Purpose: Determine overall presence of Listeria spp. on farms producing milk for artisan cheesemakers, with the primary goal of identification of management practices that accomplish mitigation of sources of contamination
Methods: Environmental sampling was conducted on five dairy farms that were either cheese producers or were supplying milk to artisan cheese producers. A double enrichment protocol was used to isolate Listeria spp. and multiplex PCR was used to identify each isolate. All L. monocytogenes isolates were ribotyped though the use of the DuPont RiboPrinter® System.
Results: Out of 266 samples, 33.0% tested positive for Listeria spp., and 16 samples tested positive for L. monocytogenes (6.0%). We compared Listeria incidence on four farms; two (Farms A and D) fed dry hay or fed cows on pasture, while two others (Farms B and C) fed silage to animals. In both Farms B and C, L. monocytogenes ribotypes DUP 1039C and 1061 were found in silage and areas of the farm environment, particularly in water sources. Presence of Listeria was not detected in bulk tank milk, but was detected in one milk filter.
Significance: Advising artisan cheesemakers making raw milk cheeses to eliminate silage feeding in favor of dry hay or pasture feeding is a strategy which shows promise to reduce presence of Listeria in milk. Testing milk filters for presence of Listeria may be more effective in identifying potential presence of Listeria in milk. Listeria is rarely present in raw milk used for artisan cheese production. When we have found presence, it is at levels below detection limits. Testing milk filters may increase detection sensitivity and provide confidence in negative results.