Listeria monocytogenes in raw milk and cheese made from unpasteurized milk

Abstract

The objective of the present study was  the detection of the human pathogen Listeria monocytogenes in  raw milk received at dairy plants and in fresh cheese samples made from unpasteurized milk. For this purpose 50 milk samples were collected from silos of dairy plants located in the VIII^th, IX^th and X^th regions of Chile. Also 25 cheese samples were purchased from street vendors in the city of Valdivia. For the isolation of  L. monocytogenes, 25 mL or g of the sample was inoculated into a preenrichment broth, followed by an inoculation on selective media. For the identification of the Listeria species, biochemical  tests were performed as described in the Bacteriologial Analytical Manual (F.D.A.).
Results of the analysis indicated the presence of  L. monocytogenes in 11 raw milk samples (22%). The number of  positive samples was higher in the IX^th region as compared to the other two regions, in spite of a lower number of samples collected. The cheese manufactured in different locations within the X^th region did not show the presence of the pathogen. It is concluded that L. monocytogenes is present in a high percentage of the milk  received at different dairy plants. The negative results found in  cheese manufactured from unpasteurized milk could indicate that the bacterium is present in a lower  percentage in milk proceeding from small  farmers.