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The influence of the lipopolysaccharide (LPS) chain length on bacterial adhesion was investigated by measuring the collision efficiencies of three Escherichia coli K12 strains, each having a different length LPS, to silica glass beads in column tests (macroscale tests). Nanoscale interactions between the bacteria and a silicon nitride tip were probed utilizing atomic force microscopy (AFM). Adhesion results based on column tests indicated that collision efficiencies of the three bacteria were not consistently correlated to LPS length. Under conditions of low ionic strength (1 mM NaCl), collision efficiencies increased with LPS length for the three strains of E. coli. However, if cells were fixed with glutaraldehyde (2.5%), the strain with the shortest LPS chain had the greatest adhesion, while the bacterium with the mid-length LPS had the least adhesion to glass beads. Collision efficiencies increased when the solution ionic strength was increased from 1 to 100 mM as expected, and in most cases glutaraldehyde treatment also increased adhesion. AFM force curves failed to distinguish the adhesion characteristics of these bacteria measured in column tests, as all AFM force curves on the bacteria were identical. Changes in adhesion were also not predictable by more conventional measurements of bacterial properties based on ζ potential or contact angle. These results suggest that the LPS molecule length is not the sole determinant of adhesion of the three E. coli strains in porous media and that AFM force curve analysis, zeta potential, or contact angle data cannot yet be used to fully predict adhesion of these three strains to glass beads.

Hydroxyapatite (HAp) is an important component of mammal bones and teeth, being widely used in prosthetic implants. Despite the importance of HAp in medicine, several promising applications involving this material (e.g., in photo-catalysis) depend on how well we understand its fundamental properties. Among the ones that are either unknown or not known accurately, we have the electronic band structure and all that relates to it, including the bandgap width. We employ state-of-the-art methodologies, including density hybrid-functional theory and many-body perturbation theory within the dynamically screened single-particle Green's function approximation, to look at the optoelectronic properties of HAp. These methods are also applied to the calculation of defect levels. We find that the use of a mix of (semi-)local and exact exchange in the exchange-correlation functional brings a drastic improvement to the band structure. Important side effects include improvements in the description of dielectric and optical properties not only involving conduction band (excited) states but also the valence. We find that the highly dispersive conduction band bottom of HAp originates from anti-bonding σ* states along the ⋯OH-OH-⋯ infinite chain, suggesting the formation of a conductive 1D-ice phase. The choice of the exchange-correlation treatment to the calculation of defect levels was also investigated by using the OH-vacancy as a testing model. We find that donor and acceptor transitions obtained within semi-local density functional theory (DFT) differ from those of hybrid-DFT by almost 2 eV. Such a large discrepancy emphasizes the importance of using a high-quality description of the electron-electron interactions in the calculation of electronic and optical transitions of defects in HAp.

In this work, computational molecular modeling and exploration was applied to study the nature of the negative piezoelectric effect in the ferroelectric polymer polyvinylidene fluoride (PVDF), and the results confirmed by actual nanoscale measurements. First principle calculations were employed, using various quantum-chemical methods (QM), including semi-empirical (PM3) and various density functional theory (DFT) approaches, and in addition combined with molecular mechanics (MM) methods in complex joint approaches (QM/MM). Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. This computational molecular exploration clearly shows that the nature of the so-called negative piezo-electric effect in the ferroelectric PVDF polymer has a self-consistent quantum nature, and is related to the redistribution of the electron molecular orbitals (wave functions), leading to the shifting of atomic nuclei and reorganization of all total charges to the new, energetically optimal positions, under an applied electrical field. Molecular modeling and first principles calculations show that the piezoelectric coefficient d 33 has a negative sign, and its average values lies in the range of d 33 ~ -16.6 to -19.2 pC/N (or pm/V) (for dielectric permittivity ε = 5) and in the range of d 33 ~ -33.5 to -38.5 pC/N (or pm/V) (for ε = 10), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezo-response. We found that when a field is applied perpendicular to the PVDF chain length, as polarization increases the chain also stretches, increasing its length and reducing its height. For computed value of ε ~ 5 we obtained a value of d31 ~ +15.5 pC/N with a positive sign. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM/PFM). This study could be useful as a basis for further insights into other organic and molecular ferroelectrics.

Hydroxyapatite (HAp) is a widely used biomaterial for the preparation of bone and dental implants. Despite the relevance of HAp in medicine, exciting applications involving this material as a bio-compatible photocatalyst, depend on how well we understand its fundamental properties. Experimental evidence suggests that oxygen vacancies play a critical role in the production of surface radicals upon exposure of HAp to ultra-violet (UV) light. However, very little is known about the underlying physical and chemical details. We present a hybrid density-functional study of the structural and electronic properties of oxygen vacancies in large HAp supercells within the plane-wave formalism. We find that under equilibrium conditions, vacancies occur either as a simple vacant oxygen site (in the neutral charge state), or as extended structures replacing several crystalline moieties (in the double plus charge state). Large atomic relaxations upon ionization make the oxygen vacancy a negative-U defect, where the single plus charge state is metastable, being only accessible under UV excitation. From inspection of the transition levels, we find that electron promotion from the valence band top to the donor state of the vacancy, involves a zerophonon transition of 3.6-3.9 eV. This mechanism is the most likely explanation to the 3.4-4.0 eV absorption onset for the observation of photocatalysis using HAp under persistent UV illumination.

