3Bacteria, yeasts, and viruses are rapidly killed on metallic copper surfaces, and the term "contact killing" has been coined for this process. While the phenomenon was already known in ancient times, it is currently receiving renewed attention. This is due to the potential use of copper as an antibacterial material in health care settings. Contact killing was observed to take place at a rate of at least 7 to 8 logs per hour, and no live microorganisms were generally recovered from copper surfaces after prolonged incubation. The antimicrobial activity of copper and copper alloys is now well established, and copper has recently been registered at the U.S. Environmental Protection Agency as the first solid antimicrobial material. In several clinical studies, copper has been evaluated for use on touch surfaces, such as door handles, bathroom fixtures, or bed rails, in attempts to curb nosocomial infections. In connection to these new applications of copper, it is important to understand the mechanism of contact killing since it may bear on central issues, such as the possibility of the emergence and spread of resistant organisms, cleaning procedures, and questions of material and object engineering. Recent work has shed light on mechanistic aspects of contact killing. These findings will be reviewed here and juxtaposed with the toxicity mechanisms of ionic copper. The merit of copper as a hygienic material in hospitals and related settings will also be discussed.

Escherichia coli is equipped with multiple systems to ensure safe copper handling under varying environmental conditions. The Cu(I)-translocating P-type ATPase CopA, the central component in copper homeostasis, is responsible for removing excess Cu(I) from the cytoplasm. The multi-copper oxidase CueO and the multi-component copper transport system CusCFBA appear to safeguard the periplasmic space from copper-induced toxicity. Some strains of E. coli can survive in copper-rich environments that would normally overwhelm the chromosomally encoded copper homeostatic systems. Such strains possess additional plasmid-encoded genes that confer copper resistance. The pco determinant encodes genes that detoxify copper in the periplasm, although the mechanism is still unknown. Genes involved in copper homeostasis are regulated by MerR-like activators responsive to cytoplasmic Cu(I) or two-component systems sensing periplasmic Cu(I). Pathways of copper uptake and intracellular copper handling are still not identified in E. coli.

In contrast to other well-studied CBA-type heavy metal efflux systems, Cus was shown to be a tetrapartite resistance system that involves the novel periplasmic copper-binding protein CusF. These data provide additional evidence for the hypothesis that Cu(I) is directly transported from the periplasm across the outer membrane by the Cus complex.

In this article, a mechanism of arsenite [As(III)] resistance through methylation and subsequent volatization is described. Heterologous expression of arsM from Rhodopseudomonas palustris was shown to confer As(III) resistance to an arsenic-sensitive strain of Escherichia coli. ArsM catalyzes the formation of a number of methylated intermediates from As(III), with trimethylarsine as the end product. The net result is loss of arsenic, from both the medium and the cells. Because ArsM homologues are widespread in nature, this microbial-mediated transformation is proposed to have an important impact on the global arsenic cycle.As(III) ͉ ArsM ͉ methylation A s genomes are sequenced, it is becoming clear that nearly all bacteria and archaea have arsenic-resistance (ars) operons that confer resistance to arsenite [As(III)] and arsenate [As(V)] (1). The widespread occurrence of ars genes reflects the fact that arsenic is a ubiquitous environmental toxic metal. In most cases, these operons encode transport proteins that extrude As(III) from cells. In eukaryotes, As(III) detoxification involves glutathionylation coupled to removal of the As(GS) 3 complex from the cytosol by ABC transporters, such as the Saccharomyces cerevisiae Ycf1p vacuolar pump (2) or mammalian biliary extrusion pump MRP2 (3). In many mammals, including humans, an alternate metabolic fate of As(III) is methylation in the liver, followed by urinary excretion of the methylated species (4). In the past, this process was considered a detoxification mechanism (5), but more recent data suggest that the methylation actually increases toxicity by producing the more toxic monomethylarsenite [MMA(III)] and dimethylarsenite [DMA(III)], calling into question whether the process is, in fact, a detoxification process (6). An enzyme (termed Cyt19 or As3MT) that catalyzes As(III)-S-adenosylmethyltransferase activity has been identified recently in rats and humans (7-9). The enzyme has been characterized in vitro, but its physiological role is unknown.Bacteria and fungi are known to produce volatile and toxic arsines (10) but the physiological roles of arsenic methylation in microorganisms are likewise unclear, and the biochemical basis is unknown. While examining microbial genomes, we identified large number of genes for bacterial and archaeal homologues of Cyt19. We have termed a subset of these genes arsM and their protein product ArsM (As(III) S-adenosylmethyltransferase). What sets these arsM genes apart from genes for other homologues is that they are each downstream of an arsR gene, encoding the archetypal arsenic-responsive transcriptional repressor that controls expression of ars operons (11), suggesting that these ArsMs evolved to confer arsenic resistance.The gene for the 283-residue ArsM (29,656 Da) (accession no. NP948900.1) was cloned from Rhodopseudomonas palustris and expressed in an arsenic-hypersensitive strain of Escherichia coli. As(III)-resistance cells in E. coli expressing recombinant arsM correlated with conversion of medium arsenic to the methy...

Metal ions are essential for many reactions, however, metal excess can be toxic. In bacteria, metal limitation activates pathways for import and mobilization of metals, whereas metal excess induces efflux and storage. In this Review, we highlight recent insights into metal homeostasis, including protein- and RNA-based sensors that interact directly with metals or metal-containing cofactors. The resulting transcriptional response to metal stress is deployed in a stepwise manner, and is reinforced by post-transcriptional regulatory systems. Metal limitation and intoxication by the host is an evolutionarily ancient strategy to limit bacterial growth. The details of the resulting growth restriction are beginning to be understood, and appear to be organism-specific.