We use optical density (OD) at a wavelength of 600nm as a quick approximation of cell abundance in liquid bacterial culture in lieu of counting the cells on a microscope. Different bacterial strains can have different relationships between OD and cell abundance depending on the amounts of polysaccharides or other secondary metabolites they might extrude into the culture medium or coat themselves with, which may vary with temperature or growth medium, for example.

A former student in the lab, A. Huston, measured OD600 and microscope counts of Colwellia psychrerythraea strain 34H in Marine Broth 2216 at two temperatures: -1C and its optimal temperature, +8C. Although the growth rate of the bacterium at +8C is several times that at -1C, there appears to be little difference in the relationship between cell abundance and OD at the two temperatures.

Some linear equations were provided to convert from OD to cell abundance, but here I’ve taken the original data and used a logistic function to get a better conversion. The equation below has an r^2 of 0.98 for the combined -1 and +8C temperature data.

(Thanks to Hamline University Physics Department Latex Equation Editor for the online LaTeXing, and to the open source graphing program QtiPlot)

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