Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Thrust tectonics and Cretaceous intracontinental shortening in Southeast Alaska

An imbricate thrust belt extending along strike for more than 2000 km overprints the tectonic boundary between two of the largest allochthonous crustal fragments (Intermontane and Insular superterranes) in the North American Cordillera, and affects rocks west of the Coast Plutonic Complex in southeast Alaska and western British Columbia. Deformation was broadly coeval with mid-Cretaceous magmatism and involved the emplacement of west-directed thrust nappes over a structurally intact and relatively unmetamorphosed basement. The Palaeozoic and lower Mesozoic Alexander terrane forms structural basement for much of the thrust belt along a moderately northeast-dipping decollement. There were two main episodes of mid-Cretaceous deformation, which were contemporaneous with the emplacement of tabular plutonic bodies. Older structures record ductile southwest-vergent folding and faulting, and regional metamorphism, associated with a well-developed axial-planar foliation. Second-generation structures include southwest-directed thrust faults that juxtapose contrasting metamorphic grades and refold earlier structures. Structural, stratigraphical and geochronologic data suggest that regional-scale deformation in southeast Alaska occurred between 113 Ma and 89 Ma. Deformation involved the imbrication of marginal basin(s) and a magmatic arc, overprinting the older tectonic boundary between the Insular superterrane and the late Mesozoic western margin of North America (i.e. the Intermontane supertcrrane). Contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism, and thus records intra-arc tectonism. Late Palaeocene to Early Eocene deformation and uplift may mark the transition from contractional to extensional tectonism, and perhaps records the collapse of tectonically thickened crust