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

Synthesis and structure of Zn(II) and Cu(II) complexes derived from 2-(aminomethyl)benzimidazole and glycine

<div><p>Reactions of 2-(aminomethyl)benzimidazole di-hydrochloride (<b>1</b>·2HCl) and glycine with 3Zn(OH)₂·2ZnCO₃ or Cu(OAc)₂·H₂O led to the synthesis of the quaternary coordination complexes <b>2</b> and <b>3</b>. X-ray diffraction showed that these complexes are composed of <b>2a</b> = [Zn(L)Cl(L′)] and <b>2b</b> = [Zn(L)(H₂O)₂(L′)], and of <b>3a</b> = [Cu(L)(H₂O)_0.25Cl(L′)] and <b>3b</b> = [Cu(L)(H₂O)_1.5(L′)], respectively, where L = 2-(aminomethyl)benzimidazole and L′ = glycinate. Zn(II) in <b>2a</b> has an intermediate geometry between a square-pyramid and a trigonal bipyramid structure. However, the geometry about the metal ion of units <b>2b</b>, <b>3a</b>, and <b>3b</b> is distorted octahedral. Moreover, the supramolecular structures for <b>2</b> and <b>3</b> were assembled through N–H⋯O and O–H⋯Cl hydrogen bonds. In these complexes, H₂O and N–H groups serve as proton donors, whereas chloride and C=O groups serve as proton acceptors. Also <i>π</i>–<i>π</i> stacking interactions between aromatic rings contribute to the stabilization of the supramolecular structure of <b>2</b> and <b>3</b>. The Zn and Cu complexes were studied by infrared and Raman spectroscopy, which indicated that <b>2</b> and <b>3</b> have similar molecular structures in the solid state. Ultrasound activation at the end of the reaction was necessary to yield <b>2</b>.</p></div