Oxoid Chromogenic Listeria Agar (Ocla), for the isolation, enumeration and presumptive identification of Listeria from food, conforms to the formulation described by Ottaviani and Agosti in ISO 11290
The selective chromogenic medium, comprised of Oxoid Chromogenic Listeria Agar (Ocla) Base (ISO) [CM1084] and Listeria Selective Supplement (ISO) [SR0226E], detects the glucosidase activity common to all Listeria species resulting in distinct blue colonies.
The addition of phosphatidylinositol at 2g/L, as recommended in the ISO formulation, detects phosphatidylinositol phospholipase C (PIPLC) produced by pathogenic Listeria species.
Alternatively, the addition of Oxoid Chromogenic Listeria Differential Supplement [SR0228E] detects phosphatidycholine phospholipase C (PCPLC).
Either supplementation results in a clearly visible, opaque white halo around pathogenic Listeria colonies.
Both PIPLC and PCPLC are associated with the virulence of Listeria, but detection of either enzyme is sufficient for the identification of pathogenicity.
Listeria monocytogenes is the most common pathogenic Listeria species, but some strains of L ivanovii may also possess these enzymes and have been shown to be pathogenic in animals and humans.
Pathogenic L ivanovii will also produce blue colony halos on Ocla (ISO).
