COMPARISON OF METHODS FOR DETERMINING THE PRESENCE OF ESCHERICHIA-COLI O157-H7 IN APPLE JUICE

Six methods were compared for detection of three strains of Escherichi a coli O157:H7 in enrichments of inoculated apple juice. Juice was ino culated at levels varying from 0.1 to 100 CFU/ml and centrifuged after overnight storage at 4 degrees C, and pellets were incubated at 37 de grees C in nonselective enrichment broth. At hourly intervals between 5 and 10 h and at 24 h, the enrichments were tested for E. coli O157:H 7 by direct fluorescent antibody (DFA), antibody-direct epifluorescent filter technique (Ab-DEFT), direct selective plating on sorbitol MacC onkey agar (SMA), immunomagnetic separation coupled to either selectiv e plating (IMS-SMA) or the polymerase chain reaction (IMS-PCR), and fl ow cytometry (FC). The most consistent detection of 0.1 CFU/ml of the slowest growing strain of the pathogen was provided by the IMS-SMA and IMS-PCR after 8 h of enrichment. The time required for detection at t he level of 0.1 CFU/ml for each assay was Ab-DEFT, 11 h; IMS-PCR, 16 h ; FC, 24 h; IMS-SMA, 32 h; and SMA, 48 h. Absolute detection limits (w ithout enrichment) were: IMS-PCR, 10(3) CFU/ml; Ab-DEFT and IMS-SMA, 1 0(4) CFLT/ml; SMA, 10(5) CFU/mL; and DFA, 10(6) CFU/ml. Recovery of th e pathogen (10 CFU/ml) in apple juice after 28 days of 4 degrees C sto rage was possible by means of an 8-h enrichment and Ab-DEFT, IMS-PCR, or IMS-SMA.