Investigating Contamination of Bulk Tank Milk with Listeria monocytogenes on a Dairy Farm

The objectives were to identify the source of Listeria monocytogenes in bulk tank milk (BTM), and to assess characteristics of Petrifilm Environmental Listeria (PEL) for detection of this pathogen in farm samples. Environmental and milk samples were collected from a dairy during two sampling periods. Follow-up samples of daily BTM and milk filters were collected. Isolates of L. monocytogenes were compared by use of Pulsed Field Gel Electrophoresis. Samples were plated on PEL and results classified into positive or negative. Of samples collected during the two sampling periods, L. monocytogenes was isolated from 66% of milk filters (19 of 29), 16% of BTM (7 of 44), 6% of water samples (two of 33) and one of 18 in-line milk samples. Except for one isolate, all were identical and of the same molecular type. Contamination of BTM with L. monocytogenes most likely originated from a common source, and results indicate that farms can develop persistent sources of contamination. The sensitivity of the PEL was high (100 and 74.1% for environmental and milk samples, respectively), but there was a high proportion of false-positive results and low specificity. These limitations need to be considered when using the PEL for on-farm screening of L. monocytogenes. Investigating Contamination of Bulk Tank Milk with Listeria monocytogenes on a Dairy Farm