Related Experiment Videos
Persistence of Listeria monocytogenes in yogurt as determined by direct plating and enrichment methods
1University of Arkansas, Fayetteville 72703.
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.
Insights
Listeria Selective Isolation Agar (LSI) and Modified Acriflavin Ceftazidime Esculin Agar (MACE) effectively detected Listeria monocytogenes in yogurt. An optimized enrichment method improved recovery of acid-stressed Listeria cells.
Area of Science:
- Food Microbiology
- Bacteriology
- Food Safety
Background:
- Listeria monocytogenes is a foodborne pathogen of significant public health concern.
- Accurate detection of Listeria in acidic food products like yogurt is crucial for food safety.
Purpose of the Study:
- To compare the efficacy of Listeria Selective Isolation Agar (LSI) and Modified Acriflavin Ceftazidime Esculin Agar (MACE) for recovering Listeria monocytogenes Scott A from yogurt.
- To determine an optimal enrichment and recovery method for acid-stressed Listeria monocytogenes in yogurt.
Main Methods:
- Comparison of three selective agars: LSI, MACE, and McBride Listeria Agar minus Blood (MLA-B).
- Inoculation of commercial yogurt (pH 4.1) with Listeria monocytogenes Scott A at different levels.
- Storage of inoculated yogurt at 5°C and periodic sampling over 12 days.
- Evaluation of direct plating and an optimized enrichment procedure (5°C incubation in phosphate buffer followed by 30°C incubation in tryptic soy broth + yeast extract).
Main Results:
- LSI and MACE demonstrated superior inhibition of background yogurt organisms and other Gram-positive bacteria compared to MLA-B.
- Direct plating detected Listeria monocytogenes at high inoculum levels (10^7 cells/g) up to day 6, with higher counts on LSI.
- The optimized enrichment method enabled recovery of viable Listeria cells from day 9 samples that were negative by direct plating.
Conclusions:
- LSI and MACE are effective selective agars for Listeria monocytogenes detection in yogurt.
- An optimized low-temperature enrichment followed by a higher-temperature incubation significantly enhances the recovery of acid-stressed Listeria monocytogenes from yogurt.