Modified Oxford Medium (MOX) is a selective and differential agar used for the isolation and detection of Listeria monocytogenes from food, environmental, and clinical samples. The formulation is derived from Columbia agar and supplemented with selective agents such as lithium chloride, colistin, and moxalactam that inhibit competing Gram negative and many Gram positive bacteria. The diagnostic feature of the medium is esculin hydrolysis. Listeria species hydrolyze esculin to esculetin, which reacts with ferric ammonium citrate to produce a black complex in the surrounding medium. Typical colonies of Listeria monocytogenes appear small and gray to black with a distinctive dark halo after incubation. This medium is widely used in food safety laboratories for monitoring Listeria contamination in processing environments.