Impact of inoculum preparation and storage conditions on the response of Escherichia coli O157:H7 populations to undercooking and simulated exposure to gastric fluid

Abstract

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 degrees C) water followed by lactic acid (2%, 55 degrees C), vacuum packaged, stored at 4 (28 days) or 12 degrees C (16 days), and periodically transferred to aerobic storage (7 degrees C for 5 days). During storage, samples were exposed to sequential heat (55 degrees 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 degrees C was lower than that of cells on nondecontaminated meat stored at 12 degrees C, where growth occurred during storage. Decontamination of fresh beef resulted in injury that inhibited subsequent growth of surviving cells at 12 degrees 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 degrees 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 degrees 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.

FIG. 1.

Diagrammatic layout detailing (A) inoculation of untreated meat with a four-strain composite of Escherichia coli O157:H7 prepared under different environmental conditions, TSB, WASH, WETB, or DRYB, left untreated, or decontaminated by sequential dipping (30 s each) in hot water (75°C) and lactic acid (2%, 55°C), followed by vacuum packaging and storage, and (B) exposure to heat (55°C; 20 min) followed by simulated gastric fluid (pH 1.0; 90 min) of samples of the above treatments after vacuum or aerobic storage.