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

Worldwide Phylogeography of Rough-Toothed Dolphins (steno Bredanensis) Provides Evidence for Subspecies Delimitation

Rough-toothed dolphins ( Steno bredanensis ) have a global tropical and subtropical distribution with oceanic, neritic, and island-associated populations. To inform conservation and management for this species, we used sequences from the mtDNA control region ( n = 360 ), mitogenomes ( n = 19 ), and six nuclear introns (n = 35) to provide multiple lines of evidence to critically evaluate the potential taxonomic status of rough-toothed dolphins. Using samples from the Pacific, Indian, and Atlantic Oceans, we examined the null hypothesis that rough-toothed dolphins are one panmictic species and the alternate hypothesis of oceanic subspecies. Phylogenetic analyses of mitogenomes revealed a private Atlantic clade sister to a larger cosmopolitan clade including individuals from all tropical and subtropical oceans. We dated the split between the Atlantic clade and the cosmopolitan clade to 890,000 years ago. We determined that Atlantic rough-toothed dolphins could be correctly diagnosed with 98% accuracy with the mtDNA control region and calculated the net nucleotide divergence as 0.02. Population level analyses revealed significant genetic differentiation using mtDNA among most regions, while significant differentiation using nuclear markers occurred only between the Atlantic and the Indian/Pacific regions. Therefore, the oceanic divergence and diagnosability of rough-toothed dolphins in the Atlantic and the Indian/Pacific Oceans meet proposed criteria for recognition as two subspecies

Raising to ergative: Remarks on applicatives of unaccusatives

Applicatives of unaccusatives provide a crucial test case for the inherent-case view of ergativity. If ergative is assigned only to external arguments, in their θ-positions, there can be no “raising to ergative” in applicative unaccusatives; an internal argument subject can never receive ergative case. In this article, I present evidence from Nez Perce (Sahaptian) that this prediction is false. In Nez Perce applicative unaccusatives, the theme argument raises over the applicative argument and is accordingly marked with ergative case. Nez Perce thus demonstrates raising to ergative. Departing from Baker’s (2014) conclusions for similar phenomena in Shipibo (Panoan), I argue that apparently nonlocal movement of the theme in the raising-to-ergative pattern involves not a covert adpositional structure, but rather a response to independently motivated constraints on antilocal movement and remnant movement