Enhanced haemolysis agar (EHA)—an improved selective and differential medium for isolation of Listeria monocytogenes

Abstract

Enhanced Haemolysis Agar (EHA) medium has been developed for the selective isolation of Listeria monocytogenes from food enrichments. It is based on the selective agents of the Oxford medium (Curtis et al. 1989a), with the addition of sheep blood (5%), 4-methylumbelliferyl-β- d-glucoside (50 mg l −1) and sphingomyelinase (10 units/l −1). The medium contains no esculin or ferric citrate and the lithium chloride concentration is reduced to 1%. L. monocytogenes colonies are distinguished by their light blue fluorescence and yellow centre (in UV 366nm), colony morphology and the presence of a distinct zone of haemolysis around the colony, indicating the production of the CAMP factor. Listeria innocua does not produce haemolysis on this medium and therefore cannot be confused with L. monocytogenes. Haemolysis positive Listeria seeligeri, although found relatively rarely in foods, can be distinguished by performing a xylose fermentation test after isolation on EHA. Strains of Listeria ivanovii from our collection failed to grow on this medium. Although haemolysis-negative and CAMP-negative L. monocytogenes may not be detected by this medium, their apparent lack of pathogenicity makes them of limited interest to food microbiologists.