Use of the Petrifilm Test Kit-HEC Direct Blot Enzyme Immunoassay Method without Swab Pre-Enrichment to Screen for Low Levels of Escherichia coli O157 : H7 on Beef Carcasses by Surface Swabbing.

A total of 750 beef carcasses were assayed to detect E. coli O157:H7 by surface swabbing. Each swab represented a 200-cm composite surface area. Escherichia coli O157:H7 was not detected in any of the carcass samples. Assays were conducted from carcass swab suspensions by direct plating on Petrifilm E. coli Count plates followed by an E. coli O157:H7 assay using the Petrifilm Test Kit-HEC direct blot enzyme immunoassay without sample pre-enrichment, and by enrichment using a modified U.S. Department of Agriculture (USDA) Petrifilm E. coli O157:H7 procedure. Additionally, beef slabs were inoculated with E. coli O157 :H7 to verify that recovery by the direct swab technique was at least as efficient as enrichment of excised tissue samples, which is the usual USDA method. Escherichia coli O157:H7 inoculation of 4,32, and 180 CFU/25 cm were designated low, medium, and high surface contamination levels, respectively. The percentage of recovery of E. coli O157:H7 using the direct swab technique without sample pre-enrichment was 60, 80, and 100% for low, medium, and high surface inoculation levels, respectively. In comparison, recovery from 25-g enrichments of ground excised samples from beef slabs containing 1.4,9.6, and 50 CFU of E. coli O157:H7 was 0, 20, and 73%, respectively. Similarly, the percent recovery of this organism from nonground excised samples containing 56 CFU/25 g enrichment was 68%, versus 100% by direct swab technique without sample pre-enrichment from beef containing 190 CFU/25 cm Direct surface swab tests with the 3M Petrifilm Test Kit-HEC without sample pre-enrichment proved to be a sensitive, nondestructive, alternate means of evaluating beef carcasses for the presence of E. coli O157:H7.