On the Origin of Listeria monocytogenes in Raw Bulk-Tank Milk

Four dairy farms with a known history of shipping raw bulk-tank milk contaminated with Listeria monocytogenes were investigated to determine possible sources of the organism and other Listeria species. Samples tested included bulk-tank milk, individual cow milk, aseptic quarter milk, feed, feces, manure, soil, bedding, and swabs of physical units. The incidence of L. monocytogenes was lowest in aseptic quarter samples, 1/262, and highest in rectal fecal samples, 10/69 (14.5%). The organism was also found in feed and environmental samples. A comparative study of three farms with no history of L. monocytogenes in raw bulk milk revealed that the organism was present in 15/114 (13.2%) rectal fecal samples. The use of Fraser Broth proved to be a valuable screening test for the presence of Listeria species in milk samples. Attempts to differentiate the isolates of L. monocytogenes on the basis of biochemical tests, antimicrobic sensitivity and plasmid profiles were unsuccessful. The significance of the environmental nature of L. monocytogenes is discussed in relation to other pathogens in raw milk.