Evolution of monolayer terrace topography on (100) GaAs annealed under an arsine/hydrogen ambient

The topographical evolution of the (100) GaAs surface annealed under an arsine/hydrogen ambient is studied by in situ orientation-resolved light scattering and ex situ atomic force microscopy (AFM). The light scattering system provides real-time monitoring of the magnitude and crystal orientation of topographical features of 0.3 mum scale. The AFM images of the GaAs surface, quenched at various annealing temperatures, vividly depict the randomly oriented high density monolayer steps evolving into an atomically smooth terracelike structure

Large Coherence Area Thin-Film Photonic Stop-Band Lasers

We demonstrate that the shift of the stop band position with increasing oblique angle in periodic structures results in a wide transverse exponential field distribution corresponding to strong angular confinement of the radiation. The beam expansion follows an effective diffusive equation depending only upon the spectral mode width. In the presence of gain, the beam cross section is limited only by the size of the gain area. As an example of an active periodic photonic medium, we calculate and measure laser emission from a dye-doped cholesteric liquid crystal film

Liposomes Recruit IpaC to the Shigella flexneri Type III Secretion Apparatus Needle as a Final Step in Secretion Induction

Shigella flexneri contact with enterocytes induces a burst of protein secretion via its type III secretion apparatus (TTSA) as an initial step in cellular invasion. We have previously reported that IpaD is positioned at the TTSA needle tip (M. Espina et al., Infect. Immuno. 74:4391-4400, 2006). From this position, IpaD senses small molecules in the environment to control the presentation of IpaB to the needle tip. This step occurs without type III secretion induction or IpaC recruitment to the S. flexneri surface. IpaC is then transported to the S. flexneri surface when target cell lipids are added, and this event presumably mimics host cell contact. Unlike IpaB mobilization, IpaC surface presentation is closely linked to secretion induction. This study demonstrates that sphingomyelin and cholesterol are key players in type III secretion induction and that they appear to interact with IpaB to elicit IpaC presentation at the TTSA needle tip. Furthermore, IpaB localization at the needle tip prior to membrane contact provides the optimal set of conditions for host cell invasion. Thus, the S. flexneri type III secretion system can be induced in a stepwise manner, with the first step being the stable association of IpaD with the needle tip, the second step being the sensing of small molecules by IpaD to mobilize IpaB to the tip, and the third step being the interaction of lipids with IpaB to induce IpaC localization at the needle tip concomitant with translocon insertion into the host membrane and type III secretion induction. Shigella flexneri, the causative agent of shigellosis, is responsible for more than 1 million deaths each year, especially among children in developing regions (www.who.int/vaccines-documents/DocsPDF99/www9947.pdf). Once ingested, S. flexneri crosses M cells and passes into the underlying gut-associated lymphoid tissues of the colon (20), where it kills macrophages (29) and then invades epithelial cells by macropinocytosis (17). The S. flexneri invasive phenotype localizes genetically to a 31-kb region of its large virulence plasmid and is absolutely tied to its type III secretion system (TTSS) (6, 23). TTSSs are used by numerous gram-negative bacteria to introduce bacterially derived effector proteins into the membrane and cytoplasm of a target cell, resulting in the subversion of normal cell functions (8). Linking the bacterium and host cell in this process is the type III secretion apparatus (TTSA), which structurally resembles a molecular needle and syringe. The system is controlled by a basal body (the syringe) that spans both bacterial membranes and an external needle that provides a conduit from the basal body to the sensory needle tip complex (8, 28). The needle in S. flexneri is comprised of a polymer of MxiH and is approximately 50 nm long and 7 nm in diameter, with a central channel that is about 2.5 nm in diameter (5). At the top of the MxiH needle resides the tip protein IpaD, most likely as a pentamer, which serves as an environmental sensor for the MxiH-IpaD tip complex (4, 7). When the presence of bile salts such as deoxycholate (DOC) is sensed by IpaD, the first translocator protein, IpaB, is mobilized to the TTSA needle tip to form an MxiH-IpaD-IpaB ternary complex. At this stage, the TTSA structure is primed for subsequent host cell contact (19, 24). In previous studies IpaC had not been found to localize to the S. flexneri surface of the log-phase bacterium (7, 19). As a next step in describing the process of type III secretion, we show here that liposomes trigger mobilization of IpaC to the needle tip complex, where it is immediately inserted into the host cell membrane, along with IpaB, to complete the TTSA conduit into the host cell just prior to initiating host cytoskeleton rearrangements. IpaC is most efficiently recruited to the S. flexneri surface with a defined liposome composition that includes phospholipids, sphingomyelin (SM), and cholesterol (Chol). Furthermore, IpaC recruitment occurs concomitantly with induction of type III secretion of IpaB, IpaC, and IpaD into the S. flexneri culture supernatant. It thus appears that IpaB mobilization to the S. flexneri TTSA needle tip represents a second discrete step in TTSA assembly, with the final third step being IpaC recruitment to the needle tip, which occurs after IpaB contacts and inserts into the host cell membrane

