Simon Silver

Silver as biocides in burn and wound dressings and bacterial resistance to silver compounds

Silver products have been used for thousands of years for their beneficial effects, often for hygiene and in more recent years as antimicrobials on wounds from burns, trauma, and diabetic ulcers. Silver sulfadiazine creams (Silvazine and Flamazine) are topical ointments that are marketed globally. In recent years, a range of wound dressings with slow-release Ag compounds have been introduced, including Acticoat, Actisorb Silver, Silverlon, and others. While these are generally accepted as useful for control of bacterial infections (and also against fungi and viruses), key issues remain, including importantly the relative efficacy of different silver products for wound and burn uses and the existence of microbes that are resistant to Ag+. These are beneficial products needing further study, although each has drawbacks. The genes (and proteins) involved in bacterial resistance to Ag have been defined and studied in recent years.

BACTERIAL RESISTANCES TO TOXIC METAL IONS : A REVIEW

Bacterial plasmids encode resistance systems for toxic metal ions, including Ag+, AsO2-, AsO4(3-), Cd2+, Co2+, CrO4(2-), Cu2+, Hg2+, Ni2+, Pb2+, Sb3+, TeO3(2-), Tl+ and Zn2+. The function of most resistance systems is based on the energy-dependent efflux of toxic ions. Some of the efflux systems are ATPases and others are chemiosmotic cation/proton antiporters. The Cd(2+)-resistance ATPase of Gram-positive bacteria (CadA) is membrane cation pump homologous with other bacterial, animal and plant P-type ATPases. CadA has been labeled with 32P from [alpha-32P] ATP and drives ATP-dependent Cd2+ (and Zn2+) uptake by inside-out membrane vesicles (equivalent to efflux from whole cells). Recently, isolated genes defective in the human hereditary diseases of copper metabolism, namely Menkes syndrome and Wilsons disease, encode P-type ATPases that are more similar to bacterial CadA than to other ATPases from eukaryotes. The arsenic resistance efflux system transports arsenite [As(III)], alternatively using either a double-polypeptide (ArsA and ArsB) ATPase or a single-polypeptide (ArsB) functioning as a chemiosmotic transporter. The third gene in the arsenic resistance system, arsC, encodes an enzyme that converts intracellular arsenate [As(V)] to arsenite [As(III)], the substrate of the efflux system. The triple-polypeptide Czc (Cd2+, Zn2+ and Co2+) chemiosmotic efflux pump consists of inner membrane (CzcA), outer membrane (CzcC) and membrane-spanning (CzcB) proteins that together transport cations from the cytoplasm across the periplasmic space to the outside of the cell.

Genes and Enzymes Involved in Bacterial Oxidation and Reduction of Inorganic Arsenic

The human use of toxic heavy metals is here to stay. In addition to intentional poisoning with arsenic ([40][1]) (arsenic levels in the hair of Napoleon Bonaparte approached 40 ppm, more than 1,000 times above allowable levels), medical, agricultural, and industrial uses of arsenic present major

A bacterial view of the periodic table: genes and proteins for toxic inorganic ions.

Essentially all bacteria have genes for toxic metal ion resistances and these include those for Ag+, AsO2−, AsO43−, Cd2+, Co2+, CrO42−, Cu2+, Hg2+, Ni2+, Pb2+, TeO32−, Tl+ and Zn2+. The largest group of resistance systems functions by energy-dependent efflux of toxic ions. Fewer involve enzymatic transformations (oxidation, reduction, methylation, and demethylation) or metal-binding proteins (for example, metallothionein SmtA, chaperone CopZ and periplasmic silver binding protein SilE). Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers. For example, Cd2+-efflux pumps of bacteria are either inner membrane P-type ATPases or three polypeptide RND chemiosmotic complexes consisting of an inner membrane pump, a periplasmic-bridging protein and an outer membrane channel. In addition to the best studied three-polypeptide chemiosmotic system, Czc (Cd2+, Zn2+, and Co2), others are known that efflux Ag+, Cu+, Ni2+, and Zn2+. Resistance to inorganic mercury, Hg2+ (and to organomercurials, such as CH3Hg+ and phenylmercury) involve a series of metal-binding and membrane transport proteins as well as the enzymes mercuric reductase and organomercurial lyase, which overall convert more toxic to less toxic forms. Arsenic resistance and metabolizing systems occur in three patterns, the widely-found ars operon that is present in most bacterial genomes and many plasmids, the more recently recognized arr genes for the periplasmic arsenate reductase that functions in anaerobic respiration as a terminal electron acceptor, and the aso genes for the periplasmic arsenite oxidase that functions as an initial electron donor in aerobic resistance to arsenite.

