USE OF THE PETRIFILM(TM) TEST KIT HEC DIRECT BLOT ENZYME-IMMUNOASSAY METHOD WITHOUT SWAB PREENRICHMENT 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 b y surface swabbing. Each swab represented a 200-cm(2) composite surfac e area. Escherichia coli O157:H7 was not detected in any of the carcas s samples. Assays were conducted from carcass swab suspensions by dire ct plating on Petrifilm(TM) E. coli Count plates followed by an E. col i O157:H7 assay using the Petrifilm(TM) Test Kit-HEC direct blot enzym e immunoassay without sample pre-enrichment, and by enrichment using a modified U.S. Department of Agriculture (USDA) Petrifilm(TM) E. coli O157:H7 procedure. Additionally, beef slabs were inoculated with E. co li 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(2) were designated low, medium, and high surface con tamination 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 le vels, respectively. In comparison, recovery from 25-g enrichments of g round 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 pe rcent recovery of this organism from nonground excised samples contain ing 56 CFU/25 g enrichment was 68%, versus 100% by direct swab techniq ue without sample pre-enrichment from beef containing 190 CFU/25 cm(2) . Direct surface swab tests with the 3M Petrifilm(TM) Test Kit-HEC wit hout sample pre-enrichment proved to be a sensitive, nondestructive, a lternate means of evaluating beef carcasses for the presence of E. col i O157:H7.