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23 November 2005

Improved Identification of Listeria SPP

Oxoid Limited has launched a new selective chromogenic medium, Oxoid Chromogenic Listeria Agar (OCLA, [product codes CM1080, SR0227E & SR0228E]), for the improved isolation, enumeration and presumptive identification of Listeria monocytogenes and other Listeria spp. from food samples.

This innovative chromogenic medium detects B-glucosidase activity common to all Listeria species, resulting in distinct blue colonies. Selective agents within the medium inhibit other organisms that possess this enzyme, such as enterococci, in addition to background flora that may be present in the sample.

The addition of lecithin to the medium permits the further differentiation of
L. monocytogenes and pathogenic L. ivanovii, both of which have the ability to produce the phospholipase enzyme, lecithinase (PCPLC). The activity of this enzyme produces a clearly visible, opaque white halo around L. monocytogenes and pathogenic L. ivanovii colonies.

The presence of PCPLC and another phospholipase enzyme (PIPLC) are required for virulence, although detection of one is sufficient for the identification of pathogenicity. L. monocytogenes is the most common pathogenic Listeria spp. and has been found in humans and animals. Pathogenic strains of L. ivanovii (those that possess lecithinase activity) are primarily found in animals but have also been shown to cause infection in humans.

Comparative studies have found OCLA to be superior to PALCAM or Oxford medium for the isolation of L. monocytogenes.

OCLA has been validated and approved for use by AFNOR. For further information about OCLA and other Oxoid products for the isolation and identification of Listeria spp., please contact Val Kane, tel: +44 (0) 1256 841144, fax: +44 (0) 1256 329728, email val.kane@oxoid.com