OD600

OD600 is the optical density of a cell suspension measured at 600 nm, serving as a rapid and convenient proxy for microbial cell density ¹.

How It Works

When a beam of 600 nm light passes through a cuvette or microplate well containing a cell suspension, cells scatter the light, reducing the intensity reaching the detector. The degree of light attenuation, reported as absorbance or optical density, correlates with cell concentration. The 600 nm wavelength is chosen because it minimizes absorption by culture media components and cellular pigments.

At low cell densities (OD600 below approximately 0.4), the relationship between OD and cell number is roughly linear. At higher densities, light scattering becomes nonlinear and measurements require dilution to remain in the linear range. The absolute relationship between OD600 and cell count varies by organism, strain, cell morphology, and instrument.

In synthetic biology, OD600 measurements are ubiquitous for monitoring culture growth, timing inductions, normalizing reporter assays per cell, and standardizing inoculation densities across experiments.

Computational Considerations

Converting OD600 to absolute cell counts requires instrument-specific calibration using microsphere standards or direct cell counting methods ². Computational tools apply calibration factors, correct for path-length differences between cuvettes and microplates, and account for nonlinear scattering at high densities. Standardized calibration protocols improve reproducibility of growth and expression measurements across different laboratories and plate reader instruments.

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