Measuring bacterial growth

The most common method for measure the growth of bacteria in liquid culture is to exploit the ability of bacterial cells to scatter light.

Light is electromagnetic.   As a photon approaches a molecule, it causes the electrons in the molecule to move. Moving electrons radiate photons. The light is scattered.

In air or water, the random distribution of molecules results in the cancellation of these secondary radiations and a beam of light emerges largely unaltered.  This is why water and glass are transparent.

A single molecule scatters light based on its size, its molecular weight.  A cluster of molecules scatters light as the square of its molecular weight.

As the size of an aggregate approaches the wavelength of visible light (about 0.5 µm), the two ends of the aggregate will be out of phase with respect to the photon's electric field.

Under these conditions, light scattering becomes anisotropic -- that means that light is preferentially scattered away from its original direction.

Anisotropic scattering reduces the intensity of the light beam that emerges from the solution.

The reduction in light intensity due to anisotrophic scattering can be used to measure the total particle mass (w) in the path of the light beam.

The constant A relates absorbance of light (that is reduction in the intensity of light entering a solution compared to the intensity of the light leaving the solutIon) to bacterial cell mass.  

The value of A is a function of the wavelength of light used and the width of the bacterial solution through which the light passes.

Particles much smaller than bacteria, like viruses, or much larger, like animal cells, do not obey this simple rule.

Different methods must be used to measure their mass.