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gram negative

METPO:1000699 · CLASS · REVIEWED

A gram stain in which bacteria do not retain crystal violet dye and appear pink or red after staining, indicating a thin peptidoglycan layer and presence of an outer membrane.

Gram-negative outer-membrane dye-loss mechanism

Evidence-backed causal sketch linking Gram-negative staining to outer membrane architecture, lipopolysaccharide, thin peptidoglycan, decolorization, and counterstain uptake.

Gram-negative outer-membrane dye-loss mechanism Interactive directed graph showing evidence-backed causal relationships for gram negative.

Edge evidence

  • outer membrane defines gram negative METPO:2007500

    The outer membrane is a defining feature of the Gram-negative cell envelope.

    • DOI:10.1038/s41579-019-0201-x defining feature of the Gram-negative cell envelope Supports outer membrane as the key Gram-negative envelope feature.
  • lipopolysaccharide localized in outer membrane biolink:located_in

    LPS is found on the outer surface of the Gram-negative outer membrane.

    • DOI:10.1038/s41579-019-0201-x lipopolysaccharide exclusively found on the surface Supports LPS as a surface-exposed outer-membrane component.
  • thin peptidoglycan layer localized in periplasmic space biolink:located_in

    Gram-negative peptidoglycan is a thin layer in the periplasm.

    • DOI:10.1016/j.bbalip.2016.10.010 periplasm ... containing a thin layer of peptidoglycan Supports thin periplasmic peptidoglycan in Gram-negative envelopes.
  • thin peptidoglycan layer fails to retain crystal violet-iodine complex

    Thin peptidoglycan and outer-membrane disruption allow dye complex loss during decolorization.

    • DOI:10.3109/10520299609117151 cell wall ... responsible for retention Inference from Gram-positive retention mechanism; thinner Gram-negative peptidoglycan contributes to non-retention.
  • counterstain confers gram negative METPO:2007700

    Counterstain gives decolorized Gram-negative cells their pink/red appearance.

    • DOI:10.3109/10520299609117151 mechanism of the Gram stain Supports Gram stain procedure context; counterstain appearance follows dye-complex loss.
  • lipopolysaccharide occupies outer membrane outer leaflet

    LPS occupies the outer leaflet of the Gram-negative outer membrane (OM asymmetry).

    • DOI:10.1146/annurev-micro-032521-014507 LPS occupies the outer leaflet while phospholipids occupy the inner leaflet.
  • divalent cations (Mg2+/Ca2+) stabilizes lipopolysaccharide

    Divalent cations (Mg2+/Ca2+) stabilize LPS packing in the OM outer leaflet.

    • DOI:10.3390/pathogens13100889 Divalent cations (Mg2+/Ca2+) stabilize LPS packing.
  • lipopolysaccharide contributes to outer membrane barrier function RO:0002326

    Tight LPS packing with OM proteins creates the impermeable OM barrier.

    • DOI:10.1146/annurev-micro-032521-014507 The tight packing of LPS and OM proteins creates an impermeable barrier.
  • Lpt pathway transports lipopolysaccharide METPO:2007812

    The Lpt machinery transports LPS across the periplasm for insertion into the OM.

    • DOI:10.1007/s12275-024-00137-w LPS transported to the OM by the ATP-driven Lpt machinery; moves LPS across periplasm for OM insertion.
  • BAM complex inserts beta-barrel outer membrane proteins

    The BAM complex folds and inserts beta-barrel OMPs into the outer membrane.

    • DOI:10.3390/pathogens13100889 The Bam complex (BamA-E) folds uOMPs into beta-barrels.
  • Mla pathway maintains outer membrane lipid asymmetry

    The Mla pathway counteracts phospholipid flipping to maintain OM lipid asymmetry.

    • DOI:10.3390/pathogens13100889 Mla system performs retrograde transport to counteract PL flipping to the outer leaflet.
  • Tol-Pal system safeguards cell envelope connectivity

    Tol-Pal safeguards connectivity between the three layers of the Gram-negative envelope.

    • DOI:10.1038/s44259-024-00065-0 Tol-Pal safeguards connectivity between the three layers of the Gram-negative cell envelope.

Provenance

Source
METPO (2025-11-25)
Author
Luke Wang
Definition source
DOI:10.1038/s41579-019-0201-x

Parent traits (1)

Synonyms (2)

  • G_negative RELATED_SYNONYM · metpo.owl
  • negative RELATED_SYNONYM · metpo.owl

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1000699 [+365.243, +263.306, -35.927, -345.448, …]

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/morphology/gram_negative-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: Gram-negative trait

**Trait:** gram negative  
**Identifier:** `METPO:1000699`  
**Category / kind:** MORPHOLOGY / CLASS  
**Recommended interpretation:** an **assay-observed Gram-stain phenotype**, not a taxonomic clade and not an exact synonym of “diderm,” “outer-membrane-bearing,” or “LPS-producing.”

