Performance of enhanced haemolysis agar compared to Oxford medium for the isolation of Listeria monocytogenes from food enrichment broths

Abstract

Combinations of tryptone soya broth plus yeast extract trypaflavine and nalidixic acid (TSByeAB), a modified Listeria enrichment broth (MLEB), Oxford medium and Enhanced Haemolysis Agar (EHA) were used to analyse 99 food samples from supermarkets in continental Europe. A total of 48 samples were positive for Listeria spp. using all combinations of enrichment broth, enrichment time and isolation medium. TSByeAB enrichments tended to result in loss of detection of positives over 48 h enrichment, whereas MLEB showed an improved detection. Oxford medium detected 33 (68·7%) of the 48 Listeria spp. positive samples, compared to 46 (95·8%) of the 48 detected by EHA. Listeria monocytogenes was detected in 36 of the 48 samples, 15 (41·6%) being detected on Oxford medium compared to 35 (97%) on EHA. Similar tendencies were observed for both isolation media irrespective of the enrichment broth used, EHA being the superior medium. In 16 enrichment broth/time combinations, Listeria innocua and L. monocytogenes were detected together on the same plate. This occurred 13 times on EHA but only once on Oxford medium. In 8 of the 16 cases, L. innocua or nothing was detected on the Oxford plate when EHA was positive for both organisms. On only one occasion was Oxford medium positive for both organisms when L. innocua or nothing was present on the EHA plate. These results show EHA to have advantages for both the isolation of Listeria spp. and the discrimination between haemolytic and non-haemolytic species of this genus.