INCREASED INCIDENCE OF LISTERIA SPECIES IN DAIRY PROCESSING PLANTS HAVING ADJACENT FARM FACILITIES

The processing environments of 30 dairy facilities were surveyed for t he presence of Listeria species. Two different primary enrichment medi a - University of Vermont Modified Listeria Enrichment Broth and Liste ria Repair Broth - were employed to increase the probability of identi fying positive samples. Samples were also tested using both an enzyme- linked immunosorbent-based (ELISA-based) assay and a gene probe assay. A total of 346 sponge samples were evaluated for the presence of List eria. Listeria spp. were identified via one or more of the assays 122 (35.3%) times. Fifty-five of the positive samples (37.2%) contained Li steria monocytogenes and 93 (62.8%) contained Listeria innocua. Of the 30 plants tested, 9 had a dairy farm contiguous to the processing fac ilities. Our results show that these plants are more likely to be cont aminated (9/9) than those plants without on site dairy farms (17/21). Analysis of the Listeria spp. results indicated that contamination was significantly higher (alpha = 0.1) at those plants with an on-site da iry farm (x = 50.1%) than those plants without an on-site dairy farm ( x = 33.5%). Plants producing dairy ingredients, frozen milk products o r fluid milk were all shown to have significantly higher incidence rat es than expected. Conversely, plants producing cultured dairy foods, o r a combination of cultured dairy foods and fluid milk were found to h ave significantly lower incidence rates than expected. There was no st atistically significant difference in contamination by area within the plants.