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This monograph has been reviewed by members of the IUMS–ICFMH Working Party on Culture Media and given ‘Proposed’ status.

Developed by Ottaviani et al. (1997), this is a selective differential medium for the isolation and direct counting of Listeria monocytogenes and other Listeria spp. from food and environmental samples. This chromogenic medium performed better than other chromogenic media tested in trials organised by the International Organization for Standardization (ISO) and was introduced in ISO 11290 in 2004.

Agar Listeria according to Ottaviani and Agosti contains the chromogenic compound X-glucoside, a substrate for the detection of β-glucosidase, common to all Listeria, which appear as blue-coloured colonies. Differentiation of Listeria monocytogenes from other Listeria spp. is achieved through the production of a phosphatidylinositol-specific phospholipase C (PI-PLC) by Listeria monocytogenes which hydrolyses the specific purified substrate added to the medium producing an opaque halo around the colonies (see Figure P1). Most Listeria ivanovii also produce an opaque halo after 24 h incubation. A few Listeria ivanovii strains produce a vague halo after 48 h incubation. Selectivity is obtained by the addition of lithium chloride, nalidixic acid and cycloheximide (Vlaemynck et al., 2000).

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