Ferroelectric Behavior in Exfoliated 2D Aurivillius Oxide Flakes of Sub‐Unit Cell Thickness

Ferroelectricity in ultrasonically exfoliated flakes of the layered Aurivillius oxide Bi5Ti3Fe0.5Co0.5O15 with a range of thicknesses is studied. These flakes have relatively large areas (linear dimensions many times the film thickness), thus classifying them as 2D materials. It is shown that ferroelectricity can exist in flakes with thicknesses of only 2.4 nm, which equals one‐half of the normal crystal unit cell. Piezoresponse force microscopy (PFM) demonstrates that these very thin flakes exhibit both piezoelectric effects and that the ferroelectric polarization can be reversibly switched. A new model is presented that permits the accurate modeling of the field‐on and field‐off PFM time domain and hysteresis loop responses from a ferroelectric during switching in the presence of charge injection, storage, and decay through a Schottky barrier at the electrode–oxide interface. The extracted values of spontaneous polarization, 0.04(±0.02) C m−2 and electrostrictive coefficient, 2(±0.1) × 10−2 m4 C−2 are in good agreement with other ferroelectric Aurivillius oxides. Coercive field scales with thickness, closely following the semi‐empirical scaling law expected for ferroelectric materials. This constitutes the first evidence for ferroelectricity in a 2D oxide material, and it offers the prospect of new devices that might use the useful properties associated with the switchable ferroelectric spontaneous polarization in a 2D materials format

Curriculum Making as the Enactment of Dwelling in Places

This article uses an account of dwelling to interrogate the concept of curriculum making. Tim Ingold's use of dwelling to understand culture is productive here because of his implicit and explicit interest in intergenerational learning. His account of dwelling rests on a foundational ontological claim-that mental construction and representation are not the basis upon which we live in the world-which is very challenging for the kinds of curriculum making with which many educators are now familiar. It undermines assumptions of propositional knowledge and of the use of mental schemas to communicate and share. At the level of critique, then, dwelling destabilizes contemporary ideas of curriculum as textual, pre-specified content for transmission or pre-defined objectives or standardized activity. The positive claims of dwelling are equally challenging, for these are that the world is a domain of relational entanglement in which an organism can be no more than a point of growth for an emergent ‘environment', and meaning only inheres in these relations. The paper articulates how differentiation (of learner, salient meanings, knowledge, skill and place) are possible in such an ontology, and how curriculum making can be understood from this perspective as being the remaking of relationships between these

Functional Analysis of the Cytoskeleton Protein MreB from Chlamydophila pneumoniae

In rod-shaped bacteria, the bacterial actin ortholog MreB is considered to organize the incorporation of cell wall precursors into the side-wall, whereas the tubulin homologue FtsZ is known to tether incorporation of cell wall building blocks at the developing septum. For intracellular bacteria, there is no need to compensate osmotic pressure by means of a cell wall, and peptidoglycan has not been reliably detected in Chlamydiaceae. Surprisingly, a nearly complete pathway for the biosynthesis of the cell wall building block lipid II has been found in the genomes of Chlamydiaceae. In a previous study, we discussed the hypothesis that conservation of lipid II biosynthesis in cell wall-lacking bacteria may reflect the intimate molecular linkage of cell wall biosynthesis and cell division and thus an essential role of the precursor in cell division. Here, we investigate why spherical-shaped chlamydiae harbor MreB which is almost exclusively found in elongated bacteria (i.e. rods, vibrios, spirilla) whereas they lack the otherwise essential division protein FtsZ. We demonstrate that chlamydial MreB polymerizes in vitro and that polymerization is not inhibited by the blocking agent A22. As observed for MreB from Bacillus subtilis, chlamydial MreB does not require ATP for polymerization but is capable of ATP hydrolysis in phosphate release assays. Co-pelleting and bacterial two-hybrid experiments indicate that MreB from Chlamydophila (Chlamydia) pneumoniae interacts with MurF, MraY and MurG, three key components in lipid II biosynthesis. In addition, MreB polymerization is improved in the presence of MurF. Our findings suggest that MreB is involved in tethering biosynthesis of lipid II and as such may be necessary for maintaining a functional divisome machinery in Chlamydiaceae

Identification of Streptococcus pneumoniae by a real-time PCR assay targeting SP2020.

Real-time PCR targeting lytA (the major autolysin gene) and piaB (permease gene of the pia ABC transporter) are currently used as the gold-standard culture-independent assays for Streptococcus pneumoniae identification. We evaluated the performance of a new real-time PCR assay - targeting SP2020 (putative transcriptional regulator gene) - and compared its performance with the assays previously described. A collection of 150 pneumococci, 433 non-pneumococci and 240 polymicrobial samples (obtained from nasopharynx, oropharynx, and saliva; 80 from each site) was tested. SP2020 and lytA-CDC assays had the best performance (sensitivity of 100% for each compared to 95.3% for piaB). The specificity for lytA and piaB was 99.5% and for SP2020 was 99.8%. Misidentifications occurred for the three genes: lytA, piaB and SP2020 were found in non-pneumococcal strains; piaB was absent in some pneumococci including a serotype 6B strain. Combining lytA and SP2020 assays resulted in no misidentifications. Most polymicrobial samples (88.8%) yielded concordant results for the three molecular targets. The remaining samples seemed to contain non-typeable pneumococci (0.8%), and non-pneumococci positive for lytA (1.7%) or SP2020 (8.7%). We propose that combined detection of both lytA-CDC and SP2020 is a powerful strategy for the identification of pneumococcus either in pure cultures or in polymicrobial samples

Live cell dynamics of production, explosive release and killing activity of phage tail-like weapons for Pseudomonas kin exclusion.

Interference competition among bacteria requires a highly specialized, narrow-spectrum weaponry when targeting closely-related competitors while sparing individuals from the same clonal population. Here we investigated mechanisms by which environmentally important Pseudomonas bacteria with plant-beneficial activity perform kin interference competition. We show that killing between phylogenetically closely-related strains involves contractile phage tail-like devices called R-tailocins that puncture target cell membranes. Using live-cell imaging, we evidence that R-tailocins are produced at the cell center, transported to the cell poles and ejected by explosive cell lysis. This enables their dispersal over several tens of micrometers to reach targeted cells. We visualize R-tailocin-mediated competition dynamics between closely-related Pseudomonas strains at the single-cell level, both in non-induced condition and upon artificial induction. We document the fatal impact of cellular self-sacrifice coupled to deployment of phage tail-like weaponry in the microenvironment of kin bacterial competitors, emphasizing the necessity for microscale assessment of microbial competitions