Which of the following is a selective and differential medium used for the isolation of Salmonella and Shigella species?

Question

Accepted Answer

D. Hektoen Enteric (HE) agar. ## Selective and Differential Media for Enterobacteria

**Key Point:** Hektoen Enteric (HE) agar is a selective and differential medium specifically designed for the isolation and differentiation of Salmonella and Shigella from mixed bacterial populations, particularly in clinical specimens.

### Composition and Function

HE agar contains:
- **Bile salts** — selective agent that inhibits gram-positive bacteria and many coliforms
- **Acid fuchsin and bromothymol blue** — differential indicators
- **Lactose, sucrose, and salicin** — fermentation substrates

### Colony Characteristics on HE Agar

| Organism | Colony Appearance | Fermentation Pattern |
|----------|-------------------|----------------------|
| Salmonella | Blue-green, opaque, may have black center (H₂S producer) | Non-lactose fermenter |
| Shigella | Blue-green, opaque, smooth | Non-lactose fermenter |
| E. coli | Yellow-orange | Lactose fermenter |
| Proteus | Brown/tan with black center | Variable fermentation |

**High-Yield:** HE agar is superior to XLD (Xylose Lysine Deoxycholate) agar in recovering Shigella species, making it the preferred medium in many clinical laboratories.

### Why Other Options Are Not Ideal

- **MacConkey agar** — selective but not optimally differential for Salmonella/Shigella; lactose fermenters appear pink
- **Blood agar** — non-selective, non-differential; supports growth of all bacteria
- **Nutrient agar** — non-selective, non-differential; general-purpose medium

**Clinical Pearl:** In clinical practice, stool specimens are typically cultured on both HE agar and XLD agar simultaneously to maximize recovery of Salmonella and Shigella.

Answer

A. MacConkey agar

Answer

B. Blood agar

Answer

C. Nutrient agar

Which of the following is a selective and differential medium used for the isolation of Salmonella and Shigella species?

Explanation

Selective and Differential Media for Enterobacteria

Composition and Function

Colony Characteristics on HE Agar

Why Other Options Are Not Ideal

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Organism	Colony Appearance	Fermentation Pattern
Salmonella	Blue-green, opaque, may have black center (H₂S producer)	Non-lactose fermenter
Shigella	Blue-green, opaque, smooth	Non-lactose fermenter
E. coli	Yellow-orange	Lactose fermenter
Proteus	Brown/tan with black center	Variable fermentation