Screening Listeria monocytogenes from bulk tank milk and in-line milk filter samples for the routine monitoring of the dairy farms
Increased consumption of raw milk has been observed among the urban population in Europe along with an increasing consumer interest in foods that are less processed and locally produced. Raw milk and raw milk products have been associated with listeriosis caused by Listeria monocytogenes. These new trends in consumer behavior increase the need for more effective surveillance methods for L. monocytogenes at farm level. In Finland, raw milk sales are allowed directly from the farms to the consumer. The renewed Finnish national legislation stipulates that screening of L. monocytogenes on farms that sell raw milk includes five replicates of 25-ml bulk tank milk (BTM) samples. Due to dilution to the high volume of BTM, it is unlikely to detect low levels of bacterial contamination from the milk samples. Testing milk samples is also expensive and logistically difficult for continuous monitoring of L. monocytogenes at the farms. An alternative approach could be the testing of milk filters. Theoretically, milk filters may be more sensitive than BTM as a sampling material since all of the milk in the bulk tank has to pass through the filter. The objective of this longitudinal study is to compare milk filters and BTM for the best sampling material suitable for routine monitoring of L. monocytogenes in the dairy farms selling raw milk. Five BTM samples and a milk filter sample from two farms were tested for L. monocytogenes each week, for a period of 10 months, using NMLK 136:2010, ISO 11290-1:1996 and ISO 11290-2:1998 methods. The isolation of L. monocytogenes and other Listeria species were identified by API biochemical assay and multiplex PCR. The results indicate that milk filters are ideal as a sampling material in detecting L. monocytogenes than BTM samples. However, the presence of L. monocytogenes in milk filters was not associated to the presence of L. monocytogenes in BTM. Thus, milk filters could be used as indicators of the farm’s contamination status. Along with these findings, seasonal variation in detection, and advantages of identification methods were also discussed in this thesis.
Raw milk, Milk filter, routine monitoring
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