Reference entry
Commensal
Plain-English definitions of microbiome research vocabulary — the terms you'll see across articles, testing services, and probiotic claims.
Glossary context
Defined
in the site-wide microbiome glossary
Definition
An organism that benefits from a relationship with another organism without causing harm. Most gut bacteria are commensals — they gain food and shelter while contributing to digestion, immunity, and barrier function.
Related terms
Microbiota
The actual living microorganisms within an environment. Often used interchangeably with microbiome, though microbiome technically includes their genes as well.
Pathobiont
A normally harmless commensal organism that can become pathogenic under certain conditions, such as immune suppression, antibiotic use, or disrupted gut barrier. Many gut bacteria are pathobionts rather than true pathogens.