Direct optical density determination of bacterial cultures in microplates for high-throughput screening applications.

A convenient and most abundantly applied method to determine the growth state of a bacterial cell culture is to determine the optical density (OD) spectrophotometrically. Dilution of the samples, which is necessary to measure within the linear range of the spectrophotometer, is time-consuming and not compatible with high-throughput applications. Here we present a direct approach to estimate the OD at 578 nm (OD578) of bacterial cultures in microplates without the need for sample dilution. This could be advantageous for high-throughput analysis of bacterial cells in microplates for example when optimizing growth conditions, screening for new substrates of a bacterial strain or monitoring enzymatic activity after enzyme evolution. Pseudomonas putida cells were grown in shake flasks. The OD578 was determined in parallel in a microplate directly without dilution and in a spectrophotometer cuvette after dilution. The resulting data set was used to identify a conversion formula, which enables direct and reliable transformation of OD measurements of undiluted samples into the corrected OD values as would have been obtained for diluted samples measured in a standard spectrophotometer. Subsequently we could show that just a few OD calibration points are required to adjust this conversion formula and make it suitable for other suspensions or cultures of bacterial strains different than P. putida. The OD calibration points can be obtained by any combination of microplate reader and cuvette spectrophotometer. For this purpose, conversion formulas for a formazine standard suspension and a suspension of Escherichia coli BL21(DE3) cells were successfully generated. The OD values calculated by both conversion formulas turned out to be identical with the values as obtained by the control measurements in the spectrophotometer. This indicates the general applicability of the conversion formula as described.