Q.36 The enriched media used to facilitate the growth of fastidious microorganisms
are
(A) Selenite F broth
(B) Blood agar
(C) Chocolate agar
(D) Loeffler’s serum
Blood agar, Chocolate agar, and Loeffler’s serum are enriched media that support fastidious microorganisms requiring extra nutrients like blood or serum. Selenite F broth serves selective enrichment instead. The correct answer is (B), (C), and (D).
Option Analysis
Selenite F broth (A) acts as selective enrichment for Salmonella and Shigella from fecal samples. Sodium selenite inhibits coliforms and gram-positive bacteria, favoring enteric pathogens over fastidious ones.
Blood agar (B) provides 5% defibrinated blood added to a peptone-tryptose base, supplying hemin, NAD, and growth factors for fastidious streptococci and Neisseria. It enables hemolysis observation (alpha, beta, gamma) while supporting nutritionally demanding bacteria.
Chocolate agar (C) derives from heated blood agar, lysing red blood cells to release intracellular factors (X and V) essential for Haemophilus influenzae and Neisseria gonorrhoeae. This non-selective enriched medium isolates these fastidious pathogens from mixed specimens.
Loeffler’s serum (D) incorporates beef serum, peptone, and glucose (often with coagulated egg) to enhance isolation of Corynebacterium diphtheriae from throat swabs. The serum restores virulence and supports fastidious growth, proteolytic activity assessment, and metachromatic granule visualization.
Enriched media for fastidious microorganisms provide essential blood, serum, or lysed cell factors to grow bacteria with complex nutritional needs, such as Haemophilus and Neisseria species. These media outperform basic nutrient agar by supplying hemin (factor X), NAD (factor V), and amino acids absent in simple formulations. Ideal for CSIR NET Life Sciences aspirants studying microbiology culture techniques.
Key Enriched Media Types
-
Blood agar: Adds 5-10% sheep blood to basal agar for streptococci (S. pyogenes, S. pneumoniae) and initial fastidious growth; distinguishes via hemolysis patterns.
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Chocolate agar: Heated blood agar releases growth factors for H. influenzae and N. gonorrhoeae; used in respiratory and genital specimen isolation.
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Loeffler’s serum: Serum-rich slope for Corynebacterium diphtheriae rapid identification; detects diphtheria toxin via tellurite reduction.
Comparison Table
| Medium | Key Supplements | Target Fastidious Organisms | Primary Use |
|---|---|---|---|
| Blood agar (B) | 5% defibrinated blood | Streptococci, some Neisseria | Hemolysis differentiation |
| Chocolate agar (C) | Heated/lysed blood | Haemophilus, Neisseria gonorrhoeae | Factor X/V requiring pathogens |
| Loeffler’s serum (D) | Bovine serum, glucose | Corynebacterium diphtheriae | Throat swab isolation |
| Selenite F broth (A) | Sodium selenite | Salmonella (not fastidious) | Selective enteric enrichment |
CSIR NET Exam Relevance
These enriched media questions test classification: enriched (solid, nutrient-supplemented) vs. selective/enrichment (liquid, inhibitory). Blood agar, Chocolate agar, and Loeffler’s serum classify as enriched for fastidious growth, excluding Selenite F broth. Practice distinguishes them for microbiology unit scoring.
