exosporium

GO:0043592 · Spore · Envelope layer · Proposed

The outermost layer of a bacterial endospore, which is loosely attached and located outside of the endospore coat. It is generally composed of protein, carbohydrate, and perhaps lipid. [GO:0043592]

Synonyms: epispore; exospore; perispore

Part of: endospore external encapsulating structure (GO:0043591); GO:0005618 — cell wall (no local record)

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
exosporium proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	bclA, bxpB, exsY		UNKNOWN	Protein constituents of the crystalline basal layer and external hairlike nap that make up the Bacillus exosporium surface. `DOI:10.1128/MMBR.00050-15` Stewart 2015 reviews the Bacillus cereus-group exosporium as a basal shell carrying an external nap whose principal collagen-like glycoprotein is BclA, and discusses BxpB and ExsY as exosporium structural proteins. `DOI:10.1111/mmi.13650` Terry et al. 2017 biochemically dissected Bacillus cereus exosporium architecture and related ExsY, BclA, and ExsFA/BxpB to basal-layer and nap assembly.

Taxonomic distribution

NCBITaxon:1239 Bacillota — VARIABLE: Bacterial exosporia are a patchy outer-spore layer best characterized in Bacillus cereus-group species, present in selected spore-forming Bacillota and absent from Bacillus subtilis spores. [DOI:10.1128/MMBR.00050-15]

Canonical examples

NCBITaxon:1392 Bacillus anthracis — Bacillus cereus-group model whose mature spores carry a loose basal-layer and nap exosporium used for exosporium assembly and surface-architecture studies. [DOI:10.1128/MMBR.00050-15]

Functions

outer spore surface interface — The exosporium forms the outermost surface of exosporium-bearing bacterial endospores and presents a semipermeable layer for environmental and host interactions.
- DOI:10.1128/MMBR.00050-15 Stewart 2015 reviews the bacterial exosporium as the outer spore layer that interacts with the environment and, in pathogens, host innate-immune surfaces.
- DOI:10.1111/mmi.13650 Terry et al. 2017 connect Bacillus cereus-group exosporium basal-layer architecture with surface exposure, semipermeability, and channels through the layer.

Mechanism graphs

Exosporium proteins build the external spore surface (FUNCTION)

The bacterial exosporium is a loose outer layer outside the endospore coat. Bacillus cereus-group exosporia combine a protein basal layer with a BclA-rich external nap to form a surface that faces host and environmental contacts.

Subject	Predicate	Object	Evidence
exosporium proteins	assemble into	exosporium	`DOI:10.1128/MMBR.00050-15` Stewart 2015 reviews the Bacillus cereus-group exosporium as a basal layer surrounded by an external BclA-rich hairlike nap. `DOI:10.1111/mmi.13650` Terry et al. 2017 relate ExsY, BclA, and ExsFA/BxpB to the Bacillus cereus-group exosporium structure.
exosporium	lies outside	endospore coat	`GO:0043592` GO:0043592 defines the exosporium as the bacterial endospore outermost layer outside the endospore coat.
exosporium	is optional outer layer of	endospore external encapsulating structure	`GO:0043591` GO:0043591 defines the endospore external encapsulating structure as including an exosporium in some species. `GO:0043592` GO:0043592 identifies the exosporium as a bacterial endospore component outside the coat.
exosporium	faces	host or environmental surface	`DOI:10.1128/MMBR.00050-15` Stewart 2015 reviews exosporium-mediated contacts with environmental and host surfaces.

Discussions

INTERPRETATION (RESOLVED): Keep the bacterial exosporium separate from the microsporidian-type exospore and generic UniProt Perispore location. GO:0043592 explicitly denotes the bacterial endospore exosporium, whereas GO:0090642 denotes a microsporidian spore-wall exospore and UniProt SL-0367 uses Perispore generically for bacterial and eukaryotic outer spore surfaces.

Evidence

GO:0043592 Gene Ontology cellular-component term for the bacterial endospore exosporium.
GO:0005618 — cell wall Gene Ontology cellular-component term for cell wall; GO places the exosporium as part of a cell wall.
GO:0043591 Gene Ontology cellular-component term for the endospore external encapsulating structure, whose definition includes an exosporium in species that build one.
GO:0043593 Gene Ontology cellular-component term for the endospore coat beneath the exosporium.
GO:0090642 Gene Ontology cellular-component term for the distinct microsporidian-type exospore.
uniprot.location:SL-0367 — Perispore UniProt Subcellular Location SL-0367 is a generic Perispore location spanning bacterial and eukaryotic spores; it is retained as record-level boundary evidence, not as an exact xref.
DOI:10.1128/MMBR.00050-15 Stewart 2015, The Exosporium Layer of Bacterial Spores: a Connection to the Environment and the Infected Host, Microbiology and Molecular Biology Reviews.
DOI:10.1111/mmi.13650 Terry et al. 2017, Molecular tiling on the surface of a bacterial spore - the exosporium of the Bacillus anthracis/cereus/thuringiensis group, Molecular Microbiology.

Curation history

2026-09-23T00:28:48Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T00:30:10Z codex · EXPANDED_RECORD — Added GO exosporium synonyms, cell-wall parthood, a label-only Bacillus exosporium protein cohort, Bacillota distribution, a Bacillus anthracis example, surface-interface function, GO/UniProt/DOI evidence, a nonmechanistic topology graph, and a boundary discussion separating the bacterial exosporium from microsporidian exospores and generic perispore locations.
2026-09-23T00:45:02Z codex · ADDRESS_REVIEW — Resolved PR #1338 review issue #1339 by linking the exosporium to the endospore external encapsulating structure in part_of, record evidence, the topology graph, and the bacterial exosporium boundary discussion.

Source: data/structures/spore/exosporium.yaml