Paper I — 2023 March (Supplementary) (2010 Scheme) — Q4
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Paper I
2023 March (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Selective media with two examples.
Q42 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 (e.g., stool, skin swab) that normally contains a mixture of commensal/other organisms.
Examples:
MacConkey agar — contains bile salts and crystal violet, which inhibit most Gram-positive organisms while allowing growth of Gram-negative enteric bacilli (also differential, since it contains lactose and a pH indicator, distinguishing lactose fermenters like E. coli from non-fermenters like Salmonella/Shigella).
Thayer-Martin/Modified Thayer-Martin medium — chocolate agar base with added antibiotics (vancomycin, colistin, nystatin, trimethoprim) to suppress commensal throat/genital flora, selectively allowing growth of Neisseria gonorrhoeae.
(Other valid examples: TCBS agar for Vibrio cholerae; DCA/XLD agar for Salmonella/Shigella; Lowenstein-Jensen medium with malachite green for Mycobacterium tuberculosis.)
