Spectral Analysis of Laser Light Scattered from Motile Microorganisms

The theoretical basis of laser scattering from motile microorganisms is examined. Spectra of swimming particles are compared with spectra arising from brownian motion. For mixtures of motile and resting organisms, that part of the spectrum related to the motile organisms is enhanced when V_s/|k|D is large, where V_s is the mean swimming speed of the motile microorganisms,|k| is the Bragg wave vector, and D is the diffusion coefficient of the nonmotile particles. When the directed motion of swimming microorganisms persists for periods which are much longer than τ = (|k|V_s)^-1, the scattering spectrum is given as S(k, ω) ∞ (| ω - ω₀]/k|), where is the probability distribution obtained by two-dimensional integration over the swimming speed distribution. A computation of scattering from bull spermatozoa, based on published velocity distributions, is investigated in detail.