coccus shaped

Deep research

Generated by just research-trait; source: research/traits/morphology/coccus_shaped-deep-research-falcon.md

# TraitMech curation report: coccus-shaped morphology

## 1. Scope summary

**Target trait:** **coccus shaped**  
**Identifier:** **METPO:1000668**  
**Category:** morphology; class; reviewed  
**Definition used:** a spherical or nearly spherical microbial cell whose dimensions are approximately equal in all directions.

This is an **assay-observed single-cell morphology**, not a metabolic capacity or environmental preference. For curation, the terminal phenotype should be a roughly spherical cell measured by microscopy or equivalent morphological observation. In the best-characterized “true coccus,” *Staphylococcus aureus*, growth is dominated by division-associated peptidoglycan (PG) synthesis at midcell. FtsZ organizes the divisome, but PG polymerization and cross-linking supply the indispensable progression of septum closure. Recent work also shows that *S. aureus* is not a geometrically invariant sphere: it undergoes limited pre-division elongation mediated by RodA–PBP3 and regulated by GpsB. Thus, **coccus-shaped means nearly spherical at the organism/phenotype level, not complete absence of all elongation**. (battaje2023modelsversuspathogens pages 1-3, costa2024theroleof pages 13-14, puls2023inhibitionofpeptidoglycan pages 4-5)

### Boundary cases

1. **Ovococci are not strict synonyms.** *Streptococcus pneumoniae* and *S. suis* combine septal and peripheral PG synthesis and commonly have an ovoid/ellipsoid shape. Their elongasome-like machinery is concentrated near midcell rather than distributed along a long lateral cylinder. They may satisfy a broad “nearly spherical” assay threshold, but their mechanism should be represented in a separate ovococcal graph or explicitly qualified. (battaje2023modelsversuspathogens pages 4-5, battaje2023modelsversuspathogens pages 1-3)
2. **“Rounder” mutants are not automatically cocci.** A statistically lower aspect ratio establishes movement toward sphericity, not necessarily attainment of METPO:1000668.
3. **Cell arrangement is separate from shape.** Diplococci, tetrads, chains, and grape-like clusters describe post-division arrangement, not the shape of each cell.
4. **Transient coccoid states require qualification.** Stationary-phase rounding, stress-induced coccoid conversion, spores, dormant bodies, and pleomorphic forms should not be treated as constitutive coccus shape without condition and life-stage nodes.
5. **Wall-less spheres are mechanistically distinct.** Protoplasts, spheroplasts, and L-forms can become spherical through loss of the load-bearing PG sacculus; they should not be merged with the septal-growth mechanism of walled cocci.
6. **Taxonomic descriptors are observations, not mechanisms.** A statement that *S. aureus* is a Gram-positive coccus supports the phenotype–taxon association but does not by itself support any molecular edge.

## 2. Current mechanistic model

The strongest curation model is:

**midcell selection → FtsZ/Z-ring and divisome assembly → recruitment/organization of septal PG machinery → PBP/SEDS-dependent PG synthesis and cross-linking → centripetal septum constriction → hydrolase-mediated septum splitting → nearly spherical daughter cells.**

True cocci lack the canonical MreB-directed, sidewall-distributed elongation program characteristic of many rods. Nevertheless, the older formulation that cocci synthesize PG **only** at the septum is now too absolute for *S. aureus*: RodA–PBP3 supports limited elongation at the outer septal edge, while GpsB controls the septal-versus-peripheral distribution of PBP2 and PBP4. Loss of GpsB shifts PBP activity toward the periphery, increases peripheral cross-linking/stiffness, impairs mild elongation, and produces smaller, rounder cells. (costa2024theroleof pages 13-14, battaje2023modelsversuspathogens pages 3-4)

A key 2023 result refined the mechanical model of cytokinesis. Vancomycin and telavancin halted constriction while the Z-ring remained present; only 3 of 735 treated cells divided, versus 98% of controls. Active septum progression fell below 3% within 10 minutes, and all tested PG inhibitors increased mean Z-ring diameter by 30–44%. Therefore, FtsZ treadmilling organizes early division but is insufficient to close the septum when PG synthesis is blocked. (puls2023inhibitionofpeptidoglycan pages 4-5, puls2023inhibitionofpeptidoglycan pages 3-4, puls2023inhibitionofpeptidoglycan pages 2-3)

## 3. Candidate graph nodes

Identifiers below are suggested only where grounding is sufficiently clear. Taxon-specific proteins should be represented with the organism/gene context; label-only nodes are preferable to an unverified UniProt accession.

### Trait and taxa

- **coccus shaped** — **METPO:1000668**
- **parent morphology trait** — **METPO:1000666**
- *Staphylococcus aureus* — **NCBITaxon:1280**; principal true-coccus model
- *Streptococcus pneumoniae* — **NCBITaxon:1313**; ovococcal boundary model
- *Streptococcus suis* — **NCBITaxon:1307**; ovococcal boundary model

### Complexes, structures, and localizations

- FtsZ cytokinetic ring / Z-ring — label-only complex node; taxon-specific FtsZ gene product
- divisome — label-only complex node
- elongasome — label-only complex node; generally absent as a canonical MreB-guided sidewall machine in true cocci
- division septum / midcell — label-only localization
- cell periphery / peripheral wall — label-only localization
- peptidoglycan sacculus — **CHEBI:8005**
- cytoplasmic membrane — **GO:0005886**

### Genes and proteins

- **ftsZ / FtsZ** — tubulin-like GTPase and divisome organizer
- **ftsW / FtsW** — septal SEDS-family PG glycosyltransferase component
- **pbp2 / PBP2** — major bifunctional PG synthase in *S. aureus*; crucial for septum closure
- **pbp1 / PBP1** and **FtsW–PBP1 pair** — candidate septal PG module in *S. aureus*
- **rodA / RodA** and **pbp3 / PBP3** — limited elongation-associated SEDS–PBP pair in *S. aureus*
- **pbp4 / PBP4** — PG cross-linking enzyme whose localization is regulated by GpsB

Showing the first 60 of 226 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.