Decoupling the role of stress and corrosion in the intergranular cracking of noble-metal alloys.

Intergranular stress-corrosion cracking (IGSCC) is a form of environmentally induced crack propagation causing premature failure of elemental metals and alloys. It is believed to require the simultaneous presence of tensile stress and corrosion; however, the exact nature of this synergy has eluded experimental identification. For noble metal alloys such as Ag-Au, IGSCC is a consequence of dealloying corrosion, forming a nanoporous gold layer that is believed to have the ability to transmit cracks into grain boundaries in un-dealloyed parent phase via a pure mechanical process. Here using atomic-scale techniques and statistical characterizations for this alloy system, we show that the separate roles of stress and anodic dissolution can be decoupled and that the apparent synergy exists owing to rapid time-dependent morphology changes at the dealloyed layer/parent phase interface. We discuss the applicability of our findings to the IGSCC of important engineering Fe- and Ni-based alloys in critical applications.

Apparent inverse Gibbs-Thomson effect in dealloyed nanoporous nanoparticles.

The Gibbs-Thomson effect (the reduction of local chemical potential due to nanoscale curvature) predicts that nanoparticles of radius r dissolve at lower electrochemical potentials than bulk materials, decreasing as 1/r. However, we show here that if the particle is an alloy--susceptible to selective dissolution (dealloying) and nanoporosity evolution--then complete selective electrochemical dissolution and porosity evolution require a higher electrochemical potential than the comparable bulk planar material, increasing empirically as 1/r. This is a kinetic effect, which we demonstrate via kinetic Monte Carlo simulation. Our model shows that in the initial stages of dissolution, the less noble particle component is easily stripped from the nanoparticle surface, but owing to an increased mobility of the more noble atoms, the surface of the particle quickly passivates. At a fixed electrochemical potential, porosity and complete dealloying can only evolve if fluctuations in the surface passivation layer are sufficiently long-lived to allow dissolution from percolating networks of the less-noble component that penetrate through the bulk of the particle.

Length scales in alloy dissolution and measurement of absolute interfacial free energy.

De-alloying is the selective dissolution of one or more of the elemental components of an alloy. In binary alloys that exhibit complete solid solubility, de-alloying of the less noble component results in the formation of nanoporous metals, a materials class that has attracted attention for applications such as catalysis, sensing and actuation. In addition, the occurrence of de-alloying in metallic alloy systems under stress is known to result in stress-corrosion cracking, a key failure mechanism in fossil fuel and nuclear plants, ageing aircraft, and also an important concern in the design of nuclear-waste storage containers. Central to the design of corrosion-resistant alloys is the identification of a composition-dependent electrochemical critical potential, Vcrit, above which the current rises dramatically with potential, signalling the onset of bulk de-alloying. Below Vcrit, the surface is passivated by the accumulation of up to several monolayers of the more noble component. The current understanding of the processes that control Vcrit is incomplete. Here, we report on de-alloying results of Ag/Au superlattices that clarify the role of pre-existing length scales in alloy dissolution. Our data motivated us to re-analyse existing data on critical potentials of Ag-Au alloys and develop a simple unifying picture that accounts for the compositional dependence of solid-solution alloy critical potentials.

Self-assembly of paired nanoribbons.

Self-assembly of surface phase domains is a promising route to fabricate stable nanometer-scale structures. This Letter reports a novel labyrinth structure of orthogonal nanoscale ribbons of Cu4Pb3 ordered-alloy on Cu(100) formed by electrochemical deposition. The labyrinth develops as loops of Cu4Pb3 ribbons elongate as closely spaced paired stripes. The structure is explained in terms of elastic interactions between anisotropic surface stress domains, wherein stripes of different phase variants form attractive dipoles. An energetic analysis determines the physical conditions necessary for the structure to form.

Anion adsorption induced reversal of coherency strain.

Experimental results are presented for stress evolution, in vacuum and electrolyte, for the first monolayer of Cu on Au(111). In electrolyte the monolayer is pseudomorphic and the stress-thickness change is -0.60 N/m, while conventional epitaxy theory predicts a value of +7.76 N/m. In vacuum, the monolayer is incoherent with the underlying gold. Using a combination of first-principles based calculations and molecular dynamic simulations we analyzed these results and demonstrate that in electrolyte, overlayer coherency is maintained owing to anion adsorption.

Electrocapillarity behavior of Au111 in SO4(2-) and F-.

