Assessment of different selective agar media for enumeration and isolation of Listeria from dairy products.

Different selective agar media were compared for the recovery and isolation of five species of Listeria from raw milk and cheese. The selective media examined were Beerens medium, MacBride medium and that described by Dominguez et al. (1984) with 6 mg/l acriflavine, listeria selective agar medium (LSAM), and LSAM with 12 mg/l acriflavine (LSAM X 2A); a non-selective yeast glucose Lemco agar was included for comparison. When the difference between listeria and the natural microflora of raw milk and cheese was 10(2) cfu/ml, listeria could be isolated by direct plating on all media tested. When it was lower than 10(3)-10(4) cfu/ml, listeria were isolated by direct plating only on LSAM and LSAM X 2A. When the difference was greater than 10(4) cfu/ml, a previous enrichment was necessary to isolate them. LSAM and LSAM X 2A media performed better than the other media tested for isolating listeria by direct plating and improved their isolation from dairy products. This superior performance was evaluated by the ability of these media to support colony formation of different species of Listeria tested, the easy recognition of these colonies from those formed by other microorganisms and by their capacity to inhibit the natural microflora of these foods.