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Candida albicans is the most frequently isolated human fungal pathogen among species causing biofilm-related clinical infections. Mechanical properties of Candida biofilms have hitherto been given no attention, despite the fact that mechanical properties are important for selection of treatment or dispersal of biofilm organisms due to a bodily fluid flow. The aim of this study was to identify the factors that determine the compression strength of Candida biofilms. Biofilms of C. albicans wild-type parental strain Caf2-1, mutant strain Chk24 lacking Chk1p [known to be involved in regulation of morphogenesis (yeast-to-hyphae transition)] and gene-reconstructed strain Chk23 were evaluated for their resistance to compression, along with biofilms of Candida tropicalis GB 9/9 and Candida parapsilosis GB 2/8, derived from used voice prosthetic biofilms. Additionally, cell morphologies within the biofilm, cell-surface hydrophobicities and extracellular polymeric substance composition were determined. Our results suggest that the hyphae-to-yeast ratio influences the compression strength of C. albicans biofilms. Biofilms with a hyphal content .50 % possessed significantly higher compressive strength and were more difficult to destroy by vortexing and sonication than biofilms with a lower hyphal content. However, when the amount of extracellular DNA (eDNA) in biofilms of C. albicans Caf2-1 and Chk24 increased, biofilm strength declined, suggesting that eDNA may influence biofilm integrity adversely. INTRODUCTIONBiofilms are populations of micro-organisms embedded in an extracellular polymeric substance (EPS) (Costerton et al., 1987). The biofilm mode of growth is beneficial for micro-organisms, as it provides a higher degree of stability than a planktonic existence, and organisms within a biofilm are more resistant to environmental challenges (Hall-Stoodley et al., 2004), such as low nutrient availability, high fluid shear and antibiotic and antimicrobial agents (Baker & Banfield, 2003;Battin et al., 2001;Donlan & Costerton, 2002;Elasri & Miller, 1999;Stewart & Costerton, 2001). The damage caused by the formation of biofilms is a widespread problem, ranging from pipeline corrosion and biofouling of ship hulls and food-processing equipment to clinical infections, such as endocarditis and cystic fibrosis pneumonia (Beech & Gaylarde, 1999;Callow & Callow, 2002;Costerton et al., 1999;Kumar & Anand, 1998). Many clinical infections due to biofilms are implant-related and occur when micro-organisms adhere to the surfaces of biomaterials used in, for example, prosthetic heart valves, voice prostheses, joint replacements, vascular grafts and urinary catheters (Costerton et al., 1999;Donlan, 2001).Biofilms can consist of bacterial or fungal species or a mixture of both. For instance, in the case of vascular catheter-related infections or voice prosthetic biofilms, the most commonly isolated microbial species are Grampositive Staphylococcus epidermidis and Staphylococcus aureus, and the fungus Candida albicans (Hampton & Sherertz, 19...

In eukaryotes, acetyl coenzyme A (acetyl-CoA) produced during peroxisomal fatty acid ␤-oxidation needs to be transported to mitochondria for further metabolism. Two parallel pathways for acetyl-CoA transport have been identified in Saccharomyces cerevisiae; one is dependent on peroxisomal citrate synthase (Cit), while the other requires peroxisomal and mitochondrial carnitine acetyltransferase (Cat) activities. Here we show that the human fungal pathogen Candida albicans lacks peroxisomal Cit, relying exclusively on Cat activity for transport of acetyl units. Deletion of the CAT2 gene encoding the major Cat enzyme in C. albicans resulted in a strain that had lost both peroxisomal and mitochondrion-associated Cat activities, could not grow on fatty acids or C 2 carbon sources (acetate or ethanol), accumulated intracellular acetyl-CoA, and showed greatly reduced fatty acid ␤-oxidation activity. The cat2 null mutant was, however, not attenuated in virulence in a mouse model of systemic candidiasis. These observations support our previous results showing that peroxisomal fatty acid ␤-oxidation activity is not essential for C. albicans virulence. Biofilm formation by the cat2 mutant on glucose was slightly reduced compared to that by the wild type, although both strains grew at the same rate on this carbon source. Our data show that C. albicans has diverged considerably from S. cerevisiae with respect to the mechanism of intracellular acetyl-CoA transport and imply that carnitine dependence may be an important trait of this human fungal pathogen.