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Applied and Environmental Microbiology, January 2006, p. 672-679, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.672-679.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Impact of Inoculum Preparation and Storage Conditions on the Response of Escherichia coli O157:H7 Populations to Undercooking and Simulated Exposure to Gastric Fluid

Jarret D. Stopforth,¹ Panagiotis N. Skandamis,¹ Laura V. Ashton,¹ Ifigenia Geornaras,¹ Patricia A. Kendall,² Keith E. Belk,¹ John A. Scanga,¹ Gary C. Smith,¹ and John N. Sofos^1*

Department of Animal Sciences,¹ Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, Colorado 80523²

Received 23 June 2005/ Accepted 1 November 2005

This study evaluated the impact of inoculum preparation and storage conditions on the response of Escherichia coli O157:H7 exposed to consumer-induced stresses simulating undercooking and digestion. Lean beef tissue samples were inoculated with E. coli O157:H7 cultures prepared in tryptic soy broth or meat decontamination runoff fluids (WASH) or detached from moist biofilms or dried biofilms formed on stainless steel coupons immersed in inoculated WASH. After inoculation, the samples were left untreated or dipped for 30 s each in hot (75°C) water followed by lactic acid (2%, 55°C), vacuum packaged, stored at 4 (28 days) or 12°C (16 days), and periodically transferred to aerobic storage (7°C for 5 days). During storage, samples were exposed to sequential heat (55°C; 20 min) and simulated gastric fluid (adjusted to pH 1.0 with HCl; 90 min) stresses simulating consumption of undercooked beef. Under the conditions of this study, cells originating from inocula of planktonic cells were, in general, more resistant to heat and acid than cells from cultures grown as biofilms and detached prior to meat inoculation. Heat and acid tolerance of cells on meat stored at 4°C was lower than that of cells on nondecontaminated meat stored at 12°C, where growth occurred during storage. Decontamination of fresh beef resulted in injury that inhibited subsequent growth of surviving cells at 12°C, as well as in decreases in resistance to subsequent heat and acid stresses. The shift of pathogen cells on beef stored under vacuum at 4°C to aerobic storage did not affect cell populations or subsequent survival after sequential exposure to heat and simulated gastric fluid. However, the transfer of meat stored under vacuum at 12°C to aerobic storage resulted in reduction in pathogen counts during aerobic storage and sensitization of survivors to the effects of sequential heat and acid exposure.

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* Corresponding author. Mailing address: Department of Animal Sciences, Colorado State University, 1171 Campus Delivery, Fort Collins, CO 80523-1171. Phone: (970) 491-7703. Fax: (970) 491-0278. E-mail: John.Sofos{at}colostate.edu

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Applied and Environmental Microbiology, January 2006, p. 672-679, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.672-679.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.