Use of plasmid profiles and restriction endonuclease digest in environmental studies of Listeria spp. from raw milk.

Forty-eight isolates of Listeria spp. (19 L. monocytogenes, 27 L. innocua, 2 L. welshimeri) from bulk raw milk were screened for plasmid DNA. These isolates were collected over a period of 1 year. Only L. innocua harboured plasmids (8/27 strains) which ranged in size 10-44 megadaltons. The plasmid bearing strains could be arranged into three groups 10 Md, 44 Md and 44 + 10 Md. In two farms where Listeria innocua were persistent in bulk raw milk different plasmids profiles were found (farm 1 and 3). In species carrying similar plasmid profiles (44 Md) isolated from the same bulk tank raw milk samples over a period of 2-4 months (farm 2) no similarities in restriction endonuclease digest patterns of the plasmids were observed. This study suggests there may be a constant influx of Listeria spp. into raw milk supplies on the farm.