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 cells 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 of E. coli O157:H7 cells were injured in salted cabbage, grated radish, seaweed, and tomato samples. In an experiment to detect E. coli O157:H7 in food samples artificially contaminated with freeze-injured E. coli O157:H7 cells, the organism was 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 selectively enriched at 42 degrees C for 18 h by adding bile salts and novobiocin. Our 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 that the resuscitation step is very effective for isolating E. coli O157:H7 from frozen foods contaminated with injured E. coli O157:H7 cells.
Hara-Kudo
Y
Y
Department of Biomedical Food Research, National Institute of Infectious Diseases, Shinjuku-ku, Tokyo 162-8640, Japan. ykudo@nih.go.jp
Ikedo
M
M
Kodaka
H
H
Nakagawa
H
H
Goto
K
K
Masuda
T
T
Konuma
H
H
Kojima
T
T
Kumagai
S
S
D015169:Colony Count, Microbial; D003470:Culture Media; D004927:Escherichia coli Infections / Q000382:microbiology; D019453:Escherichia coli O157 / Q000254:growth & development* / Q000302:isolation & purification*; D005511:Food Handling; D005516:Food Microbiology; D005615:Freezing; D006801:Humans; D008461:Meat Products / Q000382:microbiology; D014675:Vegetables / Q000382:microbiology
D003470:Culture Media