a microorganism tolerates low or transient molecular oxygen without requiring O₂ for growth. The strongest current mechanistic model is a layered defense in which O₂ is first scavenged by high-affinity reductases, ROS are detoxified, damaged proteins are repaired, and redox/stress regulators adjust these systems to oxygen concentration. The best direct causal evidence retrieved is from *Clostridioides difficile*, where deletion and complementation experiments resolve different O₂-reducing enzymes across 0.1–21% O₂. Broader sulfate-reducer evidence identifies the same functional modules under realistic redox fluctuations, but is primarily metagenomic/metatranscriptomic and should remain provisional. (caulat2024physiologicalroleand pages 2-5, caulat2024physiologicalroleand pages 5-7, dyksma2024growthofsulfatereducing pages 1-2)

## 1. Trait scope and boundary cases

### Operational definition

For TraitMech, curate `METPO:1000610` when an organism:

1. does **not require O₂** for its defining metabolism or growth;
2. survives, maintains viability, or sometimes continues limited anaerobic growth during **low or transient O₂ exposure**; and
3. has phenotype evidence tied to an explicit O₂ concentration, exposure duration, and endpoint.

The endpoint matters. In *C. difficile*, low-O₂ growth and post-exposure CFU survival were separately measured, and the organism remained unable to grow aerobically despite surviving physiological O₂ tensions. Thus, “O₂ tolerance,” “growth at low O₂,” and “O₂-dependent respiration” must not be treated as interchangeable. (caulat2024physiologicalroleand pages 1-2, caulat2024physiologicalroleand pages 5-7)

### Nearby phenotypes

| Nearby term | Distinction from microaerotolerant |
|---|---|
| **Microaerophilic** | Requires O₂ for optimal growth but at concentrations below air; O₂ is a metabolic requirement rather than merely tolerated. |
| **Aerotolerant anaerobic** | Does not use O₂ but tolerates relatively broad or atmospheric exposure. Microaerotolerance is narrower and should ordinarily require low-O₂ evidence. |
| **Facultative anaerobic** | Can switch to aerobic respiration or otherwise grow using O₂; this exceeds mere tolerance. |
| **Obligately anaerobic** | Describes lack of aerobic growth. It does not imply immediate death upon O₂ exposure; an obligate anaerobe can nevertheless be microaerotolerant. |
| **Oxygen-resistant spore** | Spore survival should not establish vegetative-cell microaerotolerance unless the assay explicitly tests vegetative cells. |

No universal numerical cutoff emerged. Relevant studies used 0.1–4% O₂ for low/intermediate exposure, 21% for air, and 133 µM dissolved O₂ for periodic ecological stress. Therefore, oxygen concentration and duration belong on the evidence association rather than in a universal trait threshold. (caulat2024physiologicalroleand pages 1-2, dyksma2024growthofsulfatereducing pages 1-2)

## 2. Current mechanistic model

The graph should distinguish four modules:

1. **O₂ removal:** flavodiiron proteins, reverse rubrerythrins, cytochrome-bd oxidase, and rubredoxin:oxygen oxidoreductase lower intracellular O₂.
2. **ROS detoxification:** catalase-peroxidase, alkyl-hydroperoxide reductase, rubrerythrin, superoxide-defense systems, and thiol peroxidases limit peroxide/superoxide injury.
3. **Damage repair:** thioredoxin/thioredoxin reductase, methionine-sulfoxide reductase, and chaperones restore oxidized or misfolded proteins.
4. **Regulatory matching:** σB, OseR/Spx-family regulation, σA, and Rex tune defenses to O₂ tension and cellular NADH/NAD⁺ state.

The 2024 *C. difficile* study shows that the O₂-removal layer is not a single generic mechanism. revRbr2 is associated with <0.4% O₂, FdpA with approximately 0.4–1%, revRbr1 with 0.1–4%, and FdpF with >4% and air exposure. This concentration partitioning is a major advance over a simple “antioxidant gene present” model. (caulat2024physiologicalroleand pages 1-2)

## 3. Candidate nodes

### Trait, environmental, and assay nodes

- microaerotolerant — `METPO:1000610`
- parent oxygen-preference trait — `METPO:1000601`
- low O₂ exposure — label-only environmental/experimental condition
- periodic oxic–anoxic transition — label-only
- atmospheric O₂ exposure — label-only
- growth under defined O₂ tension — assay node, label-only
- CFU survival after O₂ exposure — assay node, label-only
- dissolved-O₂ bioreactor exposure — assay node, label-only

### Chemicals and redox species

Showing the first 60 of 242 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.

Curation history

  1. · SEEDED_FROM_METPO · seed_from_metpo

    imported from data/raw/metpo.owl (CLASS)

  2. · CURATED_WITH_LITERATURE · codex

    Added source-backed evidence for microaerotolerant oxygen exposure survival.

  3. · ADDED_ORGANISM_EXAMPLE · codex

    Added Simulacricoccus ruber organism example with PMID-backed evidence.

  4. · ADDED_CAUSAL_GRAPH · codex

    Added DOI-backed causal graph for low-oxygen survival and oxidative stress defense in microaerotolerant organisms.

  5. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007407×1).

  6. · REMOVE_REDUNDANT_SYNONYM · claude

    Removed 1 synonym(s) whose text duplicated the label (seeder redundancy; no information lost).

  7. · ENRICH_CAUSAL_GRAPH · claude

    Added 6 evidence-backed generic edges (8 new nodes) from the deep-research report.

  8. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 3 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2000017×2, METPO:2000202×1).

  9. · GROUND_CAUSAL_NODES · claude

    Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:18421×1, CHEBI:16240×1).

  10. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (UniProtKB:A0A090JWT2×1).

  11. · RETRACT_DEAD_UNIPROT_GROUNDINGS · claude

    Retracted 1 UniProtKB grounding(s) whose accessions are deleted from UniProt; nodes demoted to label-only pending re-grounding (docs/GROUNDING_POLICY.md)

  12. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (GO:0050605×1).

  13. · MIGRATE_MICROBE_DOMAIN_EDGES_PART2 · claude

    Re-grounded 3 causal edge(s) off microbe-domain METPO predicates onto their causal-graph counterparts (1 to produces, 2 to reduces), issue 301 part 2. The previous predicates are transitively rdfs:subPropertyOf METPO:2000001, whose rdfs:domain is METPO:1000525 (microbe), so a causal-graph subject entailed that the subject IS a microbe; CausalNodeTypeEnum has no organism member, so no such edge could ever satisfy the domain. Each replacement is a 1:1 mirror of its source predicate that changes only the domain, so the claim each edge makes is unchanged and directions are unchanged. The replacements are proposed in proposals/metpo_traitmech_v9 and are placeholder ids until METPO mints them.