Exceptional Concentrations of Gold Nanoparticles in 1,7 Ga Fluid Inclusions From the Kola Superdeep Borehole, Northwest Russia

Gallic acid exerts antibiofilm activity by inhibiting methicillin-resistant Staphylococcus aureus adhesion

Methicillin-resistant Staphylococcus aureus (MRSA) is a serious threat to patients with nosocomial infections, and infection is strongly associated with biofilm formation. Gallic acid (GA) is a natural bioactive compound found in traditional Chinese medicines that exerts potent antimicrobial activity. However, the anti-MRSA biofilm efficacy of GA remained to be determined. This study investigated the antimicrobial activities of GA against MRSA and the mechanisms involved. The results revealed the significant antibacterial and antibiofilm activities of GA. The minimal inhibitory concentration of GA against MRSA was 32 μg/mL and a growth curve assay confirmed the significant inhibitory effect of GA on planktonic MRSA. Crystal violet and XTT assays showed that 8 µg/mL GA effectively inhibited the formation of new biofilms and disrupted existing biofilms by reducing both biofilm biomass and metabolic activities. Alkaline phosphatase and β-galactosidase leakage assays and live/dead staining provided evidence that GA disrupted the integrity of bacterial cell walls and membranes within the biofilm. Scanning electron microscopy observations showed that GA significantly inhibited bacterial adhesion and aggregation, affecting the overall structure of the biofilm. Bacterial adhesion, polysaccharide intercellular adhesion (PIA) production and real-time quantitative PCR assay confirmed that GA inhibited bacterial adhesion, PIA synthesis, and the expression of icaAD and sarA. These results suggested that GA inhibited biofilm formation by inhibiting the expression of sarA, then downregulating the expression of icaA and icaD, thereby reducing the synthesis of PIA to attenuate the adhesion capacity of MRSA. GA is therefore a promising candidate for development as a pharmaceutical agent for the prevention and treatment of bacterial infections caused by MRSA.

A track record of Au-Ag nanomelt generation during fluid-mineral interactions

Recent studies have reported the significant role of Au-bearing nanoparticles in the formation of hydrothermal gold deposits. Despite the ever-increasing understanding of the genesis and stability of Au-bearing nanoparticles, it is still unknown how they behave when exposed to hydrothermal fluids. Here, we study the nanostructural evolution of Au-Ag nanoparticles hosted within Co-rich diarsenides and sulfarsenides of a natural hydrothermal deposit. We use high-resolution transmission electron microscopy to provide a singular glimpse of the complete melting sequence of Au-Ag nanoparticles exposed to the hydrothermal fluid during coupled dissolution-precipitation reactions of their host minerals. The interaction of Au-Ag nanoparticles with hydrothermal fluids at temperatures (400-500 ºC) common to most hydrothermal gold deposits may promote melting and generation of Au-Ag nanomelts. This process has important implications in noble metal remobilization and accumulation during the formation of these deposits.

Colloidal transport and flocculation are the cause of the hyperenrichment of gold in nature

Hydrothermal veins supply much of the Earth’s gold. Their propensity to contain “bonanza” occurrences of gold that have concentrations millions of times greater than the concentration of gold in Earth’s crust makes them important targets for exploration and resource development. The mechanisms by which such hyperenrichment occurs are enigmatic. The accepted wisdom is that this enrichment reflects the saturation and precipitation of gold from hydrothermal fluids. Laboratory experiments and measurements of active hydrothermal systems, however, have shown that the solubility of this noble metal is exceptionally low. Here, we demonstrate that this issue is resolved by the physical transport of gold in the solid state as nanoparticles and their flocculated aggregates, thereby explaining the paradox of bonanza gold ore formation.

Aqueous complexation has long been considered the only viable means of transporting gold to depositional sites in hydrothermal ore-forming systems. A major weakness of this hypothesis is that it cannot readily explain the formation of ultrahigh-grade gold veins. This is a consequence of the relatively low gold concentrations typical of ore fluids (tens of parts per billion [ppb]) and the fact that these “bonanza” veins can contain weight-percent levels of gold in some epithermal and orogenic deposits. Here, we present direct evidence for a hypothesis that could explain these veins, namely, the transport of the gold as colloidal particles and their flocculation in nanoscale calcite veinlets. These gold-bearing nanoveinlets bear a remarkable resemblance to centimeter-scale ore veins in many hydrothermal gold deposits and give unique insight into the scale invariability of colloidal flocculation in forming hyperenriched gold deposits. Using this evidence, we propose a model for the development of bonanza gold veins in high-grade deposits. We argue that gold transport in these systems is largely mechanical and is the result of exceptionally high degrees of supersaturation that preclude precipitation of gold crystals and instead lead to the formation of colloidal particles, which flocculate and form much larger masses. These flocculated masses aggregate locally, where they are seismically pumped into fractures to locally form veins composed largely of gold. This model explains how bonanza veins may form from fluids containing ppb concentrations of gold and does not require prior encapsulation of colloidal gold particles in silica gel, as proposed by previous studies.