METHODS USED FOR DETECTION AND RECOVERY OF ESCHERICHIA-COLI O157H7 ASSOCIATED WITH A FOOD-BORNE DISEASE OUTBREAK

The current Food Safety and Inspection Service method for detection an d recovery of Escherichia coli O157:H7, (including modified EC broth w ith novobiocin (mEC+n) and a direct blot ELISA), was used to analyze b eef and environmental samples during an investigation of a food-borne disease outbreak attributed to consumption of undercooked hamburger pa tties. Double-modified trypticase soy broth (dmTSB) and a commercially available dipstick immunoassay were also used to improve detection/re covery of E. coli O157:H7. A total of 1,115 beef and environmental sam ples was screened with the direct blot ELISA and the dipstick immunoas say; 178 presumptive-positive samples (by either or both of the screen ing methods) were subjected to recovery/isolation procedures. Toxigeni c E. coli O157:H7 was recovered from 45 samples: 40 hamburger-patty sa mples produced on the epidemiologically identified date, 3 hamburger-p atty samples produced on another date, and 2 beef briskets. The organi sm was not recovered from environmental samples. Limited quantitative analyses indicated that contaminated hamburger patties contained fewer than 4.3 CFU of E. coli O157:H7 per g. Atypical, toxigenic ornithine decarboxylase-negative E. coil O157:H7 and nontoxigenic sorbitol-posit ive E. coli O157:H29 were also recovered. Both enrichment broths gave strong positive reactions with the two immunoassay screening methods, but E. coli 0157:H7 was recovered more often from mEC+n broth than fro m dmTSB. Both screening methods gave positive results for 44 of the 45 beef samples found to contain E. cole O157:H7. False-positive results were frequently observed with both screening methods.