Heat and pressure resistance of Escherichia coli

Abstract

Strains of Escherichia coli may survive heat or pressure stress, acquire specific virulence genes and cause severe human diseases. The locus of heat resistance (LHR) has been identified as an important heat resistant element in E. coli. The objective of this thesis was to explore the role of the LHR on heat and pressure resistance of E. coli, as well as its relationship with protein folding and aggregation, and evaluate the effects of food matrix and antimicrobials on pressure resistance of E. coli. To explore the heat and pressure resistance related to protein folding and aggregation, the role of LHR was investigated in E. coli MG1655 expressing ibpA-yfp fusion. A total 10 proteins of LHR were detected through proteomic analysis using mass spectrometry, including two small heat shock proteins, two heat shock proteases, proteins of the YfdX family, thioredoxin, and a sodium/hydrogen antiporter. Microscopic observations showed that LHR reduced the inclusion bodies after heat or the pressure treatment, indicating that LHR proteins function to re-fold and turnover aggregated proteins. The proteomic analysis confirmed that the phenotype of pressure resistance of LHR occurred through stress regulation, mitigation of protein aggregation and reduction of oxidative stress. To evaluate the effect of food matrix on the pressure resistance of E. coli, the resistance of two five-strain cocktails to pressure in bruschetta, tzatziki, yogurt and ground beef was compared. Pressure reduced E. coli in plant and dairy products by more than 5 logs (cfu/mL) but not in ground beef. Food components calcium, magnesium, glutamate, caffeic acid and acetic acid exhibited a protective effect on E. coli after pressure treatment and during storage at 4 °C. Further study assessed the combined effect of antimicrobials and pressure on enterohaemorrhagic E. coli (EHEC) in pressure treated beef steaks. The thiol-reactive allyl-isothiocyanate (AITC) and cinnamaldehyde exhibited synergistic activity with pressure on E. coli. However, the membrane-active essential oil components carvacrol and thymol showed antagonistic or no synergistic effect with pressure. In conclusion, the LHR confers pressure resistance of E. coli, and the resistance is related to protein folding and aggregation. Moreover, pressure resistance of E. coli is affected by food matrix and antimicrobials.