Selective enrichment with a resuscitation step for isolation of freeze-injured Escherichia coli O157 : H7 from foods

We studied injury of Escherichia coli O157:H7 cells in 11 food items during freeze storage and methods of isolating freeze-injured E. coli O157:H7 cel ls from foods. Food samples inoculated with E. coli O157:H7 were stored for 16 weeks at -20 degrees 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 off. coli O157:H7 cells were injured in salted cabbage, grated radish, seaweed, and tomato sa mples. In an experiment to detect E. coli O157:H7 in food samples artificia lly contaminated with freeze-injured E. coli O157:H7 cells, the organism wa s recovered most efficiently after the samples were Incubated in modified E . coli broth without bile salts at 25 degrees C for 2 h and then selectivel y enriched at 42 degrees C for 18 h by adding bile salts and novobiocin, Ou r enrichment method was further evaluated by isolating E. coli O157:H7 from frozen foods inoculated with the organism prior to freezing. Two hours of resuscitation at 25 degrees C in nonselective broth improved recovery of E, coli O157:H7 from frozen grated radishes and strawberries, demonstrating t hat the resuscitation step is very effective for isolating E. coli O157:H7 from frozen foods contaminated with injured E. coli O157:H7 cells.