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Applied and Environmental Microbiology, January 2004, p. 69-75, Vol. 70, No. 1
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.1.69-75.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Rapid and Specific Detection of Salmonella spp. in Animal Feed Samples by PCR after Culture Enrichment

Charlotta Löfström,1,2 Rickard Knutsson,1 Charlotta Engdahl Axelsson,2 and Peter Rådström1*

Department of Applied Microbiology, Lund Institute of Technology, Lund University, SE-221 00 Lund,1 AnalyCen Nordic AB, SE-531 19 Lidköping, Sweden2

Received 22 May 2003/ Accepted 14 October 2003

A PCR procedure has been developed for routine analysis of viable Salmonella spp. in feed samples. The objective was to develop a simple PCR-compatible enrichment procedure to enable DNA amplification without any sample pretreatment such as DNA extraction or cell lysis. PCR inhibition by 14 different feed samples and natural background flora was circumvented by the use of the DNA polymerase Tth. This DNA polymerase was found to exhibit a high level of resistance to PCR inhibitors present in these feed samples compared to DyNAzyme II, FastStart Taq, Platinum Taq, Pwo, rTth, Taq, and Tfl. The specificity of the Tth assay was confirmed by testing 101 Salmonella and 43 non-Salmonella strains isolated from feed and food samples. A sample preparation method based on culture enrichment in buffered peptone water and DNA amplification with Tth DNA polymerase was developed. The probability of detecting small numbers of salmonellae in feed, in the presence of natural background flora, was accurately determined and found to follow a logistic regression model. From this model, the probability of detecting 1 CFU per 25 g of feed in artificially contaminated soy samples was calculated and found to be 0.81. The PCR protocol was evaluated on 155 naturally contaminated feed samples and compared to an established culture-based method, NMKL-71. Eight percent of the samples were positive by PCR, compared with 3% with the conventional method. The reasons for the differences in sensitivity are discussed. Use of this method in the routine analysis of animal feed samples would improve safety in the food chain.

* Corresponding author. Mailing address: Department of Applied Microbiology, Lund Institute of Technology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden. Phone: 46 46-222 34 12. Fax: 46 46-222 42 03. E-mail: Peter.Radstrom{at}tmb.lth.se.

Applied and Environmental Microbiology, January 2004, p. 69-75, Vol. 70, No. 1
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.1.69-75.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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