Evaluation of enhanced haemolysis agar for detection of Listeria spp. and L-monocytogenes from production lines of fresh to cold-smoked fish

Enhanced haemolysis agar (EHA) was compared to the two conventional Listeri a isolation agars Oxford and PALCAM for its ability to detect Listeria spp. from production lines of fresh to cold-smoked fish. The ability of EHA for distinguishing L. monocytogenes colonies from other Listeria spp. was also evaluated. A total of 243 fish and environmental samples were analysed. Ov erall, 42 samples were found to contain Listeria spp. Only 34 samples were positive simultaneously by the three plating media. Two samples considered to be negative by the two conventional agars were found to be positive afte r isolation on EHA. All three selective agars were shown to be less effecti ve in recovering Listeria spp. after primary enrichment in half-Eraser brot h, compared to secondary enrichment in Fraser broth after 24 and 48 h. From 79 Listeria but presumptive negative L. monocytogenes colonies, EHA id entified correctly 76 Listeria spp, and presented three false-negative resu lts_three colonies further identified as L. monocytogenes but showing no no ticeable haemolysis on EHA. Twenty-three of the thirty-three L. monocytogen es presumptive positive colonies, were confirmed positive and ten were iden tified as L. seeligeri. Despite its ability of distinguishing L. monocytogenes from the other Liste ria spp., unless it is produced as a commercial medium, EHA cannot be an al ternative to time-consuming classical identification because the preparatio n of this medium is both time and labour intensive. (C) 2001 Elsevier Scien ce B.V. All rights reserved.