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United States Department of Agriculture

Agricultural Research Service


item Karns, Jeffrey
item Van Kessel, Jo Ann
item Mccluskey, Brian
item Perdue, Michael

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2005
Publication Date: 10/1/2005
Citation: Karns, J.S., Van Kessel, J.S., Mccluskey, B.J., Perdue, M.L. 2005. Prevalanece of Salmonella Enterica in bulk tank milk from U.S. Dairies as determined by pcr. Journal of Dairy Science. 88:3475-3479.

Interpretive Summary: Raw milk may contain bacteria that are pathogenic to humans. Although pasteurization will kill these bacteria, many people still drink raw milk or eat raw milk products. The objective of this study was to determine how often raw milk from U.S. dairies is contaminated with the pathogenic bacterium Salmonella enterica. A real-time assay based on the polymerase chain reaction (PCR) was used to detect Salmonella enterica in milk samples from across the United States taken as part of the NAHMS 2002 Dairy survey. We had previously analyzed these samples using conventional bacteriological culture techniques and found that 2.6% of them contained Salmonella enterica. When assayed using the real-time PCR assay we found that a substantially higher number (11.8%) contained Salmonella enterica. Thus, Salmonella in raw milk represents a greater potential risk to consumers of raw milk and raw milk products than previously believed. Real-time PCR may be more sensitive than traditional culture methods for the detection of Salmonella spp. in raw milk. In addition, enrichment followed by RT-PCR yields results in 24 h as opposed to 48 to 72 h for traditional culture.

Technical Abstract: Samples of bulk tank milk from dairies across the United States, taken as part of the NAHMS Dairy 2002 survey were analyzed for the presence of Salmonella enterica using a commercially available real-time PCR kit. Samples from 854 farms in 21 states were collected and enriched in tetrathionate broth to amplify any salmonellae present and DNA was isolated from the resulting biomass. One-hundred and one samples (11.8%) were shown to contain traces of Salmonella enterica using the real-time PCR assay whereas conventional culture techniques detected the pathogen in only 22 (2.6%) of the samples. A conventional PCR assay targeting a different gene from Salmonella enterica confirmed the presence of the organism in 94 of the real-time PCR positive samples. Thus, assay of milk samples by real-time PCR indicates that the prevalence of Salmonella enterica in US bulk tank milk is substantially higher than previously reported.

Last Modified: 10/19/2017
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