Which of the following media is both selective and differential, commonly used for isolation of Salmonella and Shigella from clinical specimens?

Explanation

## MacConkey Agar: Selective and Differential Medium **Key Point:** MacConkey agar is a selective and differential medium that inhibits gram-positive bacteria while allowing gram-negative bacteria to grow and be differentiated based on lactose fermentation. ### Composition and Selectivity | Component | Function | |-----------|----------| | **Bile salts (oxgall)** | Inhibit gram-positive bacteria and most gram-negative cocci | | **Crystal violet dye** | Enhances selectivity against gram-positive organisms | | **Lactose** | Fermentable carbohydrate for differentiation | | **Neutral red indicator** | pH indicator; turns red in acidic conditions (lactose fermentation) | | **Peptone & beef extract** | Nutrient sources | ### Differential Properties **High-Yield:** MacConkey agar differentiates bacteria based on lactose fermentation: - **Lactose fermenters (LF):** Pink/red colonies (e.g., *Escherichia coli*, *Klebsiella*) - **Non-lactose fermenters (NLF):** Colorless/transparent colonies (e.g., *Salmonella*, *Shigella*, *Proteus*) ### Clinical Applications **Mnemonic:** **SMACK** — *Salmonella*, *Shigella*, *Citrobacter*, *Klebsiella* (all grow on MacConkey) - Salmonella and Shigella appear as colorless (NLF) colonies - Allows presumptive identification based on colony morphology - Further biochemical tests required for definitive identification **Clinical Pearl:** MacConkey agar is the gold standard for screening enteric pathogens in stool cultures. The colorless colonies of Salmonella and Shigella stand out against the pink background of lactose fermenters.