Guest Editorial

Some metals are toxic and have no beneficial purposes, as far as we know. Others are essential nutrients, but even these are toxic in excess. Bacteria have evolved systems to handle both the toxicity of metals and to ensure that sufficient of each of the essential metals is received. In some cases, such as Hg(II), different bacteria handle the metal in essentially the same way; in others, such as Cu(II), the detailed mechanisms for regulating metal concentrations are different between bacteria. Intracellular metal availability must be carefully regulated to prevent metals from populating the ‘wrong’ binding sites or engaging in uncontrolled redox reactions. Bacteria generally have no discrete subcellular compartments (cyanobacteria being a notable exception), and metal concentration is altered by proteins which sequester or transport the metal. In Gram-negative bacteria surplus metal is commonly exported into the periplasm where, at least for some metals and in some strains, binding proteins retain the excess possibly for subsequent import during deficiency. The amount of each protein produced is regulated genetically, as is the case with nearly all bacterial systems. Two of the reviews in this issue describe families of metal-responsive regulatory proteins. Other regulatory systems are described in the reviews dealing with specific metal systems. Metal-dependent gene regulation, metal sequestration and metal transport must all be highly specific for the relevant metal, otherwise inducible efflux systems, for example, may act to eliminate the wrong metal and cause cell death. In addition to their intrinsic interest and as models for understanding metal speciation in higher organisms, bacteria–metal interactions are finding application in biotechnology. Reduction alters the valence state, and may alter the toxicity and environmental mobility of a metal. This is finding increasing use in bioremediation. Ralstonia metallidurans CH34, for example, has a large number of metal resistance determinants and is being studied as a potentially important bacterium for bioremediation of metal-contaminated wastes. The study of bacteria–metal interactions covers a variety of interests, from the detailed understanding of the coordination chemistry of protein–metal interactions, through gene regulation and microbial physiology, to large-scale environmental bioremediation. We cannot hope to cover all the possible topics in a single issue of a general reviews journal. However, we have gathered a selection of experts to provide up-to-date exemplary accounts of the field. It is now over a decade since a collection of reviews on bacteria–metal interactions appeared as a single issue of a journal [1]. With the rapid generation of bacterial genome sequences and increasing knowledge of the mechanisms by which bacteria interact with metals, the time is opportune for another such collection. We are pleased to be associated with it.