All of the following are characteristics of selective media EXCEPT:

Question

Accepted Answer

B. Support the growth of all microorganisms present in a mixed culture equally. ## Selective Media: Definition and Function

**Key Point:** Selective media are formulated to allow growth of desired microorganisms while inhibiting unwanted ones. The incorrect option states that selective media support growth of ALL microorganisms equally — this is the opposite of their purpose.

### Characteristics of Selective Media

| Feature | Details |
|---------|----------|
| **Purpose** | Inhibit unwanted organisms; allow target organisms to grow |
| **Inhibitory agents** | Antibiotics (e.g., vancomycin in Thayer-Martin), dyes (e.g., methylene blue), high salt (e.g., 6.5% NaCl for enterococci) |
| **Clinical use** | Primary isolation of pathogens from mixed flora (e.g., blood, CSF, respiratory specimens) |
| **Differential property** | NO — selective media only inhibit/allow; they do NOT differentiate based on biochemical reactions |

### Examples of Selective Media

- **Thayer-Martin medium** — Neisseria gonorrhoeae (contains antibiotics to suppress normal flora)
- **Löffler's serum medium** — Corynebacterium diphtheriae (selective for diphtheria bacillus)
- **Mannitol salt agar (MSA)** — Staphylococcus aureus (6.5% NaCl inhibits most Gram-negatives)
- **Sabouraud dextrose agar** — Fungi (acidic pH and antibiotics inhibit bacteria)

**High-Yield:** Selective media ≠ Differential media. Selective media INHIBIT unwanted organisms. Differential media ALLOW all organisms to grow but DIFFERENTIATE them by color/morphology based on biochemical reactions.

**Clinical Pearl:** When culturing from mixed flora (e.g., throat, sputum, stool), selective media are essential to suppress normal flora and recover the pathogen.

Answer

A. Used for primary isolation of pathogenic bacteria from clinical specimens

Answer

C. Contain substances such as antibiotics, dyes, or high salt concentrations

Answer

D. Inhibit the growth of unwanted microorganisms while allowing desired ones to grow

All of the following are characteristics of selective media EXCEPT:

Explanation

Selective Media: Definition and Function

Characteristics of Selective Media

Examples of Selective Media

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Feature	Details
Purpose	Inhibit unwanted organisms; allow target organisms to grow
Inhibitory agents	Antibiotics (e.g., vancomycin in Thayer-Martin), dyes (e.g., methylene blue), high salt (e.g., 6.5% NaCl for enterococci)
Clinical use	Primary isolation of pathogens from mixed flora (e.g., blood, CSF, respiratory specimens)
Differential property	NO — selective media only inhibit/allow; they do NOT differentiate based on biochemical reactions