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

Abstract

A total of 750 beef carcasses were assayed to detect E. coli O157:H7 by surface swabbing. Each swab represented a 200-cm2 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 cm2 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 cm2 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.