Refraction and absorption of light in bacterial suspensions

Abstract

The apparent average refractive indices of three types of microorganism in liquid suspension have been estimated by two methods. In the first, the refractive index of the aqueous suspension is measured in an interferometer, the bacterial cell volume fraction is estimated from the dilution of an added inert solute, and the refractive index of the disperse phase is calculated with the aid of a mixture rule. In the second method, optical density spectra of suspensions in several concentrated solutions of nonpenetrating solutes are measured as a function of the refractive index,n1, of the solution. The value ofn1 when the optical density,E, has its smallest value is equated to the average cell refractive index and the constants of the parabolaE (n1) are related to the heterogeneity of the cell population in respect to refractive index. The preliminary results obtained by the two methods are in satisfactory agreement and are consistent with the specific refractive increments of protein and nucleic acid and their concentrations in the cell. It is pointed out that measurements of optical density spectra can also give useful information concerning the number and size of cells in a suspension and their optical dispersion constants. To obtain such information, however, a spectrophotometer specially designed to exclude forward-scattered light must be used and an acceptable theory must be available. Finally, the apparent spectral absorption of “clarified” cell suspensions is presented and compared with spectra obtained by other methods. It is pointed out that because of refractive heterodispersity even “clarified” suspensions are turbid, so that this method does not provide a completely satisfactory technique for cell spectrophotometry.