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 **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.