Enumerating Bacteria

How to measure cell density or number of viable cells in liquid media.

There are two standard ways of measuring cell density in the lab.

Using a spectrophotometer

Measuring the optical density (OD), or turbidity, of a liquid cell culture with either the NanoDrop (cuvette mode) or in a 96-well microplate in the TECAN Spark. Usually the sample is measured at a wavelength of 600 nm, referred to as the OD600 value. The selection of 600 nm is because there is minimal absorption at that wavelength and you are only measuring light scattering. You are looking for a direct relationship between cell biomass and change in light transmission, that does not vary with physiology of the cell, the production of metabolites, or the media that is used. See the NanoDrop page for the protocol to measuring cell density.

Counting colonies

This is an old school approach that can give a very reliable measurement for the number of viable cells in a liquid culture. The protocol is as follows.