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The ability of microorganisms to withstand large environmental perturbations enables survival in a wide range of habitats including foodstuffs. The importance of elucidation of such survival strategies has been stressed. In order that microorganisms survive in such harsh environments control mechanisms must exist which enable the cell to grow under these conditions. Survival under such extremes would indicate adaptation. The mechanisms involved in such adaptation ultimately come from within the bacterial genome and are thought to be due to alterations in gene expression. The effect of altered external and internal pH was observed upon the recovery and habituation of wild type cells over a period of time and indicated that cells possess the ability to habituate. Using lacZ fusion strains and DNA supercoiling measurements enabled the effect of stress on <i>ompF, ompC</i> and <i>proU</i> gene expression to be assessed. It was demonstrated that <i>ompC</i> is expressed in response to both acidification of the external medium and the cytoplasm. The response of the cell to external and internal acidification is both quantitatively and qualitatively different. Only the acidification of the cytoplasm results in transient differential gene expression typical of gene induction. This process is carbon source dependent. In parallel studies it has been demonstrated that the expression of the supercoiling-dependent <i>proU</i> locus is repressed by acid (both cytoplasmic and environmental). Consistent with this observation is that reporter plasmids are more supercoiled when isolated from cells incubated under acid conditions and would lead to a reduction in gene expression. From these studies it can be concluded that regulation of <i>ompC</i> does not lie at the level of DNA supercoiling but is dependent on the effects exerted on the <i>EnvZ/OmpR</i> regulatory system and also suggests the role of a secondary internal sensing mechanism.

Chapter 1 discusses the history, applications and effects (both beneficial and hazardous) of food irradiation and the techniques developed to detect irradiated foods. The basic radiation chemistry of major food components is reviewed. The preparation of 3,5-dimethyl-4-nitrosopyridine-1-oxide (DMNPO) and its dichloro- and dibromo- derivatives is described. Spin trapping of a variety of free radicals by DMNPO showed it to be an efficient trap for carbon-centred radicals but it did not form stable adducts with oxygen-centred radicals at room temperature as had been previously reported. The esr spectra were complicated by couplings arising from the hydrogens and nitrogen of the trap. The dichloro- and dibromo-analogues gave simpler spectra but were much more difficult to isolate. The effects of ionising radiation on spices and the methods used to detect these effects are reviewed. The measurement of free radical concentrations by esr spectroscopy was examined as a dosimetric method of identifying γ-irradiated spices. This proved to be impracticable because of the relatively rapid decay of the radicals and the variation in base-line radical concentrations. The short lived free radicals produced by irradiation were found to arise from the oleoresins. Identification of these radicals was attempted by irradiating the oleoresins and their major components, followed by spin trapping. In some cases, hydrogen abstraction gave the same radicals as those produced by irradiation. Separation of the spin adducts, formed from oleoresin components, by reverse phase HPLC is also described. Irradiation of food is known to cause destruction of vitamins. However, the processes involved are not fully understood. The solid state radiation chemistry of vitamins was investigated by spin trapping and esr spectroscopy. Irradiation of bones produces stable free radicals with characteristic esr spectra. It was shown that measurement of free radical concentrations in irradiated bones by esr spectroscopy could provide a dosimetric technique as the radical concentrations were stable and proportional to the applied dose.

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