Infrared scattering: a method for evaluating the mass and size of bacteria

In the realm of biophysics and biochemistry, light scattering is used extensively to evaluate the molecular weight and size of macromolecules and particles in suspension. Bacteria scatter strongly but are too large for the conventional procedures. By extending the wavelength lambda of the incident radiation into the infrared, we show that the effective size of the bacteria (relative to lambda) is reduced, and the usual Zimm plot measurements and procedure can be applied to evaluate the molecular weight and size. Details of the apparatus, its alignment, calibration and use are given along with data for aqueous suspensions of the three species Serratia marcescens, Escherichia coli and Thiobacillus ferrooxidans. The method has the advantages of being suitable for rod-like or ellipsoid-like bacteria as well as spheres, for polydisperse samples and for monitoring the effects of environment, antibodies and chemical therapeutic agents on the bacteria.