  We present a fast, high-throughput method for characterizing the motility of
microorganisms in 3D based on standard imaging microscopy. Instead of tracking
individual cells, we analyse the spatio-temporal fluctuations of the intensity
in the sample from time-lapse images and obtain the intermediate scattering
function (ISF) of the system. We demonstrate our method on two different types
of microorganisms: bacteria, both smooth swimming (run only) and wild type (run
and tumble) Escherichia coli, and the bi-flagellate alga Chlamydomonas
reinhardtii. We validate the methodology using computer simulations and
particle tracking. From the ISF, we are able to extract (i) for E. coli: the
swimming speed distribution, the fraction of motile cells and the diffusivity,
and (ii) for C. reinhardtii: the swimming speed distribution, the amplitude and
frequency of the oscillatory dynamics. In both cases, the motility parameters
are averaged over \approx 10^4 cells and obtained in a few minutes.
