title: Persistence of Listeria monocytogenes in yogurt as determined by direct plating and enrichment methods.
abstract: 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.
authors.affiliation: University of Arkansas, Fayetteville 72703.
authors.forename: G R
authors.initials: GR
authors.lastname: Siragusa
authors.forename: M G
authors.initials: MG
authors.lastname: Johnson
mesh_terms: D000818:Animals; D002417:Cattle; D015169:Colony Count, Microbial; D003470:Culture Media; D003611:Dairy Products; D005516:Food Microbiology; D006801:Humans; D008089:Listeria monocytogenes / Q000254:growth & development / Q000302:isolation & purification*; D010944:Plants; D011817:Rabbits; D013696:Temperature; D015014:Yogurt
chemical_list: D003470:Culture Media