Selective Enrichment with a Resuscitation Step for Isolation of Freeze-Injured Escherichia coli O157:H7 from Foods

We studied injury ofEscherichia coli O157:H7 cells in 11 food items during freeze storage and methods of isolating freeze-injuredE. coli O157:H7 cells from foods. Food samples inoculated withE. coli O157:H7 were stored for 16 weeks at −20°C in a freezer. Noninjured and injured cells were counted by using tryptic soy agar and sorbitol MacConkey agar supplemented with cefixime and potassium tellurite. Large populations ofE. coli O157:H7 cells were injured in salted cabbage, grated radish, seaweed, and tomato samples. In an experiment to detectE. coli O157:H7 in food samples artificially contaminated with freeze-injuredE. coli O157:H7 cells, the organism was recovered most efficiently after the samples were incubated in modifiedE. coli broth without bile salts at 25°C for 2 h and then selectively enriched at 42°C for 18 h by adding bile salts and novobiocin. Our enrichment method was further evaluated by isolatingE. coli O157:H7 from frozen foods inoculated with the organism prior to freezing. Two hours of resuscitation at 25°C in nonselective broth improved recovery ofE. coli O157:H7 from frozen grated radishes and strawberries, demonstrating that the resuscitation step is very effective for isolatingE. coli O157:H7 from frozen foods contaminated with injuredE. coli O157:H7 cells.