Mobilising Knowledge: Determining key elements for success and pitfalls in developing Community Based Tourism

Community based tourism (CBT) has often been cited as an alternative to mass tourism and an approach for tourism to become more sustainable. If developed well, CBT can become a poverty alleviation mechanism and a way to access improvements in quality of life, providing empowerment and greater economic benefit to individuals in local communities. Despite the plethora of literature on CBT and evaluation of models, there is little analysis of the facilitators and barriers to achieving it. Through the use of case studies in both academic and grey literature, this paper serves as an instructive review of the CBT literature to synthesise the key elements of success and the challenges

A quantified triple bottom line for tourism: experimental results

The tradition of tourism businesses and regional tourism industries is to measure their value to the host community by jobs, wages and tax revenues even though every member of that community is impacted on a daily basis though a broad variety of impacts. This paper demonstrates a conceptual approach for measuring the relative importance of the major dimensions of community quality of life that can be influenced by the tourism industry in order to calculate an indication of overall impact on the well being of community residents. Furthermore, we have formulated a conjoint model that values this overall performance in monetary units. A conjoint model for estimating importance is successfully implemented using samples of college students and tourism industry professionals in the US and Cyprus. A monetary version of Triple Bottom Line impacts is calculated for the impacts of a specific hypothetical tourism business development

The N-terminus of IpaB provides a potential anchor to the Shigella type III secretion system tip complex protein IpaD

The type III secretion system (T3SS) is an essential virulence factor for Shigella flexneri, providing a conduit through which host-altering effectors are injected directly into a host cell to promote uptake. The type III secretion apparatus (T3SA) is comprised of a basal body, external needle, and regulatory tip complex. The nascent needle is a polymer of MxiH capped by a pentamer of invasion plasmid antigen D (IpaD). Exposure to bile salts (e.g. deoxycholate) causes a conformational change in IpaD and promotes recruitment of IpaB to the needle tip. It has been proposed that IpaB senses contact with host cell membranes, recruiting IpaC and inducing full secretion of T3SS effectors. While the steps of T3SA maturation and their external triggers have been identified, details of specific protein interactions and mechanisms have remained difficult to study due to the hydrophobic nature of the IpaB and IpaC translocator proteins. Here we explored the ability for a series of soluble N-terminal IpaB peptides to interact with IpaD. We found that DOC is required for the interaction and that a region of IpaB between residues 11–27 is required for maximum binding, which was confirmed in vivo. Furthermore, intramolecular FRET measurements indicated that movement of the IpaD distal domain away from the protein core accompanied the binding of IpaB11-226. Together these new findings provide important new insight into the interactions and potential mechanisms that define the maturation of the Shigella T3SA needle tip complex and provide a foundation for further studies probing T3SS activation

Tourism, inclusive growth and decent work: a political economy critique

This paper interrogates the ideas of ‘sustained’ and ‘inclusive’ growth that are intrinsic to one of three UN Sustainable Development Goals (SDG 8 - Decent Work and Growth) adopted by the UN World Tourism Organisation’s (UNWTO) 2030 sustainable tourism agenda. It provides a Marxian-inspired political economy critique of the UNWTO’s embrace of SDG8 and highlights the blind spot within the UNWTO’s inclusive growth-led SDG agenda with respect to questions of equity and social justice. The paper contends that the UNWTO’s SDG-led agenda is contradicted by the logics of growth, competitiveness and profit-making that drive the continued expansion and development of tourism. Rather than addressing the structural injustices that entrench inequalities and reproduce exploitative labour practices, the notion of sustained and inclusive growth reinforces the primacy of capital and market notions of justice and continues to perpetuate a growth driven tourism development model. The paper contributes to a critical theorization of sustainable tourism and offers an informed critique of the current political agenda for sustainable tourism and its potential outcomes