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.
ABSTRACT
The Petrifilm E. coli Count plate (PEC) method was compared to the AOAC MPN method to determine the efficacy of the PEC method to detect E. coli and coliforms in 115 inoculated cheese samples, 94 vegetables samples, and in 100 naturally contaminated poultry samples. The PEC method was compared to two other coliform plate count methods. The 24 h PEC method is as good as or better than the AOAC MPN method for the detection of E. coli. In addition, qualitative results suggest that the PEC method may be more sensitive than the 9 tube MPN method for the detection of very low numbers of E. coli. Comparable coliform results were obtained.
ABSTRACT
A screening method was devised incorporating a commercially available reactive disc blot ELISA for Escherichia coli 0157 antigen, into a cultural screening program for the isolation of E. coli 0157:H7 from meat and poultry products. The method includes the inoculation of a raw or cooked meat sample into an enrichment broth, incubation with shaking at 37°C for 6 to 8 h, followed by inoculation of 3M Petrifilm™ E. coli Count plates with dilutions of the enrichment culture. The Petrifilm plates were incubated at 42°C for 18 h and tested for the presence of the 0157 antigen. The enrichment cultures were reincubated static at 35°C after the initial shaken incubation. Isolation was attempted from the positive Petrifilm plates by both a direct picking and streaking method and by the 3M Prompt™ isolation method. Isolation also was attempted from the 24-h enrichment cultures by spread plating serial dilutions on 150 × 15 mm MacConkey sorbitol agar (MSA) and MSA with 5-bromo-4-chloro-3-indoxyl-ß-D-glucuronic acid cyclohexylammonium salt (BCIG). This fast and efficient screening procedure identifies negative and presumptive positive samples in 26-28 h. Isolation and confirmation of the presumptive positive isolates require an additional 3 to 4 d.
