Non-thermal processing refers to emerging food preservation technologies with the application of mechanisms other than traditional heating to inactivate food-borne microorganisms. These include high pressure for Ultra-High Pressure (UHP) processing or electric pulses for Pulsed Electric Field (PEF) processing. Application of UHP or PEF processing is aiming to produce high quality and safe food without compromising its nutritional, functional and sensory attributes. Progress has been made in identifying the major targets in microbial cells treated with UHP or PEF. However, there are some challenges associated with the application of these techniques. Great variability among Escherichia coli strains in their resistance to UHP is yet to be understood. Presence of injured cells post UHP treatment may pose a hazard to food safety and quality.
The objectives of this study are (i) to investigate the relationship between intracellular free iron and UHP- or PEF-induced E. coli inactivation; (ii) to determine the transcriptional changes in E. coli O157:H7 subjected to sublethal UHP treatments and maintained under different oxygen availability conditions.
UHP treatments significantly increased the amounts of intracellular free iron in aerobically growing E. coli. Iron-overload conditions produced cell populations consistently more sensitive to the UHP treatment, regardless of the concentration of added iron. On the contrary, iron-deprivation significantly increased barotolerance of E.coli. It was proposed that high pressure-induced intracellular iron accumulation contributes to cell lethality by generating hydroxyl radicals through the Fenton reaction. The presence of oxygen might have acted as a hurdle enhancing the bactericidal effects via oxidatively leaching iron from iron-containing proteins. However, for E. coli grown anaerobically, free iron amounts were dramatically decreased after the application of UHP treatments. It was suggested that UHP-induced microbial inactivation was achieved through different mechanisms depending on the availability of oxygen.
Microarray analysis indicated that, in aerobically grown E. coli, genes related to stress response, cell membrane, ribosome units were significantly induced immediately after UHP treatments, however, carbohydrate metabolism related genes were significantly suppressed (p<0.01). Expression of these genes could be ascribed to UHP-induced damages on cellular membrane, proteins, ribosome units and DNAs, which had been demonstrated in previous study. For anaerobic growing E. coli, high pressure significantly up-regulated the expression of OxyR regulated genes and down-regulated Fnr regulated genes, and thus oxidative stress was proposed. Microarray analysis revealed a series of genes which are possibly regulated by UHP treatments. Reverse transcription real time PCR is recommended to confirm transcriptional changes and mutant analysis is suggested for mechanism clarification.