Molecular basis for resistance to silver cations in Salmonella

Here we report the genetic and proposed molecular basis for silver resistance in pathogenic microorganisms. The silver resistance determinant from a hospital burn ward Salmonella plasmid contains nine open reading frames, arranged in three measured and divergently transcribed RNAs. The resistance determinant encodes a periplasmic silver–specific binding protein (SilE) plus apparently two parallel efflux pumps: one, a P–type ATPase (SilP); the other, a membrane potential–dependent three–polypeptide cation/proton antiporter (SilCBA). The sil determinant is governed by a two–component membrane sensor and transcriptional responder comprising silS and silR, which are co–transcribed. The availability of the sil silver–resistance determinant will be the basis for mechanistic molecular and biochemical studies as well as molecular epidemiology of silver resistance in clinical settings in which silver is used as a biocide.

Mining with Microbes

Microbes are playing increasingly important roles in commercial mining operations, where they are being used in the “bioleaching” of copper, uranium, and gold ores. Direct leaching is when microbial metabolism changes the redox state of the metal being harvested, rendering it more soluble. Indirect leaching includes redox chemistry of other metal cations that are then coupled in chemical oxidation or reduction of the harvested metal ion and microbial attack upon and solubilization of the mineral matrix in which the metal is physically embedded. In addition, bacterial cells are used to detoxify the waste cyanide solution from gold-mining operations and as “absorbants” of the mineral cations. Bacterial cells may replace activated carbon or alternative biomass. With an increasing understanding of microbial physiology, biochemistry and molecular genetics, rational approaches to improving these microbial activities become possible

Metal-microbe interactions: contemporary approaches.

Publisher Summary This chapter discusses the methods to study interactions between metals and microorganisms, and demonstrate the necessity of adopting a multidisciplinary approach to tackle this increasingly important and diverse area of microbiology. The multidisciplinary approach surveyed in the chapter involves microbiology, particularly microbial physiology, genetics and molecular biology, bioinorganic chemistry, analytical chemistry, and the application of instrumental techniques. Commonly used methods for the determination of metal concentrations in cells (and in cell compartments or fractions)—that is, those of atomic spectroscopy are discussed. Radiochemical techniques are of particular value in studying the uptake and localization of metals and sometimes offer the only way of studying variations in concentrations of metals in cellular compartments. The use of spectroscopic methods provides information about the ligands, geometries, andthe electronic status of metals coordinated at binding sites, sometimes in living cells. The use of non-invasive techniques is of significant mechanistic potential as they may allow the direct study of ionic and metabolic processes in essentially unperturbed cells. Genetics and molecular biology hold great promise for advancing studies of metal-microbe interactions, particularly those aspects that are currently perceived as especially significant. These include gene regulation by metals (metalcontaining transcription factors and zinc fingers), control of specificity at metalbinding sites and genetic modification of microorganisms for application in biohydrometallurgy. Studies on extracellular precipitates produced by microbial activity or on the complexation of metal ions in supernatant solutions or growth media by release of extracellular ligands are also discussed.

