Establishing a quantitative link between crystal violet absorbance and biomass in biofilms

Quantifying biomass is essential for studying biofilms in biomanufacturing, yet widely used assays such as crystal violet (CV) often yield results that are difficult to compare across laboratories due to its reliance on absorbance as a subjective proxy for biomass. To address this, we present a standardized method that calibrates CV absorbance against cellular optical density (OD) and dry cell weight (DCW) using centrifuged planktonic cultures. By establishing a three-way correlation among OD, DCW, and CV absorbance, this simple method allows for quantitative, reproducible, and comparable biomass measurements. Validation across Escherichia coli strains and Rhodopseudomonas palustris showed strong linearity, particularly when using 10% acetic acid as the solvent. Seasonal and instrument-based variability were evaluated, supporting the method's robustness and broad utility. This method enables researchers to normalize CV data, improving inter-laboratory comparability and supporting more accurate assessment of biofilm productivity.