Thermal resistance of intracellular and freely suspended Listeria monocytogenes that was associated with a milkborne outbreak of listeriosis was studied by using the sealed tube and slug flow heat exchanger methods. Test temperatures for the former method were 57.
The standard selective enrichment protocols of the Food and Drug Administration (FDA) and U.S. Department of Agriculture (USDA) were compared with an experimental nonselective broth enrichment (NSB) protocol and variations of the standard cold-enrichment (CE) protocol for the recovery of heat-injured Listeria monocytogenes. Bacterial cells (107/ml) were suspended in sterile milk and heated at 71.7°C in a slug-flow heat exchanger for holding times ranging from 1 to 30 s. Surviving cells were determined (50% endpoint) by the given protocols, and the following D values were obtained: NSB, D = 2.0 0.5 s; FDA, D = 1.4 + 0.3 s; USDA, D = 0.6 0.2 s; CE, D < 1.2 s. The respective direct-plating media used in these enrichments were also analyzed for recovery, and the following D values were calculated from the enumeration of surviving cells: NSB, D = 2.7 ± 0.8 s; FDA, D = 1.3 ± 0.4 s; USDA, D = 0.7 ± 0.2 s. The low levels of heat-injured L. monocytogenes cells which were detected at inactivation endpoints on the optimal nonselective media (25°C for 7 days) failed to recover and multiply during experimental CEs (4°C for 28 days). Initial inactivation experiments in which raw whole milk was used as the heating menstruum gave much lower recoveries with all protocols. The detectable limits for uninjured cells that were suspended in raw milk were similar (0.35 to 3.2 cells per ml) for the standard CE, FDA, and USDA protocols. Recovery by the NSB procedure (68 cells per ml) was compromised by background flora. The above data suggest that any cells surviving high-temperature, short-time pasteurization will be injured and unable to multiply either during cold storage of milk or in the FDA or USDA systems. Thus, L. monocytogenes cells recovered in finished pasteurized milk products by these detection methods probably represent uninjured environmental contaminants.
The processing environments of 30 dairy facilities were surveyed for the presence of Listeria species. Two different primary enrichment media — University of Vermont Modified Listeria Enrichment Broth and Listeria Repair Broth — were employed to increase the probability of identifying positive samples. Samples were also tested using both an enzyme-linked immunosorbent-based (ELISA-based) assay and a gene probe assay. A total of 346 sponge samples were evaluated for the presence of Listeria. Listeria spp. were identified via one or more of the assays 122 (35.3%) times. Fifty-five of the positive samples (37.2%) contained Listeria monocytogenes and 93 (62.8%) contained Listeria innocua. Of the 30 plants tested, 9 had a dairy farm contiguous to the processing facilities. Our results show that these plants are more likely to be contaminated (9/9) than those plants without on site dairy farms (17/21). Analysis of the Listeria spp. results indicated that contamination was significantly higher (α = 0.1) at those plants with an on-site dairy farm (x = 50.1 %) than those plants without an on-site dairy farm (x = 33.5%). Plants producing dairy ingredients, frozen milk products or fluid milk were all shown to have significantly higher incidence rates than expected. Conversely, plants producing cultured dairy foods, or a combination of cultured dairy foods and fluid milk were found to have significantly lower incidence rates than expected. There was no statistically significant difference in contamination by area within the plants.
SUMMARY Because little is known of the phagocytes of the human colon we enumerated these cells in mucosal suspensions and studied their phagocytic activity. Phagocyte rich suspensions were made by EDTA
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