Thermal Inactivation of Shiga Toxin-Producing <i>Escherichia coli</i> in Foods

Emerging non-O157 Shiga toxin-producing Escherichia coli (STEC) were recently added to the zero tolerance policy by the USDA-FSIS. Therefore, the precise characterization of their thermal inactivation kinetics in different foods and the effect of stress on thermal inactivation are needed. This research aimed at determining the heat inactivation kinetics of non-O157 and O157 STECs in buffer and model food matrices and the effects of DnaK levels on thermal resistance after acid and heat-shock. Thermal inactivation was carried out in either in 2-ml glass vials or nylon vacuum-sealed bags for buffer and food (spinach, ground-beef, turkey deli-meat, pasta) samples, respectively. Vials or bags were immersed in a re-circulating water bath at various set temperatures for fixed time-intervals. Surviving bacteria were enumerated using Tryptic Soy Agar plates. D-values were calculated using first-order linear and Weibull (for pasta only) models. Total bacterial protein (using Bicinchonic acid assay) and the heat-shock protein (DnaK) concentration were measured (using competitive ELISA) before and after treating overnight-grown cells in Tryptic Soy Broth with either acid- (acetic acid pH 5.5 for 1 h) or heat-shock (46°C for 15 min). All experiments were performed in duplicate and replicated thrice, data were analyzed using SAS (pE. coli O157 and non-O157 in all tested samples, at 56°C D-values ranged from 5.57±0.38 to 15.39±0.14 min; at 58°C D -values ranged from 1.99±0.9 to 7.20±0.55 min and at 60°C D–values ranged from 0.99±0.07 to 2.86±0.22 min. Higher levels of DnaK were detected after sub-lethal injury with heat- or acid-shock that corresponded to enhanced thermal tolerance of all strains, except E. coli O111. D-values in buffer for un-shocked cells ranged from 1.49±0.35 to 2.21±0.17 min, heat-shocked cells from 2.04±0.35 to 2.83±0.35 min and acid-shocked cells from 2.32±0.29 to 4.09±0.29 min. Thus, acid- or heat-shock conditions that might occur during food processing need to be considered during design of thermal inactivation processes and product-formulation to prevent food-borne outbreaks. This study provides insights on the thermal inactivation parameters of O157 and non-O157 STEC in foods that would be beneficial to the food industry.