Persistence at source of Listeria spp. in raw milk

From 36 of 315 bulk tank sources of raw milk found to harbour Listeria spp., 34 were available for resampling at intervals to determine persistence of the organisms. Listeriae were reisolated from 21 sources. In 16 Listeria spp. were isolated in one retest. From the other five listeriae were obtained in more than one retest. Listerial populations were not particularly persistent. In all but one instance listeriae were not reisolated more than 5 months after initial sampling. Intermittent variations in Listeria spp. isolated were observed. Some repeat samples yielded the same species as originally identified, but sometimes only one of originally two species was isolated. On occasions completely different or additional species were found. The aetiology of listeriosis in cattle and contamination of raw milk is discussed.