Theory of the angular dependence of light scattered by bacteria and similar-sized biological objects

Abstract

For suspension of randomly oriented, asymmetric particles of slightly irregular shape, reasonably accurate estimates of the light scattering can be computed from simple Rayleigh-Gans theory, even when applied to large objects such as mammalian mitochondria or the most-studied bacteria, Escherichia coli. The calculations apply for elongated particles of diameter less than 1 μ, ranging from two to four times as long with an index of refraction averaging less than 5% greater than the surrounding medium. The simple Rayleigh-Gans approximation works quite well for these large particles because in computing the light scatter it is necessary to average over orientation, asymmetry, heterogeneity in shape, and variations in the minor dimension. These averages tend to minimize the detailed discrepancies that would be found if all particles were perfectly spherical and of the same volume. An order of magnitude discrepancy between the theory and our experimental measurement at high angles is interpreted as resulting from the light scattering from the high index of refraction of the walls of the particles or from the irregular distribution of cell materials. But for these types of biological materials, most of the light is scatttered in nearly the forward direction; and low angle light scattering measurements should yield quite accurately the average volume of the particles.