We present the first set of results measuring the change in interfacial free energy and surface stress for Au(111) electrodes in an electrolyte containing a nonspecifically adsorbing anion and compare this behavior to that in an electrolyte containing an anion known to undergo specific adsorption. Generally, we find that the surface stress is more sensitive to changes in electrode potential and adsorption then the interfacial free energy. The results obtained in fluoride electrolytes are compared to the predictions of a thermodynamic analysis.

Alterations of cell wall structure and metabolism accompany reduced susceptibility to vancomycin in an isogenic series of clinical isolates of Staphylococcus aureus.

A series of isogenic methicillin-resistant Staphylococcus aureus isolates recovered from a bacteremic patient were shown to acquire gradually increasing levels of resistance to vancomycin during chemotherapy with the drug (K. Sieradzki, T. Leski, L. Borio, J. Dick, and A. Tomasz, J. Clin. Microbiol. 41:1687-1693, 2003). We compared properties of the earliest (parental) vancomycin-susceptible isolate, JH1 (MIC, 1 microg/ml), to two late (progeny) isolates, JH9 and JH14 (vancomycin MIC, 8 microg/ml). The resistant isolates produced abnormally thick cell walls and poorly separated cells when grown in antibiotic-free medium. Chemical analysis of the resistant isolates showed decreased cross-linkage of the peptidoglycan and drastically reduced levels of PBP4 as determined by the fluorographic assay. Resistant isolates showed reduced rates of cell wall turnover and autolysis. In vitro hydrolysis of resistant cell walls by autolytic extracts prepared from either susceptible or resistant strains was also slow, and this abnormality could be traced to a quantitative (or qualitative) change in the wall teichoic acid component of resistant isolates. Some change in the structure and/or metabolism of teichoic acids appears to be an important component of the mechanism of decreased susceptibility to vancomycin in S. aureus.

Evolution of a vancomycin-intermediate Staphylococcus aureus strain in vivo: multiple changes in the antibiotic resistance phenotypes of a single lineage of methicillin-resistant S. aureus under the impact of antibiotics administered for chemotherapy.

A number of methicillin-resistant Staphylococcus aureus (MRSA) isolates were recovered over a period of several weeks from blood samples and from the heart valve of a patient who underwent extensive vancomycin chemotherapy for persistent S. aureus bacteremia. Consecutive isolates showed gradually decreasing growth rates during in vitro cultivation and increasing vancomycin MICs, from an MIC of 1 micro g/ml for the initial isolate to an MIC of 8 micro g/ml for the final MRSA isolates, which also became tolerant to vancomycin. Major changes were observed in the oxacillin resistance phenotype of several of the isolates-apparently related to in vivo exposure to imipenem, which was also used during a period of chemotherapy. Both the gradually increasing vancomycin MICs and the changes in oxacillin resistance could be reproduced by appropriate exposure of the initial MRSA isolate to antibiotics in vitro. All isolates had the same pulsed-field gel electrophoresis pattern, spaA type, and multilocus sequence type (MLST), which was identified as a single-locus variant of ST5, the MLST characteristic of previously characterized MRSA isolates with reduced susceptibility to vancomycin in the United States and Japan.

Evolution of nanoporosity in dealloying.

Dealloying is a common corrosion process during which an alloy is 'parted' by the selective dissolution of the most electrochemically active of its elements. This process results in the formation of a nanoporous sponge composed almost entirely of the more noble alloy constituents. Although considerable attention has been devoted to the morphological aspects of the dealloying process, its underlying physical mechanism has remained unclear. Here we propose a continuum model that is fully consistent with experiments and theoretical simulations of alloy dissolution, and demonstrate that nanoporosity in metals is due to an intrinsic dynamical pattern formation process. That is, pores form because the more noble atoms are chemically driven to aggregate into two-dimensional clusters by a phase separation process (spinodal decomposition) at the solid-electrolyte interface, and the surface area continuously increases owing to etching. Together, these processes evolve porosity with a characteristic length scale predicted by our continuum model. We expect that chemically tailored nanoporous gold made by dealloying Ag-Au should be suitable for sensor applications, particularly in a biomaterials context.

Gradual alterations in cell wall structure and metabolism in vancomycin-resistant mutants of Staphylococcus aureus.

In five vancomycin-resistant laboratory step mutants selected from the highly and homogeneously methicillin-resistant Staphylococcus aureus strain COL (MIC of methicillin, 800 microg/ml; MIC of vancomycin, 1.5 microg/ml), the gradually increasing levels of resistance to vancomycin were accompanied by parallel decreases in the levels of methicillin resistance and abnormalities in cell wall metabolism. The latter included a gradual reduction in the proportion of highly cross-linked muropeptide species in peptidoglycan, down-regulation of the production of penicillin-binding protein 2A (PBP2A) and PBP4, and hypersensitivity to beta-lactam antibiotics each with a relatively selective affinity for the various staphylococcal PBPs; the PBP2-specific inhibitor ceftizoxime was particularly effective.

