Regulation of cytoplasmic pH in bacteria.

In the last 30 years, bacteria have provided experimentalists with an easily manipulable system in which to study regulatory phenomena. This has led to a relatively clear understanding of the regulatory mechanisms which operate to control gene expression (35). However, with a few exceptions (e.g., chemotaxis, the control of enzyme activity) regulatory phenomena which primarily involve control of extant biochemical systems have fared less well. In particular the regulation of transport systems involved in cellular homeostasis is still poorly understood despite the early recognition of its importance (28). In recent years cytoplasmic pH has come to be recognized as an important aspect of bacterial cell physiology over which the cell exerts relatively tight regulation. In essence, regulation of cytoplasmic pH implies control over the permeability of the cell membrane to protons. It is generally accepted that this is achieved by control of the activity of ion transport systems which facilitate proton entry. The aim of this review of pH homeostasis in bacteria is to assess the role of transport systems in the regulation of cytoplasmic pH and to consider the principal questions which remain to be answered. As a result of work on a wide range of organisms, a number of generalizations can be made about bacterial pH homeostasis. First, it is clear that there is no magic value of internal pH (pHj) which all organisms aim to achieve. Thus it is recognized that acidophiles exhibit pH, values in the range of pH1 6.5 to 7.0, neutrophiles have pH, values of 7.5 to 8.0, and alkalophiles have pH, values of 8.4 to 9.0 (3, 37, 40, 41, 43, 49, 59, 80, 82, 108; D. McLaggan, Ph.D. thesis, University of East Anglia, Norwich, England, 1984). This variance in pH, values reflects a remarkable capacity to evolve enzyme systems which operate in these ranges. Second, it is clear that organisms exhibit different capacities to regulate their cytoplasmic pH. In many organisms, the pH1 varies by only 0.1 units per pH unit change in external pH, whereas in others much larger changes in pH, have been observed (3, 41). One caveat to this generalization is that only rarely has the cytoplasmic pH been measured under growth conditions where cells might be expected to show optimum regulatory capacity. Third, fermentative organisms