ENUMERATION OF VIABLE LISTERIA SPECIES AND LISTERIA-MONOCYTOGENES IN FOODS

A direct plating procedure was developed for the enumeration of Listeria spp. and Listeria monocytogenes in foods. Both naturally contaminated foods and foods spiked with L. innocua, L. seeligeri, and L. monocytogenes were studied. The enhanced hemolysis agar (EHA) developed by Cox and modified in our study resulted in two types of agar, referred to as listeria enumeration agar (LEA) no. 1 and 2, used for products of lighter and heavier background microbial populations, respectively. On LEA plates, total Listeria spp, counts were determined by fluorescence caused by the breakdown of 4-methylumbelliferyl-beta-D-glucoside contained in EHA. L. monocytogenes counts were determined by picking a representative number of hemolytic colonies and stabbing them into a xylose agar plate to distinguish L. monocytogenes from L. seeligeri. Contamination levels of >200 Listeria cells per g of food can be accurately quantified by this procedure with >80% recovery. Counts of <200 Listeria cells per g of food were considered estimates. When the level of contamination was <100 Listeria cells per g of food, the recovery was <58%. Occasionally, with low-level inocula, Listeria was not detected. Nevertheless, when the procedure was combined with incubation of the enrichment mixture (used for the 1:10 direct plating dilution) and subsequent streaking, Listeria contamination could still be detected and the level, therefore, was determined to be between 1 and 150/g.