Presumptive positive colonies of all Listeria sp. appear as pink convex colonies 1.0 to 2.0 mm diameter with or without a precipitate after 42-48 h incubation at 35°C. Enterococcus and Bacillus species, either do not grow or have a scant growth with pinpoint clear colonies. Gram (-)s, other Gram (+)s and yeasts are inhibited.

This medium was developed to replace MOX (USDA) and/or LPM (FDA) for identifying both nonpathogenic and pathogenic Listeria species, especially in environmental samples from plants producing ready-to-eat foods. The Listeria sp. chromogenic plating medium contains the chromogens Salmon-β-D-glucopyranoside and Magenta-β-D-glucopyranoside which detects the β-glucosidase enzyme found in all Listeria species. By using the correct concentrations of Salmon-β-D-glucopyranoside and Magenta-β-D-glucopyranoside in the medium all Listeria produce pink colored colonies against an opaque white background (titanium oxide).

Fish, poultry, meat, dairy, bakery and ready-to-eat foods and environmental testing in these food processing facilities

1. The highly differential medium is based on specific chromogenic substrates in the plating medium that simultaneously identifies both nonpathogenic and pathogenic Listeria species as pink colored colonies on a single plate in 42-48 hours at 35°C.