Paper I — 2014 September (Supplementary) (2010 Scheme) — Q6
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Paper I
2014 September (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Enrichment media with an example
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 the 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.
Example: Selenite F broth — used to enrich for Salmonella species from stool specimens before subculturing onto selective media such as DCA (deoxycholate citrate agar) or XLD agar; sodium selenite inhibits coliforms and other competing gut flora while Salmonella multiplies relatively unimpeded, improving isolation rates from specimens with a low pathogen load, especially in carriers.
(Other examples include tetrathionate broth for Salmonella, and alkaline peptone water at pH 8.6 for enriching Vibrio cholerae.)
