Unlike conventional antimicrobials, the study of bacterial resistance to silver nanoparticles (AgNPs) remains in its infancy and the mechanism(s) through which it evolves are limited and inconclusive. The central question remains whether bacterial resistance is driven by the AgNPs, released Ag(I) ions or a combination of these and other factors. Here, we show a specific resistance in an Escherichia coli K-12 MG1655 strain to subinhibitory concentrations of AgNPs, and not Ag(I) ions, as indicated by a statistically significant greater-than-twofold increase in the minimum inhibitory concentration occurring after eight repeated passages that was maintained after the AgNPs were removed and reintroduced. Whole-population genome sequencing identified a cusS mutation associated with the heritable resistance that possibly increased silver ion efflux. Finally, we rule out the effect of particle aggregation on resistance and suggest that the mechanism of resistance may be enhanced or mediated by flagellum-based motility.

Generating, splitting, transporting, and merging droplets are fundamental and critical unit operations for digital (droplet-based) microfluidics. State-of-the-art digital microfluidics performs such operations commonly using electrowetting-on-dielectric (EWOD) in the typical configuration of two parallel channel plates. This paper presents such operations using dielectrowetting (derived from liquid dielectrophoresis), not EWOD, with an array of interdigitated electrodes. The major and unique feature is that the present droplet manipulations are effective for conductive (water with/without surfactant) and non-conductive (propylene carbonate) fluids. An equally important aspect is that the manipulations are performed in an open space without the covering top plate. This behavior is attributed to the intrinsic nature of dielectrowetting to generate stronger wetting forces than EWOD (with the ability to achieve complete wetting with contact angle = 0° to form a thin film). Using dielectrowetting, micro-droplets of various volumes are created from a large droplet and transported. Splitting a single droplet as well as multiple droplets and merging them are also achieved, even when the droplets are smaller than the electrode pads. The above splitting, transport, and merging operations are effective for propylene carbonate as well as DI water with/without surfactant, though the creating operation is proven only for propylene carbonate at this moment. All the above manipulations are successfully carried out on a single plate, which not only simplifies the structure and operation procedure, but could also eliminate the restriction to the volume of fluid handled.

Silver nanoparticles (AgNPs) and other ionizing engineered nanomaterials (ENMs) are candidates for the development of antimicrobial agents due to their efficacy, multiple modes of bacterial inactivation, and tunability with respect to both the magnitude and mechanisms of antimicrobial activity.

bDevelopment of acetic acid-resistant Saccharomyces cerevisiae is important for economically viable production of biofuels from lignocellulosic biomass, but the goal remains a critical challenge due to limited information on effective genetic perturbation targets for improving acetic acid resistance in the yeast. This study employed a genomic-library-based inverse metabolic engineering approach to successfully identify a novel gene target, WHI2 (encoding a cytoplasmatic globular scaffold protein), which elicited improved acetic acid resistance in S. cerevisiae. Overexpression of WHI2 significantly improved glucose and/or xylose fermentation under acetic acid stress in engineered yeast. The WHI2-overexpressing strain had 5-times-higher specific ethanol productivity than the control in glucose fermentation with acetic acid. Analysis of the expression of WHI2 gene products (including protein and transcript) determined that acetic acid induced endogenous expression of Whi2 in S. cerevisiae. Meanwhile, the whi2⌬ mutant strain had substantially higher susceptibility to acetic acid than the wild type, suggesting the important role of Whi2 in the acetic acid response in S. cerevisiae. Additionally, overexpression of WHI2 and of a cognate phosphatase gene, PSR1, had a synergistic effect in improving acetic acid resistance, suggesting that Whi2 might function in combination with Psr1 to elicit the acetic acid resistance mechanism. These results improve our understanding of the yeast response to acetic acid stress and provide a new strategy to breed acetic acid-resistant yeast strains for renewable biofuel production. Lignocellulosic biomass from nonfood stocks such as agricultural and forestry residues has been identified as the prime source for production of renewable biofuels to substitute for conventional fossil fuels in the face of growing demand for energy and rising concerns about greenhouse gas emissions (1-4). Bioconversion of plant cell wall materials by microbial fermentation is typically preceded by harsh (physico)chemical hydrolysis designed to release sugars; this hydrolysis treatment also generates by-products that are toxic to fermenting microorganisms (5, 6). Since hemicellulose and lignin in the plant cell wall are ubiquitously acetylated (7,8), the typical acidic pretreatment of lignocellulosic biomass generates substantial amounts of acetic acid (with concentrations ranging from 1 g/liter to 15 g/liter) in the resulting hydrolysates (9, 10). Acetic acid severely inhibits cell growth and fermentation activity in Saccharomyces cerevisiae (5, 6, 11-13), the predominant microorganism used in industrial fermentation (14, 15). Therefore, improvement in S. cerevisiae resistance to acetic acid is highly desired and critical for achieving efficient and economically viable bioconversion of cellulosic sugars to biofuels.The toxic effects of acetic acid in S. cerevisiae have been intensively characterized, and toxicity mechanisms have been proposed (10,11,(16)(17)(18)(19)(20). When the external pH is lower than the...