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 ompF, ompC and proU gene expression to be assessed. It was demonstrated that ompC 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 proU 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 ompC does not lie at the level of DNA supercoiling but is dependent on the effects exerted on the EnvZ/OmpR regulatory system and also suggests the role of a secondary internal sensing mechanism.