Harlequin™ Listeria Medium HAL 2 Description This is a selective identification medium for the isolation of Listeria spp. from food and clinical materials based on the Oxford formulation. The aesculin hydrolysis reaction is replaced by a combination of the novel chromogen, CHE-glucoside, and ferrous gluconate. This reaction results in the formation of an intense black pigment that is retained within the Listeria colonies. The retention of the pigment against the clear agar makes screening for suspect colonies very easy. Lithium chloride is incorporated as an inhibitor of enterococci and further selectivity is achieved by adding the antibiotic supplement X122 or X123 after sterilisation of the medium. In a comparative study, using naturally contaminated food samples, the Listeria Novel Substrate Medium gave a 34% increase in positive isolations over the traditional Listeria Isolation Medium (Oxford Formulation). Formula g/litre
Method for reconstitution Weigh 73.5 grams of powder, disperse in 1 litre of deionised water. Allow the mixture to soak for 10 minutes, swirl to mix and then sterilise by autoclaving at 121°C for 15 minutes. Allow the medium to cool to 47°C, add 1 vial of selective supplement X122 or X123, mix well and pour into Petri dishes. Appearance: Yellow, slightly opaque gel. Storage of Prepared Medium: Inoculation: Incubation: GROWTH CHARACTERISTICS Organism Colony Size (mm) Shape & Surface Colour Other L. monocytogenes 0.5 - 1.0 CV.E.G Black Draughtsman or CV.CR.D colonies Bacillus spp. 0.1-1.5 CV.CR.D Grey centre with buff periphery Gram +ve cocci enterococci p.p - 1.0 CV.E.G Buff Pale Grey Usually no growth References Smith, P.A., Mellors, D., Holroyd, A., Gray, C. (2001). A new chromogenic medium for the isolation of Listeria spp. Letters in Applied Microbiology. 32. 78-82. Poster presentation, The 9th Meeting of the Microbiological Methods Innovations Forum. Campden and Chorleywood Food Research Association. |
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