Presence and persistence of Listeria in four artisanal cheese plants in Jalisco, Mexico
In order to determine the presence of Listeria in four artisanal dairies (A, B, C, D) that produce soft chee- ses called Adobera, Fresco, Panela and Requesón, 392 samples were collected from raw milk, rennet curd, cheese equipment, surfaces, brooms, and brushes used for clea- ning, over a 21-month period from February 1999 to October 2000. Isolation and identification of Lis- teria was conducted following the methodologies described by the U.S. Food and Drug Administration (FDA) and Department of Agriculture (USDA), respectively. Findings showed that the frequency of Listeria per dairy was: Plant A: 55 %; B: 33 %; C: 26 %; and D: 7 %. Surfaces, floor and cleanning utensils had the highest percentages of positive samples. The species iso- lated were: L. monocytogenes, L. innocua, L. welshimeri, and L. iva- novii. Eleven (18 %) of 59 cheese samples were positive for Listeria, and cheeses from Plants A and C represented sources of L. mono- cytogenes infection in consumers. L. monocytogenes was isolated at two plants. At one (A), L. mono- cytogenes serovars 1/2a, 1/2b, 1/2 non-motile, as well as 4b were identified. DNA macro-restriction profile analysis showed three clus- ters for L. monocytogenes serovar 1/2b, of which one was recovered throughout the 21-month period.
At the other plant (C) only L. mo- nocytogenes serovar 4b was identi- fied. Four clusters were identified by DNA macrorestriction profile analysis, of which one was present throughout a 15-month period. L. innocua was the only species found at Plant B. The low frequen- cy of Listeria at Plant D was likely related to the cleaning practice of using boiling water or whey heated to 90 °C to clean floors, equipment and surfaces.