title: Effects of using reduced volumes of nonselective enrichment medium in methods for the detection of Escherichia coli O157:H7 from raw beef.
abstract: Recent work from our laboratory revealed that tryptic soy broth (TSB) was a superior enrichment medium for use in test-and-hold Escherichia coli O157:H7 methods at levels down to a ratio of three volumes of medium to one volume of sample. Lower ratios were examined for their effect on results obtained from culture isolation, the BAX E. coli O157:H7 MP assay, and the Assurance GDS E. coli O157:H7 assay. Ground beef and boneless beef trim were inoculated with a high level (170 CFU/65 g of ground beef and 43 CFU/65 g of trim) and a low level (17 CFU/65 g of ground beef and 4 CFU/65 g of trim) of E. coli O157:H7 and enriched in 3, 1, 0.5, and 0 volumes of TSB. The volume of TSB used did not affect E. coli O157:H7 detection by culture isolation, Assurance GDS detection in ground beef or trim, or the BAX MP assay detection in ground beef. However, BAX MP assay detection of E. coli O157:H7 in beef trim was 50, 42, and 33% positive when enrichment volumes of 0.5x, 1x, and 3x, respectively, were used. Optimum results with all methods were obtained using 1 volume of TSB. We concluded that detection test results can be considered valid as long as enrichment medium is used, even when it is less than the specified 3 or 10 volumes.
authors.affiliation: U.S. Department of Agriculture, Agricultural Research Service, Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, Nebraska 68933-0166, USA. mick.bosilevac@ars.usda.gov
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Bosilevac
authors.forename: Mohammad
authors.initials: M
authors.lastname: Koohmaraie
mesh_terms: D000818:Animals; D002417:Cattle; D015169:Colony Count, Microbial / Q000379:methods*; D003257:Consumer Product Safety; D003470:Culture Media / Q000737:chemistry*; D019453:Escherichia coli O157 / Q000302:isolation & purification*; D005506:Food Contamination / Q000032:analysis*; D006801:Humans; D008460:Meat / Q000382:microbiology*; D008461:Meat Products / Q000382:microbiology
chemical_list: D003470:Culture Media