## 1. Scope summary

`METPO:1000699` should represent cells that fail to retain the crystal-violet–iodine primary stain after the decolorization step and consequently appear pink/red after counterstaining. The classical sequence is gentian/crystal violet, iodine, alcohol decolorization, and safranin counterstaining. Typical Gram-negative Proteobacteria have a thin peptidoglycan layer between inner and outer membranes; disruption of the outer membrane by decolorizer permits ready removal of the limited retained primary-dye complex. Safranin then supplies the observed pink/red endpoint. (megrian2020oneortwo pages 1-3, zerbib2025bacterialcellenvelopes pages 4-6)

The mechanistic graph should therefore distinguish:

1. **proximal assay mechanism:** staining → mordanting → envelope disruption/dehydration → primary-dye loss → counterstain uptake → pink/red observation;
2. **cell-envelope determinants:** relatively thin peptidoglycan, outer membrane, periplasm and envelope organization;
3. **upstream biogenesis machinery:** Lpt, BAM, Lol and related systems that construct a typical diderm envelope; and
4. **correlated consequences:** permeability, antimicrobial resistance and innate immune recognition, which are biologically important but do not themselves cause the Gram-stain readout.

### Boundary cases

The Gram reaction and membrane count correlate but are not equivalent. “Diderm” means two cellular membranes regardless of staining or membrane lipid composition. Some monoderm Firmicutes stain Gram-negative or Gram-variable, some diderms can stain Gram-positive, and diderms lacking LPS exist. Consequently, neither `outer membrane → gram negative` nor `LPS → gram negative` should be asserted as an exception-free universal rule. (megrian2020oneortwo pages 1-3, leonard2022wasthelast pages 1-2, zerbib2025bacterialcellenvelopes pages 4-6)

Other assay boundaries requiring explicit metadata include culture age, cell-envelope damage, fixation, decolorization time and reagent formulation. These can produce Gram-variable or false reactions and should be modeled as experimental modifiers unless directly supported for a specified organism and protocol.

## 2. Candidate nodes

### Trait and observed phenotype

- **gram negative:** `METPO:1000699`
- **parent trait:** `METPO:1000697`
- Primary-dye retention/loss — label-only candidate
- Pink/red Gram-stain appearance — label-only candidate
- Gram-variable staining — label-only candidate

### Cellular structures and localizations

- **Outer membrane:** `GO:0019867`
- **Periplasmic space:** `GO:0042597`
- **Peptidoglycan-based cell wall:** `GO:0009274`
- Inner/cytoplasmic membrane — use the verified organism-appropriate GO cellular-component term during implementation
- Outer-membrane outer leaflet — label-only candidate
- Outer-membrane inner leaflet — label-only candidate
- Diderm cell envelope — label-only candidate; do not equate automatically with the target trait

A typical diderm-LPS envelope has an asymmetric outer membrane—LPS in the surface leaflet and phospholipid in the periplasmic leaflet—with a thin but mechanically strong peptidoglycan layer in the periplasm. Reported peptidoglycan thicknesses include approximately 6 nm in *E. coli* and 2.4 nm in *Pseudomonas aeruginosa*; these are examples, not universal thresholds for Gram negativity. (zerbib2025bacterialcellenvelopes pages 4-6)

### Chemicals and assay factors

- Crystal violet — preferably ground to a verified ChEBI record at implementation
- Iodine / iodide mordant — verify exact chemical-form CURIE
- Crystal-violet–iodine complex — label-only candidate
- Ethanol — `CHEBI:16236`
- Acetone — `CHEBI:15347`
- Safranin counterstain — verify exact dye record; label-only if formulation is ambiguous
- Lipopolysaccharide — `CHEBI:16412`
- Lipid A — verify exact ChEBI class before YAML insertion
- Peptidoglycan — `CHEBI:8005`
- Phospholipid — `CHEBI:16247`
- Mg²⁺ — `CHEBI:18420`
- Ca²⁺ — `CHEBI:29108`

Showing the first 60 of 259 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_ORGANISM_EXAMPLE · codex

    Added Escherichia coli organism example with PMID-backed evidence.

  3. · CURATED_WITH_LITERATURE · codex

    Replaced PMID definition source with DOI-backed Gram-negative envelope source and added causal graph for outer membrane, lipopolysaccharide, periplasmic thin peptidoglycan, dye loss, and counterstaining.

  4. · GROUND_CAUSAL_PREDICATES · claude

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

  5. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 2 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:located_in×2).

  6. · GROUND_CAUSAL_NODES · claude

    Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (GO:0019867×1, GO:0042597×1).

  7. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007500×1).

  8. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:16412×1).

  9. · ENRICH_CAUSAL_GRAPH · claude

    Added 7 evidence-backed generic edges (10 new nodes) from the deep-research report.

  10. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (RO:0002326×1).

  11. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2000207×1).

  12. · MIGRATE_MICROBE_DOMAIN_EDGES · claude

    Re-grounded 1 causal edge(s) off microbe-domain METPO predicates (1 to confers), issue 301. 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. Edge directions are unchanged - this pass only relabels and re-grounds. RO:0002234 (has output) is used where the subject is an activity, since biolink gives it the domain 'biological process or activity'; the METPO replacements are proposed in proposals/metpo_traitmech_v8 and v9 and are placeholder ids until METPO mints them.

  13. · MIGRATE_MICROBE_DOMAIN_EDGES_PART2 · claude

    Re-grounded 1 causal edge(s) off microbe-domain METPO predicates onto their causal-graph counterparts (1 to transports), 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.