Endo Agar is a selective and differential culture medium used in microbiology for the isolation and identification of Gram-negative coliform bacteria, particularly Escherichia coli, from water, milk, food, and other samples. It contains peptone, lactose, basic fuchsin, and sodium sulfite, which inhibit Gram-positive bacteria and allow differentiation based on lactose fermentation. Lactose-positive coliforms form red colonies with a metallic sheen, while non-fermenters appear colorless. Widely used in water quality testing to assess sanitary conditions and detect fecal contamination in drinking water, wastewater, and environmental samples.