Inactivated pbp4 in highly glycopeptide-resistant laboratory mutants of Staphylococcus aureus.

Both vancomycin- and teicoplanin-resistant laboratory mutants of Staphylococcus aureus produce peptidoglycans of altered composition in which the proportion of highly cross-linked muropeptide species is drastically reduced with a parallel increase in the representation of muropeptide monomers and dimers (Sieradzki, K., and Tomasz, A. (1997) J. Bacteriol. 179, 2557-2566; and Sieradzki, K. , and Tomasz, A. (1998) Microb. Drug Resist. 4, 159-168). We now report that the distorted peptidoglycan composition is related to defects in penicillin-binding protein 4 (PBP4); no PBP4 was detectable by the fluorographic assay in membrane preparations from the mutants, and comparison of the sequence of pbp4 amplified from the mutants indicated disruption of the gene by two types of abnormalities, a 17-amino acid long duplication starting at position 305 of the pbp4 gene was detected in the vancomycin-resistant mutant, and a stop codon was found to be introduced into the pbp4 KTG motif at position 261 in the mutant selected for teicoplanin resistance. Additional common patterns of disturbances in the peptidoglycan metabolism of the mutants are indicated by the increased sensitivity of mutant cell walls to the M1 muramidase and decreased sensitivity to lysostaphin, which is a reversal of the susceptibility pattern of the parental cell walls. Furthermore, the results of high performance liquid chromatography analysis of lysostaphin digests of peptidoglycan suggest an increase in the average chain length of the glycan strands in the peptidoglycan of the glycopeptide-resistant mutants. The increased molar proportion of muropeptide monomers in the cell wall of the glycopeptide-resistant mutants should provide binding sites for the "capture" of vancomycin and teicoplanin molecules, which may be part of the mechanism of glycopeptide resistance in S. aureus.

Electrochemical defect-mediated thin-film growth

An electrodeposition technique is described that produces atomically flat epitaxial metal overlayers of quality similar to that obtained by ultrahigh vacuum techniques at elevated temperature. In this approach, a metal of interest such as silver is co-deposited with a reversibly deposited mediator metal. The mediator is periodically deposited and stripped from the surface, and this serves to significantly increase the density of two-dimensional islands of silver atoms, promoting a layer-by-layer thin-film growth mode. In situ scanning tunneling microscopy was used to demonstrate the growth process for the heteroepitaxial system silver/gold (111) with either lead or copper as the mediator.

Heterogeneously vancomycin-resistant Staphylococcus epidermidis strain causing recurrent peritonitis in a dialysis patient during vancomycin therapy.

Methicillin-resistant Staphylococcus epidermidis (MRSE) was recovered over a 2-month period from the dialysis fluid of a peritoneal dialysis (PD) patient who experienced recurrent episodes of peritonitis during therapeutic and prophylactic use of vancomycin. Characterization of five consecutive MRSE isolates by molecular and microbiological methods showed that they were representatives of a single strain, had reduced susceptibility to vancomycin, did not react with DNA probes specific for the enterococcal vanA or vanB gene, and showed characteristics reminiscent of the properties of a recently described vancomycin-resistant laboratory mutant of Staphylococcus aureus. Cultures of these MRSE isolates were heterogeneous: they contained-with a frequency of 10(-4) to 10(-5)-bacteria for which vancomycin MICs were high (25 to 50 microg/ml) which could easily be selected to "take over" the cultures by using vancomycin selection in the laboratory. In contrast, the five consecutive MRSE isolates recovered from the PD patient during virtually continuous vancomycin therapy showed no indication for a similar enrichment of more resistant subpopulations, suggesting the existence of an "occult" infection site in the patient (presumably at the catheter exit site) which was not accessible to the antibiotic.

Inactivation of the methicillin resistance gene mecA in vancomycin-resistant Staphylococcus aureus.

