Selective Enrichment with a Resuscitation Step for Isolation of Freeze Injured Escherichia Coli O157H7 From Foods

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

Appl Environ Microbiol. 2000 July; 66(7): 2866-2872.

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 

Abstract

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.