XLD Agar is a specialized medium designed for the selective isolation and identification of Salmonella and Shigella species. It achieves precise differentiation through multiple biochemical identification mechanisms: Salmonella produces pink colonies with a black center (H₂S positive) due to lysine decarboxylation yielding an alkaline substance, while Shigella forms colorless or pale pink colonies (H₂S negative). Lactose/sucrose-fermenting bacteria (e.g., Escherichia coli) produce red colonies, significantly enhancing detection specificity for target pathogens.