Methods used to isolate Listeria spp. from raw milk were compared during a 13-month period (April 1989–April 1990). Raw milk was obtained bimonthly from 12 dairy farms during this time period. Three enrichments were compared: Listeria enrichment broth (LEB); University of Vermont medium (UVM), followed by a secondary enrichment with Fraser broth; and LPALCAMY. Four selective plating media, including Listeria selective agar-Oxford formulation; Modified McBride Listeria agar (MMLA); Lithium chloride-phenylethanol-moxalactam (LPM) agar; and L-PALCAM agar, were compared. L-PALCAMY enrichment broth enhanced isolation of L. monocytogenes and other Listeria spp. The greatest numbers of positive Listeria samples were obtained on Oxford and PALCAM agars, while the poorest isolation was obtained using the combination of LEB with MMLA. L. monocytogenes was found to be present in 3.0% (9/300) of the samples analyzed. The incidence of Listeria spp. was 28.0%; Listeria innocua, 26.7%; and Listeria welshimeri, 1.7%. Listeria ivanovii was not isolated. The incidence of Listeria was highest during the warmer months of April–July, while the lowest incidences occurred in December and February. Somatic cell counts (SCC) and standard plate counts (SPC) were determined by the cooperative that supplied the raw milk. In general, a producer's milk with high SPC corresponded to a high incidence of Listeria spp., while low SPC corresponded to a low incidence. A particular producer's milk with the highest incidence of Listeria spp. also had the highest SCC.
*Published as paper number 18612 of the contribution series of the Minnesota Agricultural Experiment Station based on research conducted under Project 18–56, supported by Hatch Funds and by a grant from the Minnesota-South Dakota Dairy Foods Research Center.