24-Hour Presumptive Enumeration of Escherichia coli O157:H7 in Foods by Using the ISO-GRID Method with SD-39 Agar.

Two 24-hour presumptive enumeration methods for Escherichia coli O157:H7 organisms based on the hydrophobic grid membrane filter (ISO-GRID) and using SD-39 agar, a new selective and differential culture medium, were developed and compared to a 3-tube MPN (most probable number) method using modified tryptone soy broth enrichment. The comparative study comprised 22 combinations of storage conditions and food products, including a variety of raw and cooked meats and several dairy products. The ISO-GRID direct filtration method produced counts which were equivalent to or significantly higher than the 3-tube MPN method for all food-storage combinations except for frozen pasteurized whole egg. The ISO-GRID resuscitation method produced counts equivalent to the 3-tube MPN method for the frozen egg.