Paper I — 2020 February (Supplementary) (2010 Scheme) — Q6
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Paper I
2020 February (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Enrichment media.
Q62 marksShort Notes
Answer
Enrichment media are liquid media designed to selectively favour multiplication of a particular pathogen present in a mixed clinical specimen (e.g., stool) while inhibiting accompanying normal/commensal flora, thereby increasing the pathogen’s numbers before subculture onto a selective solid medium.
Mechanism: they contain inhibitory substances (bile salts, selenite, tetrathionate, or an altered pH) that the target pathogen tolerates but competing organisms do not, allowing preferential multiplication of the pathogen during a short incubation period, improving isolation rates from specimens with a low pathogen load (e.g., carriers).
Examples:
Selenite F broth and tetrathionate broth — enrich for Salmonella species from stool before subculturing onto selective media (DCA, XLD agar).
Alkaline peptone water (pH 8.6) — enriches for Vibrio cholerae from stool, which tolerates alkaline pH while other gut flora are inhibited.
(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.)
