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 Biological Evaluation of Novel Water-Soluble Poly-(ethylene glycol)-10-hydroxycamptothecin Conjugates

In order to improve the antitumor activity and water solubility of 10-hydroxycamptothecin (HCPT), a series of novel HCPT conjugates were designed and synthesized by conjugating polyethylene glycol (PEG) to the 10-hydroxyl group of HCPT via a valine spacer. The in vitro stability of these synthesized compounds was determined in pH 7.4 buffer at 37 °C, and the results showed that they released HCPT at different rates. All the compounds demonstrated significant antitumor activity in vitro against K562, HepG2 and HT-29 cells. Among them, compounds, 4a, 4d, 4e and 4f, exhibited 2–5 times higher potency than HCPT. The stability and antitumor activity of these conjugates were found to be closely related to the length of PEG and the linker type, conjugates with a relatively short PEG chain and carbamate linkages (compounds 4a and 4f) exhibited controlled release of HCPT and excellent antitumor in vitro activity