Occurrence of Listeria monocytogenes in Raw Milk in Nebraska

Raw milk samples were obtained from bulk storage tanks of individual dairy farms in eastern Nebraska during February and July of 1986. One hundred different farms were tested during each period. One-tenth ml of each sample was plated directly onto McBride's Listeria Agar (MLA) and 30 ml was subjected to a four-week cold enrichment procedure. Suspect colonies from MLA were subjected to biochemical tests to confirm identity. Nine percent of all raw milk samples examined were determined to be positive for Listeria species after the cold enrichment procedure. Four percent contained L. monocytogenes and five percent contained L. innocua . Six percent and two percent of samples were found to contain L. monocytogenes in February and July respectively.