RAPID’ L. mono (M131a): Incubate plates at 37° C, L. monocytogenes and L. ivanovii produce a 1-3 mm diameter, smooth, convex, blue/green colony. Typical colonies appear black in the red background of RAPID’ L. mono agar and appear blue/green when the background flora change the background color of the agar to yellow. Additionally a yellow halo will surround L. ivanovii colonies. However, caution has to be taken for L. monocytogenes-L. ivanovii differentiation because background flora in some food commodities can change the color of certain areas of this agar to yellow, and this could make L. monocytogenes appear as L. ivanovii. All other Listeria species produce a 1-2 mm, smooth, convex white colony with or without a yellow halo.