Diversity of Listeria ribotypes recovered from dairy cattle, silage, and dairy processing environments

Listeria strains isolated over the past 10 years from farms and dairy processing environments were subjected to strain-specific ribotyping using the automated Riboprinter microbial characterization system, alpha version (E. I. du Pont de Nemours & Co., Inc.). A total of 388 Listeria isolates from 20 different dairy processing facilities were examined along with 44 silage, 14 raw milk bulk tank, and 29 dairy cattle (26 udder quarter milk, 1 brain, 1 liver, and 1 aborted; fetus) isolates. These 475 isolates included 93 L. monocytogenes, 362 L. innocua, 11 L. welshimeri, 6 L. seeligeri, 2 L. grayi, and 1 L. ivanovii strains. Thirty-seven different Listeria ribotypes (RTs) comprising 16 L. monocytogenes (including five known clinical RTs responsible for foodborne listeriosis), 12 L. innocua, 5 L. welshimeri, 2 L. seeligeri, 1 L. ivanovii, and 1 L. grayi were identified. Greatest diversity was seen among isolates from dairy processing facilities with 14 of 16 (87.5%) of the L. monocytogenes RTs (including five clinical RTs) and 19 of 21 (90.5%) of the non-L. monocytogenes RTs detected. Sixty-five of the 93 L. monocytogenes isolates belonged to a group of five clinical RTs. These five clinical RTs included one RT unique to dairy processing environments, two RTs common to dairy processing environments and silage, and one RT common to dairy processing environments, silage, and dairy cattle with the last RT appearing in dairy processing environments, silage, raw milk bulk tanks, and dairy cattle. These findings, which support the link between on-farm sources of Listeria contamination (dairy cattle, raw milk, silage) and subsequent contamination of dairy processing environments, stress the importance of farm-based HACCP programs for controling listeriae.