Antimicrobial silver: Uses, toxicity and potential for resistance

This review gives a comprehensive overview of the widespread use and toxicity of silver compounds in many biological applications. Moreover, the bacterial silver resistance mechanisms and their spread in the environment are discussed. This study shows that it is important to understand in detail how silver and silver nanoparticles exert their toxicity and to understand how bacteria acquire silver resistance. Silver ions have shown to possess strong antimicrobial properties but cause no immediate and serious risk for human health, which led to an extensive use of silver-based products in many applications. However, the risk of silver nanoparticles is not yet clarified and their widespread use could increase silver release in the environment, which can have negative impacts on ecosystems. Moreover, it is shown that silver resistance determinants are widely spread among environmental and clinically relevant bacteria. These resistance determinants are often located on mobile genetic elements, facilitating their spread. Therefore, detailed knowledge of the silver toxicity and resistance mechanisms can improve its applications and lead to a better understanding of the impact on human health and ecosystems.

Molecular microbiology of heavy metals

Molecular Physiology of Metal-Microbe Interactions According to Mechanisms.- Understanding How Cells Allocate Metals.- Metalloregulators: Arbiters of Metal Sufficiency.- Transcriptomic Responses of Bacterial Cells to Sublethal Metal Ion Stress.- Bacterial Transition Metal Homeostasis.- Biosensing of Heavy Metals.- A Glossary of Microanalytical Tools to Assess the Metallome.- Molecular Physiology of Metal-Microbe Interactions According to Groups.- Acquisition of Iron by Bacteria.- New Transport Deals for Old Iron.- Manganese: Uptake, Biological Function, and Role in Virulence.- How Bacteria Handle Copper.- Microbial Physiology of Nickel and Cobalt.- Zinc, Cadmium, and Lead Resistance and Homeostasis.- Microbiology of the Toxic Noble Metal Silver.- Mercury Microbiology: Resistance Systems, Environmental Aspects, Methylation, and Human Health.- Arsenic Metabolism in Prokaryotic and Eukaryotic Microbes.- Reduction and Efflux of Chromate by Bacteria.- Molybdate and Tungstate: Uptake, Homeostasis, Cofactors, and Enzymes.

Genes for all metals--a bacterial view of the periodic table. The 1996 Thom Award Lecture.

Bacterial chromosomes have genes for transport proteins for inorganic nutrient cations and oxyanions, such as NH4+, K+, Mg2+, Co2+, Fe3+, Mn2+, Zn2+ and other trace cations, PO43-, SO42- and less abundant oxyanions. Together these account for perhaps a few hundred genes in many bacteria. Bacterial plasmids encode resistance systems for toxic metal and metalloid ions including Ag+ AsO2-, AsO43-, Cd2+, Co2+, CrO42−, Cu2+, Hg2+, Ni2+, Pb2+, TeO32−, TI+ and Zn2+. Most resistance systems function by energy-dependent efflux of toxic ions. A few involve enzymatic (mostly redox) transformations. Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers. The Cd2+-resistance cation pump of Gram-positive bacteria is membrane P-type ATPase, which has been labeled with 32P from [γ-32P]ATP and drives ATP-dependent Cd2+ (and Zn2+) transport by membrane vesicles. The genes defective in the human hereditary diseases of copper metabolism, Menkes syndrome and Wilson’s disease, encode P-type ATPases that are similar to bacterial cadmium ATPases. The arsenic resistance system transports arsenite [As(III)], alternatively with the ArsB polypeptide functioning as a chemiosmotic efflux transporter or with two polypeptides, ArsB and ArsA, functioning as an ATPase. The third protein of the arsenic resistance system is an enzyme that reduces intracellular arsenate [As(V)] to arsenite [As(III)], the substrate of the efflux system. In Gram-negative cells, a three polypeptide complex functions as a chemiosmotic cation/protein exchanger to efflux Cd2+, Zn2+ and Co2+. This pump consists of an inner membrane (CzcA), an outer membrane (CzcC) and a membrane-spanning (CzcB) protein that function together.