Direct 24-hour presumptive enumeration of Escherichia coli O157:H7 in foods using hydrophobic grid membrane filter followed by serological confirmation: collaborative study.

Fifteen laboratories took part in a collaborative study to validate a method for enumerating Escherichia coli O157:H7. The method is based on use of a hydrophobic grid membrane filter and consists of 24 h presumptive enumeration on SD-39 Agar and serological confirmation to yield a confirmed E. coli O157:H7 count. Six food products were analyzed: pasteurized apple cider, pasteurized 2% milk, cottage cheese, cooked ground pork, raw ground beef, and frozen whole egg. The test method produced significantly higher confirmed count results than did the reference method for milk, pork, and beef. Test method results were numerically higher than but statistically equivalent to reference method results for cheese, cider, and egg. The test method produced lower repeatability and reproducibility values than did the reference method for most food/inoculation level combinations and values very similar to those of the reference method for the remaining combinations. Overall, 94% of presumptive positive isolates from the test method were confirmed serologically as E coli O157:H7, and 98% of these were also biochemically typical of E. coli O157:H7 (completed test). Corresponding rates for the reference method were 69 and 98%, respectively. On the basis of the results of this collaborative study and the precollaborative study that preceded it, it is recommended that this method be adopted official first action for enumeration of E. coli O157:H7 in meats, poultry, dairy foods, infant formula, liquid eggs, mayonnaise, and apple cider.