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United States Department of Agriculture

Agricultural Research Service

Title: Comparison of Methods for Detection and Isolation of Cold-Stressed Escherichia Coli O157:h7 in Raw Ground Beef

Authors
item Fratamico, Pina
item Bagi, Lori

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: February 15, 2003
Publication Date: August 12, 2003
Citation: FRATAMICO, P.M., BAGI, L.K. COMPARISON OF METHODS FOR DETECTION AND ISOLATION OF COLD-STRESSED ESCHERICHIA COLI O157:H7 IN RAW GROUND BEEF. MEETING ABSTRACT. 2003.

Technical Abstract: A comparison was made of the relative efficiencies of two enrichment media, Rapid Check E. coli O157 Enrichment Broth (REB) and modified E. coli broth containing novobiocin (mEC+n), and four selective plating media, for detection/isolation of cold-stressed Escherichia coli O157:H7 in raw ground beef. Ground beef samples (25 g) were inoculated with 1 to 5 or 10 to 20 CFU of E. coli O157:H7 and stored at 4ºC for 72 hours prior to enrichment. After enrichment at 42ºC with aeration or at 35ºC with no aeration for 8 or 20 h, the cultures were diluted and directly plated onto BCM O157:H7(+) agar, CT-SMAC, CHROMagar O157, and Rainbow Agar O157 plates. The cultures were also tested with the Rapid Check E. coli O157 lateral flow immunoassay cassette and with a multiplex PCR assay amplifying segments of the rfbEO157:H7, fliCh7, stx1, and stx2 genes. Generally, after 8 h of enrichment at 42ºC in REB and mEC+n, E. coli O157:H7 colonies were detected on all four selective agar media, even in samples inoculated at a level of 1 to 5 CFU/25g; however, the number of CFU/ml obtained was ca. 1 to 2 log10 higher using REB compared to mEC+n. E. coli O157:H7 was not detected by plating on the selective agars when inoculated at levels of 1 to 5 CFU/g of ground beef after 8 or 20 h of enrichment in mEC+n at 35ºC without aeration. In most cases, positive results were obtained with the Rapid Check E. coli O157 lateral flow cassette when levels of E. coli O157:H7 reached ca. 4 to 5 log10 CFU/ml; however, the organism was detected at levels lower than 4 log10 CFU/ml by the multiplex PCR in both REB and mEC+n enrichments incubated at 42ºC for 8 or 20 h. Results of this study indicate that enrichment in REB at 42ºC with aeration is superior to enrichment in mEC+n at 35ºC without aeration for the detection/isolation of cold-stressed E. coli O157:H7 by direct plating, the PCR, and by an immunoassay after 8 h of enrichment when inoculated at levels of 1 to 5 CFU/25 g of ground beef.

Last Modified: 7/24/2014
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