Enrichment media are liquid media that selectively favour the multiplication of a particular pathogen while inhibiting the accompanying commensal/normal flora present in a mixed clinical specimen (e.g., stool), thereby increasing the yield of the pathogen before it is subcultured onto a solid selective medium.
Mechanism: they contain inhibitory substances (bile salts, selenite, tetrathionate, or altered pH/osmolarity) tolerated by the target pathogen but not by competing organisms, allowing the pathogen to multiply preferentially during a short incubation period.
Examples:
Selenite F broth and Tetrathionate broth — enrich for Salmonella species from stool before plating on selective media (e.g., DCA, XLD).
Alkaline peptone water (pH 8.6) — enriches for Vibrio cholerae from stool, which tolerates alkaline pH while other gut flora are inhibited.
Selenite broth is also used for isolating Shigella in some protocols.
(Enrichment media are distinct from enriched media, which supply extra nutrients such as blood or serum to support fastidious organisms rather than to suppress competing flora.)
