Location: Meat Safety & Quality Research
Title: The effect of sample size and matrix on the ability of rapid methods to detect E. coli O157:H7 Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: February 11, 2011
Publication Date: March 2, 2011
Citation: Bosilevac, J.M., Schmidt, J.W., Kalchayanand, N., Wheeler, T.L., Koohmaraie, M. 2011. The effect of sample size and matrix on the ability of rapid methods to detect E. coli O157:H7 [abstract]. 2011 Beef Industry Safety Summit, Dallas, TX. March 2-4, 2011. 2011 Beef Industry Safety Summit Research Update. Abstract No. 1. Technical Abstract: Objective. E. coli O157:H7 testing schemes and test methods are constantly evolving areas and as better ways of testing come about, there is a tendency to implement these changes while not recognizing potential problems that may result later. For example, most detection kits are validated according to AOAC protocols that use 25 g samples, but most users recognize that a larger sample provides a more sensitive test. Also, detection tests originally validated for use with ground beef are being used on different sample types such as offals. Therefore, objectives of this work were to first determine if changes in the size of a sample, the volume of media used, and the length of time of enrichment, affected the detection of E. coli O157:H7. The second objective was to determine if the matrix of the sample impacted the detection tests. Experimental Treatments. Boneless beef trim samples of 25 g, 150 g, or 325 g were inoculated with 1 to 3 CFU E. coli O157:H7 each and enriched in 1, 3, or 9 volumes of tryptic soy broth (TSB) for either 8, 12, or 16 h then tested for E. coli O157:H7. The detection tests were culture isolation, lateral flow device, and DNA amplification. A second round of experiments using the same parameters was repeated but TSB was replaced with modified TSB containing casamino acids and novobiocin (mTSBcn). Next, different sample matrices of either boneless beef trim (< 50% lean and > 95% lean), ground beef (73%, 85%, and 93%), lean finely textured beef, beef hearts, kidneys, cheek meat, liver, and sponge samples (collected from 500 cm2 of hide and 4000 cm2 of carcass) were inoculated with 1 to 3 CFU E. coli O157:H7 each. Two different sample enrichment conditions were used (25 g in 225 mL TSB incubated 8 h, and 325 g in 1 L mTSBcn incubated 16 h) for the E. coli O157:H7 detection tests. Key Results. The results showed that increasing sample size or decreasing volume affected the accuracy of detecting E. coli O157:H7 at 8, 12, and 16 h of incubation and that the 12 h and 16 h detections could be improved through the use of a more selective media such as mTSBcn. When the E. coli O157:H7 detection tests were used on samples of different matrices, all methods were equally effective at detecting E. coli O157:H7 in all sample types examined except those containing liver, which required a phosphate buffered medium for the proper detection of E. coli O157:H7. How this Information can be Applied in the Industry. These results identify situations when a revalidation is needed and when it is not. They also provide guidance on the use of modified enrichment media, and the length of time needed to detect E. coli O157:H7 by different tests in different sizes and types of samples, and can be used as supporting documents in testing programs.