Variation in resistance to high hydrostatic pressure of Escherichia coli

Abstract

Verotoxigenic Escherichia coli (VTEC) pose a risk for foodborne illness that can lead to death. Thermal food preservation to inactivate VTEC also alters food quality. High hydrostatic pressure (HHP) processing has been adopted by the food industry as an alternative to thermal preservation. However, E. coli exhibits great variability in resistance to pressure. This research evaluated the pressure resistance of VTEC and non-VTEC E. coli in laboratory media and ground beef. A strain cocktail of non-pathogenic E. coli was developed to match the pressure resistance of VTEC. The strain cocktail was validated by pressure treatments in ground beef and vegetable products. Pressure resistance of E. coli is affected by the food matrix. Therefore, the effect of the food matrix and food constituents on pressure resistance of E. coli was determined. Ground beef showed a baro-protective effect on E. coli when compared to treatments in bruschetta and tzatziki at the same pH (5.5). Divalent cations (Ca2+) exerted a baroprotective effect and may partially explain the relative resistance of E. coli in ground beef. To further elucidate mechanisms of pressure resistance, four extremely pressure resistant strains of E. coli, including one VTEC, were generated and cross-resistance to other stressors was evaluated. All four strains of E. coli evolved as extremely pressure resistant strains when treated with consecutive cycles of increasing pressure. Derivative E. coli AW1.7 became sensitive to low pH (2.5) and derivative E. coli AW1.3 exhibited increased resistance to heat and osmotic stress. Comparative analysis of the genome sequence of 3 wild type strains to the respective pressure-resistant revealed that derivative strains exhibited deletion of genetic elements. However, deletions were different without apparent consistency among the strains. In conclusion, the species E. coli contains extremely pressure resistant strains that are not readily inactivated by pressure treatment of food. Knowledge on the strain/matrix interaction during and after pressure treatment will facilitate the design, adoption, and/or combination of different intervention methods to warrant food safety.