Acquisition of high-level resistance to vancomycin in the laboratory mutant VM50 (vancomycin MIC increased from 1.5 to 100 microg/ml) was accompanied by the appearance of a heterogeneous phenotype and a virtual loss in methicillin resistance: in most cells of cultures of VM50 the methicillin MIC of the parental strain was reduced from 800 to 1.5 microg/ml with only a subpopulation (10(-5)) retaining methicillin resistance at near the parental level (MIC of 400 microg/ml). Interestingly, the vancomycin MIC of this subpopulation was less (25 microg/ml) than that of VM50 (100 microg/ml). A similar antagonism between methicillin and vancomycin resistance levels was observed upon introduction of an intact mecA into VM50 on a plasmid vector: methicillin resistance of the majority of cells increased from 1.5 to 100 microg/ml while the vancomycin MIC declined from 100 to 12/25 microg/ml. Membrane preparations from mutant VM50 showed no detectable penicillin-binding protein (PBP) 2A by the fluorographic assay. Sequencing of the mecA gene resident in mutant VM50 indicated the presence of a 19-bp duplication between nucleotide residues 280-298, leading to the generation of a stop codon TAA starting at nucleotide position 286.

Suppression of glycopeptide resistance in a highly teicoplanin-resistant mutant of Staphylococcus aureus by transposon inactivation of genes involved in cell wall synthesis.

The teicoplanin-resistant laboratory mutant TNM of Staphylococcus aureus strain COL (minimal inhibitory concentration for teicoplanin increased from 3 to 200 microg/ml) produced an abnormal peptidoglycan in which the proportion of cross-linked oligomeric muropeptides (pentameric and higher than pentameric species), representing approximately 60% of all muropeptide species in the parental strain, was reduced to approximately 17% in the mutant. In parallel, there was an increase in the representation of the monomeric muropeptides from 4% (in the parent) to 20% in the resistant strain. The mutant cell wall showed greatly increased porosity for the detergent extraction of cytoplasmic proteins, and this property was abolished in a Tn551 insertional derivative of TNM, which was selected for reduced (parental level) teicoplanin resistance. Transposon inactivation of the global regulatory genes Sigma-B and sar, and several genes involved in early steps of staphylococcal peptidoglycan synthesis, all caused extensive reduction of teicoplanin resistance in mutant TNM, in some cases to levels close to or below the MIC value of the parental strain.

Decreased susceptibilities to teicoplanin and vancomycin among coagulase-negative methicillin-resistant clinical isolates of staphylococci.

Of 41 methicillin-resistant coagulase-negative staphylococcal clinical isolates collected during a 5-month period between late 1995 and early 1996, 28 showed tube dilution teicoplanin MICs of 4 to 8 microg/ml which increased to 16 to 32 microg/ml upon prolonged incubation. Cultures of such bacteria were heterogeneous; they contained subpopulations with frequencies of 10(-5) to 10(-4) that could grow on up to 50 microg of teicoplanin per ml. The same cultures were also heterogeneous with respect to susceptibility to vancomycin; while the MICs for the majority of cells were 2 to 4 microg/ml, subpopulations that could grow on 6 to 12 microg of vancomycin per ml were also present at frequencies of 10(-5) to 10(-7). Selective enrichment of such cultures for the resistant subpopulation occurred with relative ease under laboratory conditions. Heterogeneous phenotypes for teicoplanin (but not for vancomycin) susceptibility were also identified in several Staphylococcus epidermidis isolates collected during the preantibiotic era. The addition of half the MIC of teicoplanin inhibited autolysis and caused formation of cellular aggregates which disintegrated to individual bacteria in the stationary phase when the titer of teicoplanin in the medium fell to undetectable levels, indicating removal of the antibiotic from the culture medium by the bacteria.

Suppression of beta-lactam antibiotic resistance in a methicillin-resistant Staphylococcus aureus through synergic action of early cell wall inhibitors and some other antibiotics.

We tested the effect of a number of mechanistically distinct antibacterial agents on the expression of methicillin resistance in a highly and homogeneously resistant strain of methicillin-resistant Staphylococcus aureus. The antibiotics, used at 0.25 x MIC, included inhibitors of early steps in peptidoglycan synthesis (fosfomycin, beta-chloro-D-alanine, D-cycloserine); bacitracin; teicoplanin and vancomycin; beta-lactam inhibitors chosen on the basis of their relatively selective affinities for penicillin-binding proteins 1, 2, 3 and 4 of S. aureus (imipenem, cefotaxime, cephradine and cefoxitin); compounds that inhibit various steps in protein synthesis (tetracycline, chloramphenicol, gentamicin, erythromycin and quinupristin/dalfopristin) and an inhibitor of DNA gyrase (temafloxacin). All inhibitors of early cell wall synthesis caused reduction of methicillin resistance and change from the homogeneous to the heterogeneous methicillin-resistant phenotype. Similar effects were obtained with only cephradine out of the four beta-lactams tested, and with erythromycin and quinupristin/dalfopristin as well. The other inhibitors of protein synthesis and DNA gyrase had no effect.