Presumptive positive colonies of nonpathogenic Listeria sp. appear pink, whereas Listeria monocytogenes (L.m.) and Listeria ivanovii (L.i.) appear as blue-green to blue-violet colonies after 42-48 h at 35°C. A small percent of Enterococcus sp. will grow as white colored colonies. Bacillus sp., yeasts and Gram (-)s are inhibited. A quick and simple biochemical test on our confirmation medium biplate can easily distinguish, within 6 h, L. m. (fluorogenic reaction and/or acid production) from L.i. (negative for both).

Our Listeria sp./Listeria monocytogenes medium, as in the Listeria monocytogenes medium, contains the chromogenic substrate X-inositol phosphate (X-IP) that detects the enzyme phosphatidylinositol phospholipase C, virulence factor, which is found in L.m. and L.i. producing blue-green or blue-violet colonies. Even though L.m. and L.i. are the same color, L.i. strains are very seldom found in foods. In addition, this medium will also detect other Listeria sp. by using the combination chromogenic substrates of Salmon-β-D-glucopyranoside and Magenta-β-D-glucopyranoside (detects the β-glucosidase enzyme) in which these strains will produce pink colonies against an opaque white background (titanium oxide). Our confirmatory medium biplate uses a quick fluorogenic reaction in α-mannosidase and an acid production from rhamnose to distinguish L.m. from L.i. 

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

1. Highly differential medium that works by the combination of indoxyl derivative chromogenic substrates that produce positive color reactions for colonies of nonpathogenic Listeria sp. that are pink due to their ß-glucosidase activity, and blue-green to blue-violet for the pathogenic species depending on the strain-specific balance of ß-glucosidase (pink) and phosphatidylinositol-specificphospholipase C (blue) activities.