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29 July 2003

NEW OXOID MEDIUM IMPROVES ISOLATION OF LISTERIA MONOCYTOGENES

Oxoid Limited has launched a new chromogenic medium for the improved isolation, enumeration and identification of Listeria from food samples. In addition to providing presumptive identification of Listeria species, Oxoid Chromogenic Listeria Agar ( OCLA ) Plate (PO5165A) is also able to differentiate pathogenic Listeria monocytogenes and Listeria ivanovii from other Listeria species.

The X- glucoside chromogen within the medium is cleaved by the enzyme b - glucosidase , common to all Listeria species, resulting in clearly visible blue/green colonies. (Other micro-organisms that possess b - glucosidase , such as enterococci , are inhibited by selective agents, lithium chloride, nalidixic acid and polymixin B.) In addition, phospholipase in Listeria monocytogenes (and pathogenic Listeria ivanovii ) hydrolyses lecithin in the medium to produce a distinctive opaque white halo around the colony.

The selectivity of the medium is further enhanced by the addition of ceftazidime and amphotericin B, which inhibit other background flora, such as bacilli, yeasts and moulds. 

Studies have shown the OCLA Plate to have superior performance to PALCAM and Oxford medium for the isolation of Listeria monocytogenes 1. 

To find out more about Oxoid Chromogenic Listeria Agar Plate and other products for the identification of Listeria spp, telephone: +44 (0) 1256 841144, fax: +44 (0) 1256 329728, email: val.kane@oxoid.com.