Publications

Biofilms are multicellular heterogeneous bacterial communities characterized by social-like division of labor, and remarkable robustness with respect to external stresses. Increasingly often an analogy between biofilms and arguably more complex eukaryotic tissues is being drawn. One illustrative example of where this analogy can be practically useful is the process of wound healing. While it has been extensively studied in eukaryotic tissues, the mechanism of wound healing in biofilms is virtually unexplored. Combining experiments in Bacillus subtilis bacteria, a model organism for biofilm formation, and a lattice-based theoretical model of biofilm growth, we studied how biofilms recover after macroscopic damage. We suggest that nutrient gradients and the abundance of proliferating cells are key factors augmenting wound closure. Accordingly, in the model, cell quiescence, nutrient fluxes, and biomass represented by cells and self-secreted extracellular matrix are necessary to qualitatively recapitulate the experimental results for damage repair. One of the surprising experimental findings is that residual cells, persisting in a damaged area after removal of a part of the biofilm, prominently affect the healing process. Taken together, our results outline the important roles of nutrient gradients and residual cells on biomass regrowth on macroscopic scales of the whole biofilm. The proposed combined experiment–simulation framework opens the way to further investigate the possible relation between wound healing, cell signaling and cell phenotype alternation in the local microenvironment of the wound.

Summary

Memory consolidation following learning is a dynamic and complex process comprising several transitions between distinct memory phases. Although memory consolidation has been studied extensively, it remains difficult to draw an integral description that can delimit the transition points between specific memory phases at the behavioral, neuronal, and genetic levels. To this end, we have developed a rapid and robust aversive conditioning protocol for the nematode worm Caenorhabditis elegans, tracing memory consolidation within the first hour post conditioning and then up to 18 h post conditioning. This made it possible to uncover time-dependent involvement of primary sensory neurons, transcription and translation processes, and diverse gene populations in memory consolidation. The change in neuronal valence was strong enough to induce second order conditioning, and was amenable to considerable modulation in specific mutant strains. Together, our work lends memory consolidation to detailed temporal and spatial analysis, advancing system-wide understanding of learning and memory.

We report high-level calculations of the excited states of [2,2]-paracyclophane (PCP), which was recently investigated experimentally by ultrafast pump-probe experiments on oriented single crystals [Haggag et al., ChemPhotoChem 6 e202200181 (2022)]. PCP, in which the orientation of the two benzene rings and their range of motion are constrained, serves as a model for studying benzene exciplex formation. The character of the excimer state and the state responsible for the brightest transition are similar to those in benzene dimer. The constrained structure of PCP allows one to focus on the most important degree of freedom, the inter-ring distance. The calculations explain the main features of the transient absorption spectral evolution. This brightest transition of the excimer is polarized along the inter-fragment axis. The absorption of light polarized in the plane of the rings reveals the presence of other absorbing states of Rydberg character, with much weaker intensities. We also report new transient absorption data obtained by a broadband 8 fs pump, which time-resolve strong modulations of the excimer absorption. The combination of theory and experiment provides a detailed picture of the evolution of the electronic structure of the PCP excimer in the course of a single molecular vibration.

Scholarly efforts to identify core design features for effective teacher professional development have grown rapidly in the last 25 years. Many concise lists of design principles have emerged, most of which converge on a consensus of 5-7 presumably "effective” design features (e.g., collaborative tasks, focus on content, active learning). The proliferation and convergence of reviews create the impression that this consensus is based on strong evidence from large-scale, replicated and rigorously controlled research studies. We critique the empirical foundation on which conclusions about evidence-based design features for teacher professional development have been based, by the same evidential standards that have been adopted within this field of scholarly work. We conclude that the empirical foundations for these lists are problematic and that claims to methodological rigor are misleading as they are based on flawed inferences. We further argue that the ambition to identify general features of effective professional development is also problematic, and reflect on why, despite its weaknesses and potentially adverse consequences for research and practice, we as a field continue to herald this consensus. We call for greater focus on the development, testing and refinement of theories about teacher professional learning in order to advance understanding, policy and practice in the field.

Mating success depends on many factors, but first of all, a male and a female need to meet at the same place and time. The circadian clock is an endogenous system regulating activity and sex-related behaviors in animals. We studied bumble bees (Bombus terrestris) in which the influence of circadian rhythms on sexual behavior has been little explored. We characterized circadian rhythms in adult emergence and locomotor activity under different illumination regimes for males and gynes (unmated queens). We developed a method to monitor adult emergence from the pupal cocoon and found no circadian rhythms in this behavior for either males or gynes. These results are not consistent with the hypothesis that the circadian clock regulates emergence from the pupa in this species. Consistent with this premise, we found that both gynes and males do not show circadian rhythms in locomotor activity during the first 3 days after pupal emergence, but shortly after developed robust circadian rhythms that are readily shifted by a phase delay in illumination regime. We conclude that the bumble bees do not need strong rhythms in adult emergence and during early adult life in their protected and regulated nest environment, but do need strong activity rhythms for timing flights and mating-related behaviors. Next, we tested the hypothesis that the locomotor activity of males and gynes have a similar phase, which may improve mating success. We found that both males and gynes have strong endogenous circadian rhythms that are entrained by the illumination regime, but males show rhythms at an earlier age, their rhythms are stronger, and their phase is slightly advanced relative to that of gynes. An earlier phase may be advantageous to males competing to mate a receptive gyne. Our results are consistent with the hypothesis that sex-related variations in circadian rhythms is shaped by sexual selection.

Does social cohesion explain variation in violence within divided cities? In line with insights drawn from the ethnic politics, criminology and urban geography literature we suggest that explaining variation in inter-group violence is not possible by relying on motivational elements alone, and attention to social cohesion is required as well. While cohesion can facilitate collective action that aids violent mobilization, it can also strengthen social order that contributes to the group’s capability to control and prevent unrest. We test these relationships using an application of a latent variable model to an integration of crime data, survey results and expert-coded data in order to measure cohesion in East Jerusalem neighborhoods. We then analyze its impact on riots using three original geo-located datasets recording violence in the neighborhoods between the years 2013-2015. Our results reveal that even with controls for economic and political determinants of violence, as well as for spatial clustering and temporal explanations, neighborhood-level social cohesion is a robust explanatory variable - it negatively correlates with riots.

This study explores one of the least studied components of forced migrants' journeys—the internal political conditions of the host/transit countries. Using the social navigation approach, we demonstrate how these conditions influence the progress or halt of migration journeys of asylum seekers. We interviewed asylum seekers from Eritrea and Sudan about their journey from their country of origin. Based on these life stories, we derived three salient political themes affecting their journeys: state political instability, the state of freedoms, and lasting persecution. We conclude with insights into the influence of the political component along the journey.

Book review