1,721,003 research outputs found
Relation between the structure of an aerobic biofilm and transport phenomena
No full textAn aerobic biofilm was characterized using confocal scanning laser microscopy (CSLM), O2 micro-electrodes, particle tracking and microinjection of fluorescent dyes. The biofilms were found to consist of microbial clusters of cells and Extra-cellular Polymeric Substance (EPS) separated by interstitial voids. The cell clusters were ca 300 m and the voids were ca 100 m wide. The voids were open channels connected with the bulk fluid. Fluorescein micro-injection showed that liquid could flow through the voids, but was always stagnant in the cell clusters. Consequently, in voids both diffusion and convection may contribute to mass transfer, while in cell clusters transport is determined by diffusion only. Particle tracking with CSLM showed that flow velocity inside the biofilm was proportional to the bulk flow velocity. The importance of convective mass transport in biofilms was demonstrated by oxygen distribution measurements. At high flow velocities of the bulk liquid, the mass boundary layer followed the irregular biofilm surface. At lower velocities the mass boundary layer was parallel to the substratum. Mass transfer from voids to cell clusters increased with flow velocity, as result from convective mass transport from the bulk to the voids. Convective transport was insignificant at low flow velocities, but at high flow velocities it increased the total mass transport by 200-250%. The local diffusion coefficients in biofilms were measured using microinjection of fluorescent dyes and quantitative analysis of the subsequent plume formation using CSLM. The diffusion coefficient of small, non-binding molecules in cell clusters is close to that in water. Very large molecules were impeded in their diffusion through the biofilm matrix. It was calculated that the cell cluster matrix had the characteristics of a gel network with pore diameters of 80 nm
Influence of electric fields and pH on biofilm structure as related to the bioelectric effect
No full textMixed species biofilms of Klebsiella pneumoniae, Pseudomonas fluorescens, and Pseudomonas aeruginosa were grown in a flow cell fitted with two platinum wire electrodes. The biofilm growing on the wires reached a thickness of approximately 50 microm after 3 days. When a voltage was applied with oscillating polarity, the biofilm attached to the wire expanded and contracted. The biofilm expanded by approximately 4% when the wire was cathodic but was reduced to 74% of the original thickness when the wire was anodic. The phenomenon was reproduced by alternately flushing the flow cell with media adjusted to pH 3 and pH 10 with no electric current. At pH 10 the biofilm was unaltered, but it became compacted to 69% of the original thickness at pH 3. We explained these phenomena in terms of the molecular interactions between charged acidic groups in the biofilm slime and the bacterial cell walls. Contraction of the biofilm under acidic conditions may be caused by (i) the elimination of electrostatic repulsion from neutralization of negatively charged carboxylate groups through protonation and (ii) subsequent hydrogen bonding between the carboxylic acids and oxygen atoms in the sugars. Electrostatic interactions between negatively charged groups in the biofilm and the charged wire may also be expected to cause biofilm expansion when the wire was cathodic and contraction when the wire was anodic. The consequences of the explanation of the increased susceptibility of biofilm cells to antibiotics in an electric field, the "bioelectric effect," are discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Real time microsensor measurement of local metabolic activities in ex vivo dental biofilms treated with chlorhexidine
No full textDental biofilms are characterized by structural and functional heterogeneity. Due to bacterial metabolism, gradients develop and diverse ecological microniches exist. The aims of this study were (i) to determine the metabolic activity of microorganisms in naturally grown dental biofilms ex vivo by measuring dissolved oxygen (DO) and pH profiles with microelectrodes with high spatial resolution and (ii) to analyze the impact of an antimicrobial chlorhexidine (CHX) treatment on microbial physiology during stimulation by sucrose in real time. Biofilms were cultivated on standardized human enamel surfaces in vivo. DO and pH profiles were measured in a flow cell system in sterile human saliva, after sucrose addition (10%), again after alternative treatment of the sucrose exposed biofilms with CHX (0.2%) for 1 or 10 min or after being killed with paraformaldehyde (4%). Biofilm structure was visualized by vitality staining with confocal microscopy. With saliva as the sole nutrient source oxygen consumption was high within the superficial biofilm layers rendering deeper layers (>220 mum) anoxic. Sucrose addition induced the thickness of the anaerobic zone to increase with a concurrent decrease in pH (7.1 to 4.4). CHX exposure reduced metabolic activity and microbial viability at the biofilm surface and drove metabolic activity deeper into the biofilm. CHX treatment led to a reduced viability at the biofilm surface with minor influence on overall biofilm physiology after 1 min; even after 10 min there was measurable respiration and fermentation inside the biofilm. However, the local microenvironment was more aerated, less acidogenic, and presumably less pathogenic.<br/
Powered brushing and fluoride in a streptococcus mutans typodont biofilm
No full textObjectives: to develop a typodont biofilm model to measure the effect of powered brushing on biofilm removal and the delivery of fluoride into biofilm remaining in accessible areas in an anatomically relevant geometry.Methods: streptococcus mutans biofilms were grown on sections of a typodont (A-PZ Frasaco) for three or six days. The typodont sections contained either incisors and cuspids or premolars and molars. The biofilm was observed on various surfaces before and after brushing facial surfaces for 10 seconds with a powered toothbrush (Philips Sonicare FlexCare), according to manufacturer instructions. A pH microelectrode was used to measure profiles in the biofilm. To measure profiles of free fluoride ion in the biofilm we constructed a fluoride microelectrode. The electrode was calibrated and profiles measured in an agar gel.Results: biofilm grew on the hard plastic tooth and soft gum materials of the typodont on all surfaces. The pH microelectrode showed that acid was produced in the presence of sucrose due to acid respiration. The pH dropped from approximately 7 to as low as 5.5. After hand brushing the typodont was reexamined. The facial surfaces were almost devoid of biofilm, however, there was still biofilm remaining in the interproximal spaces and molar occlusal surfaces. The fluoride microelectrode had a tip diameter of 5 µm and had a measurement range from approximately 1 to 1000 ppm F-, in a log-linear relationship.Conclusion: the typodont model is a good system for quantifying biofilm removal and measuring remaining biofilm in “hard to reach locations” in an anatomically relevant model. Microelectrodes can be used to measure local activity and zones of acid production. The fluoride microelectrode has potential for measuring the local delivery of fluoride to biofilm on various typodont surface
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