The recently launched Thermo Scientific Chromogenic Listeria Agar (OCLA), for the isolation, enumeration and presumptive identification of Listeria spp. and Listeria monocytogenes from food, conforms to the formulation described by Ottaviani and Agosti in ISO 11290 parts 1 & 2:1997 (Incorporating Amendment No.1)[1,2]



Photo: New Thermo Scientific Chromogenic Listeria Agar (OCLA) conforms to the formulation described by Ottaviani and Agosti in ISO 11290 parts 1 & 2:1997 (Incorporating Amendment No.1)

The selective chromogenic medium, comprised of OCLA Base (ISO) [CM1084] and Listeria Selective Supplement (ISO) [SR0226E], detects the b-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 Thermo Scientific 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[3]. Pathogenic L. ivanovii will also produce blue colony halos on OCLA (ISO).

For further information about OCLA (ISO) and the wide range of products available from Thermo Scientific for the growth, detection, isolation and identification of Listeria and other food-borne pathogens, please contact Thermo Scientific [details above].
References:

• 1] ISO 11290-1:1997 (Microbiology of food and animal feeding stuffs – horizontal method for the detection of Listeria monocytogenes – part 1, Incorporating Amendment 1).
  
• 2] ISO 11290-2:1997 (Microbiology of Listeria monocytogenes – part 2, Incorporating Amendment 1).
  
• 3] Cummins, A. J., Fielding, A. K., and McLauchlin, J (1994) J Infection 28:89-91.