Persistence of Listeria monocytogenes in yogurt as determined by direct plating and enrichment methods.

Listeria Selective Isolation Agar (LSI) and Modified Acriflavin Ceftazidime Esculin Agar (MACE) were compared to McBride Listeria Agar minus Blood (MLA-B) for ability to recover Listeria monocytogenes Scott A cells inoculated into commercial yogurt, pH 4.1, Yogurt was stored at 5 degrees C and sampled periodically over a 12 day period. LSI, MACE and MLA-B inhibited the growth of the two yogurt organisms but LSI and MACE gave better inhibition of other separately tested Gram-positive bacteria likely to be present in other fermented foods. Acid-stressed Listeria monocytogenes Scott A cells were optimally recovered by enrichment at 5 degrees C for 5-18 days in 0.1 M phosphate buffer, pH 7.2, followed by transfer to tryptic soy broth +0.5% yeast extract at 30 degrees C for 2 days. At low inoculum levels (10(2) cells/g yogurt), they were not detectable by direct plating or enrichment of yogurt after day 0. At high inoculum levels (10(7) cells/g yogurt), they were detectable up to day 6 but not at day 9 by direct plating on MLA-B, LSI or MACE with log counts per gram of yogurt being about 10 fold higher on LSI than on MACE or MLA-B. The above enrichment procedure permitted recovery on MLA-B, LSI, or MACE of viable Listeria cells from the day 9 samples found negative by direct plating.