## 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.
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