Classification of Minimal Separating Sets in Low Genus Surfaces

Consider a surface $S$ and let $M\subset S$. If $S\setminus M$ is not connected, then we say $M$ \emph{separates} $S$, and we refer to $M$ as a \emph{separating set} of $S$. If $M$ separates $S$, and no proper subset of $M$ separates $S$, then we say $M$ is a \emph{minimal separating set} of $S$. In this paper we use methods of computational combinatorial topology to classify the minimal separating sets of the orientable surfaces of genus $g=2$ and $g=3$. The classification for genus 0 and 1 was done in earlier work, using methods of algebraic topology.Comment: 24 pages, 5 figures, 2 tables (11 pages

The Oxidation Stability, Light Absorbing Power, and Component of Humic Acids from Different Origins, and Their Mutal Relations

Toxin-antitoxin modules are necessary for the mode of action of several antibiotics. One of the most studied toxin-antitoxin modules is the quorum sensing - dependent MazEF system in Escherichia coli. The quorum sensing factor in this system is called the extracellular death factor (EDF), a linear pentapeptide with the sequence NNWNN. In spite of the extensive research on the mazEF system and the involvement of the quorum sensing factor EDF, the effect of EDF itself on bacteria has not yet been studied. In this research, we determined the effect of EDF and variants on cell growth in the Gram-negative bacterium E. coli and the Gram-positive Bacillus globigii. By aligning the zwf gene (from where EDF originates) of different bacterial species, we found 27 new theoretical variants of the peptide. By evaluating growth curves and light microscopy we found that three EDF variants reduced bacterial cell size in B. globigii, but not in E. coli. The D-peptides did not affect cell size, indicating that the effect is stereospecific. Peptides wherein tryptophan was substituted by alanine also did not affect cell size, which indicates that the effect seen is mediated by an intracellular target. © 2013 Springer Science+Business Media Dordrecht

Scaffolding School Pupils’ Scientific Argumentation with Evidence-Based Dialogue Maps

This chapter reports pilot work investigating the potential of Evidence-based Dialogue Mapping to scaffold young teenagers’ scientific argumentation. Our research objective is to better understand pupils’ usage of dialogue maps created in Compendium to write scientific ex-planations. The participants were 20 pupils, 12-13 years old, in a summer science course for “gifted and talented” children in the UK. Through qualitative analysis of three case studies, we investigate the value of dialogue mapping as a mediating tool in the scientific reasoning process during a set of learning activities. These activities were published in an online learning envi-ronment to foster collaborative learning. Pupils mapped their discussions in pairs, shared maps via the online forum and in plenary discussions, and wrote essays based on their dialogue maps. This study draws on these multiple data sources: pupils’ maps in Compendium, writings in science and reflective comments about the uses of mapping for writing. Our analysis highlights the diversity of ways, both successful and unsuccessful, in which dialogue mapping was used by these young teenagers

Synthetic LPETG-containing peptide incorporation in the Staphylococcus aureus cell-wall in a sortase a- and growth phase-dependent manner

The majority of Staphylococcus aureus virulence- and colonization- associated surface proteins contain a pentapeptide recognition motif (LPXTG). This motif can be recognized and cleaved by sortase A (SrtA) which is a membrane-bound transpeptidase. After cleavage these proteins are covalently incorporated into the peptidoglycan. Therefore, SrtA plays a key role in S. aureus virulence. We aimed to generate a substrate mimicking this SrtA recognition motif for several purposes: to incorporate this substrate into the S. aureus cell-wall in a SrtA-dependent manner, to characterize this incorporation and to determine the effect of substrate incorporation on the incorporation of native SrtA-dependent cell-surface-associated proteins. We synthesized substrate containing the specific LPXTG motif, LPETG. As a negative control we used a scrambled version of this substrate, EGTLP and a S. aureus srtA knockout strain. Both substrates contained a fluorescence label for detection by FACScan and fluorescence microscope. A spreading assay and a competitive Luminex assay were used to determine the effect of substrate treatment on native LPXTG containing proteins deposition in the bacterial cell-wall. We demonstrate a SrtA-dependent covalent incorporation of the LPETG-containing substrate in wild type S. aureus strains and several other Gram-positive bacterial species. LPETG-containing substrate incorporation in S. aureus was growth phase-dependent and peaked at the stationary phase. This incorporation negatively correlated with srtA mRNA expression. Exogenous addition of the artificial substrate did not result in a decreased expression of native SrtA substrates (e.g. clumping factor A/B and protein A) nor induced a srtA knockout phenotype