AL medium (Agar Listeria according to Ottaviani and Agosti) is based on the simultaneous detection of 2 enzyme activities: b-glucosidase and phosphatidylinositol-specific phospholipase C (PI-PLC).
B-D-glucosidase activity, common to Listeria genus, is detected using a chromogenic substrate (X-glucoside). Its hydrolysis induces the formation of a blue to blue-green colour in all Listeria colonies.
PI-PLC is an enzyme only detected in pathogenic Listeria species: L. monocytogenes and L. ivanovii. AL medium contains phosphatidylinositol which, when it breaks down, produces an opaque halo around colonies of bacteria of these 2 species: 24 hrs for L. monocytogenes and 48 hrs for L. ivanovii.