A new chromogenic medium for the isolation of Listeria spp.

A new medium is described for the isolation of Listeria spp. from foods and environmental samples. It is based on a modified Oxford medium in which 3,4-cyclohexenoesculetin-beta-D-glucoside replaces aesculin. Positive colonies are intensely black with the advantage that the pigment does not diffuse into the medium. The medium, when tested alongside the US Department of Agriculture (spiked samples) and Food and Drug Administration (naturally contaminated samples) isolation procedures, performed significantly better than the current formulations (34% more confirmed positives from naturally contaminated samples) with a reduction of 1 d in the assay time for most samples.