Bulletin of the American Physical Society
19th Annual Meeting of the APS Northwest Section
Volume 63, Number 6
Thursday–Saturday, May 31–June 2 2018; Tacoma, Washington
Session F2: Bio and Multidisciplinary Physics I |
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Chair: Andreas Vasdekis, University of Idaho Room: Thompson Hall 193 |
Saturday, June 2, 2018 1:30PM - 2:00PM |
F2.00001: Using microfluidics to understand stress signaling in individual bacterial cells Invited Speaker: Matthew Cabeen To ensure their survival, all living organisms---including bacteria---must be able to sense environmental stressors and mount an effective response. Certain species, including our model bacterium \textit{Bacillus subtilis} and the pathogen \textit{Listeria monocytogenes}, sense environmental stress via large `stressosome' complexes that include multiple stress-sensor variants among their 80 protein constituents. Why use such stress-sensing molecular machines in combination with sensor variants? We seek a greater understanding of how stressosomes sense and integrate different input signals to produce different stress responses that help cells survive diverse environmental insults. To gain insight into stressosome function, we employ a microfluidic platform that permits us to examine stress response patterns at the single-cell level over tens or hundreds of generations with precise control of environmental conditions. In response to ethanol stress, a wild-type strain containing multiple stress sensors and strains containing each sensor in isolation showed distinct long-term response patterns. For example, one sensor closely matched the rapid and transient wild-type response, whereas another showed a slower but sustained average response composed of pulsatile activation events in single cells. We are presently asking whether stressors other than ethanol elicit distinct response patterns and whether different response patterns confer different degrees of fitness on stressed cell populations. [Preview Abstract] |
Saturday, June 2, 2018 2:00PM - 2:12PM |
F2.00002: The Effect of Cathode Sheath Position on a Self-Magnetic-Pinch Diode David Housley, Rick Spielman Pulsed electron accelerators like TriMeV emit electrons from a cathode into a vacuum gap and accelerate them toward an anode by a high voltage potential (3-MV peak, 20-ns duration) applied across the gap. Often the electron beam is applied to a Bremsstrahlung converter to produce a radiographic source whose radial extent is termed the "spot" and the "diode" is the portion which bridges the cathode and anode quintessentially being the path electrons traverse and only in one direction. Initially, emitted electrons follow the electric field lines but within a few nanoseconds an equilibrium is established between opposing radial forces that shape the temporal extent of the electron beam throughout the anode-cathode gap. Elements influencing this equilibrium are the geometry and strength of the electric field, coulomb forces between the particles, and Lorentz forces due to currents in the gap. It is thought that all of these elements can be perturbed by changes in the design of the diode which includes the hardware terminating the cathode and accessories mounted on the anode. Recent experiments investigating the effect of a cathode sheath and its position, applied to a self-magnetic-pinch diode, on the resulting spot size and radiographic dose will be presented. [Preview Abstract] |
Saturday, June 2, 2018 2:12PM - 2:24PM |
F2.00003: Robust Microbial Cell Segmentation by Quantitative Phase Imaging. Hamdah Alanazi, Amrah Canui, Adam Garman, Joshua Quimby, Andreas Vasdekis Cell imaging is an important tool in cell and molecular biology research. This is because such procedures allow us to determine the cellular function-structure relationship. However, the first step to extract cell physiology information from all forms of biological imaging experiments is to segment cells. As such, cell segmentation has attracted considerable attention in computational image processing. Despite such progress however, it remains challenging to identify a global algorithm that pertains to all cellular models ranging from mammalian lines to yeast and bacteria. To address this shortcoming, we undertook a different approach by replacing conventional imaging modalities, such as phase contrast and fluorescence with Quantitative Phase Imaging (QPI). QPI relies on the optical-phase rather than intensity to image cells and localize their contour. In this way, QPI enabled a very high cell segmentation success-rate greater than 99{\%} for yeast and 98{\%} for E. coli bacteria cells without any computationally intensive, post-acquisition processing. We attribute this improved performance to the remarkably uniform background, elimination of cell-to-cell and intracellular optical artifacts, and enhanced signal-to background ratio -- all innate properties of imaging in the optical-phase domain. [Preview Abstract] |
Saturday, June 2, 2018 2:24PM - 2:54PM |
F2.00004: Transcription leads to pervasive replisome instability in bacteria. Invited Speaker: Paul Wiggins The canonical model of DNA replication describes a highly-processive and largely continuous process by which the genome is duplicated. This continuous model is based upon~\textit{in vitro}~reconstitution and~\textit{in vivo}~ensemble experiments. Here, we characterize the replisome-complex stoichiometry and dynamics with single-molecule resolution in bacterial cells. Strikingly, the stoichiometries of the replicative helicase, DNA polymerase, and clamp loader complexes are consistent with the presence of only one active replisome in a significant fraction of cells (\textgreater 40{\%}). Furthermore, many of the observed complexes have short lifetimes (\textless 8 min), suggesting that replisome disassembly is quite prevalent, possibly occurring several times per cell cycle. The instability of the replisome complex is conflict-induced: transcription inhibition stabilizes these complexes, restoring the second replisome in many of the cells. Our results suggest that, in contrast to the canonical model, DNA replication is a largely discontinuous process~\textit{in vivo}~due to pervasive replication-transcription conflicts. [Preview Abstract] |
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