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 persistant 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. © 1990.