Harlequin Listeria Chromogenic Agar according to Ottaviani & Agosti

Listeria Chromogenic Agar (according to the formulation of Ottaviani and Agosti) is a selective medium for the isolation and presumptive identification of Listeria monocytogenes from foodstuffs and related materials as described in ISO 11290-1:2017. This medium is not intended for use in the diagnosis of disease or other conditions in humans.

Lithium chloride in the base medium and supplementary antimicrobial compounds Ceftazidime, Polymyxin, Nalidixic acid and Cycloheximide provide the medium’s selectivity. Chromogenic activity is as a result of a chromogenic substrate for the detection of the β-glucosidase enzyme, common to all Listeria spp. and to a few strains of Enterococci and Bacilli.

The specific differential activity of this agar is obtained with a proprietary lecithin substrate for the detection of the phospholipase enzyme present in the L. monocytogenes colonies growing on this media. This enzyme activity will result in a halo of precipitation surrounding the target colonies.

With the combination of both the chromogenic and phospholipase enzyme reactions, it is possible to differentiate Listeria monocytogenes (blue colonies surrounded by an opaque halo) from other Listeria spp. (blue colonies without an opaque halo).