Optical density (OD) is routinely used to estimate the density of cells in liquid culture. However, it is semi quantitative and requires a calibration with known particles of similar sizes and optical properties to be accurate. 244 laboratories participating in the iGEM Interlab study used a common lab bacteria Escherichia coli expressing a fluorescent marker GFP to perform comparative measurements.Thus cells count from fluorescence estimates and silica beads used as standard for correction were contrasted. The silica beads dilutions’ approach produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also measures instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell. This type of measurement opens the possibility for direct comparison and data fusion with flow cytometry observations. When compared to fluorescence per cell measurements we showed that there is only a 1.07-fold mean difference between plate reader and flow cytometry data.
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Nature Communications Biology (August 2020).
Beal et al; iGEM Interlab Contributors : Alice Dejoux, Deshmukh Gopaul, Léa Guérassimoff, Samuel Jaoui, Manon Madelenat and Serena Petracchini – iGEM Pasteur Paris 2018