Paper I — 2025 March (Supplementary) (2010 Scheme) — Q5

Paper I

2025 March (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Define Selective media with two examples

Q52 marksShort Notes★✓

Answer

Selective media are culture media that contain agents (inhibitory substances, antibiotics, dyes, or altered pH/salt concentration) that suppress or inhibit the growth of unwanted/competing organisms while allowing the specific target organism(s) to grow — used to isolate a particular pathogen from a specimen that normally contains a mixture of commensal/other organisms.

Two examples:

1. MacConkey agar — contains bile salts and crystal violet, inhibiting most Gram-positive organisms while allowing growth of Gram-negative enteric bacilli.
2. Thayer-Martin/Modified Thayer-Martin medium — chocolate agar base with added antibiotics (vancomycin, colistin, nystatin, trimethoprim) to suppress commensal flora, selectively allowing growth of Neisseria gonorrhoeae.

(Other valid examples: TCBS agar for Vibrio cholerae; DCA/XLD agar for Salmonella/Shigella.)

Related Concepts

1. 1Culture Media and Methods

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