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Applied and Environmental Microbiology, July 2000, p. 2866-2872, Vol. 66, No. 7
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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

Y. Hara-Kudo,1,* M. Ikedo,2 H. Kodaka,3 H. Nakagawa,4 K. Goto,5 T. Masuda,6 H. Konuma,7 T. Kojima,2 and S. Kumagai1

Department of Biomedical Food Research, National Institute of Infectious Diseases, Shinjuku-ku, Tokyo 162-8640,1 Eiken Chemical Co. Ltd., Shimotsuga-gun, Tochigi 329-0114,2 Nissui Pharmaceutical Co. Ltd., Yuki, Ibaraki 307-0036,3 Tokyo Kenbikyoin Foundation, Higashikurume, Tokyo 203-0032,4 Niigata Prefectural Research Laboratory for Health and Environments, Sowa 314-1, Niigatashi 950-215,5 Shizuoka Institute of Environment and Hygiene, Shizuoka 420-8637,6 and Division of Microbiology, National Institute of Health Sciences, Setagaya-ku, Tokyo 158-8501,7 Japan

Received 20 January 2000/Accepted 19 April 2000

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°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°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 isolating E. 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 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.


* Corresponding author. Mailing address: Department of Biomedical Food Research, National Institute of Infectious Diseases, 1-23-1 Toyama, Shinjuku-ku, Tokyo 162-8640, Japan. Phone: 81 3 5285 1111. Fax: 81 3 5285 1176. E-mail: ykudo{at}nih.go.jp.


Applied and Environmental Microbiology, July 2000, p. 2866-2872, Vol. 66, No. 7
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.



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