General approach to the statistics of microbial orientation: Lévy walks, noise, and deterministic drift

Microbial motion is typically analyzed by simplified models in which trajectories exhibit straight runs (perhaps with added Gaussian noise) followed by random, discrete tumbling events. We present the results of a statistical analysis of the angular dynamics for four different swimming microbes: tumbling and smooth-swimming strains of Bacillus subtilis and two Eukaryotic algae, Tetraselmis suecica and Euglena gracilis. We show that the angular statistics closely resemble a Voigt profile, the convolution of a Gaussian (Lévy index α=2) and Lorentzian (Lévy index α=1) distribution. This distribution is ubiquitous for all four microbes. Rather than modeling tumbling as a discrete process, we model tumbling dynamics as a continuous process: Lévy flights in the orientational dynamics using a Lorentzian noise model. This model is analytically solvable. Each individual microbe trajectory has both stochastic behavior (noise) and varying drift behavior, such as helices of different sizes and frequencies and circular arcs with different radii. We model the distribution of different drift behavior via an ensemble theory. The drift behavior (e.g., circular arcs) comes from physical observations of the swimming behavior and explains many of the qualitative features of the experimental data that cannot be explained by a pure noise model. From this theory, we estimate the strength of Lorentzian noise, the physical rotational diffusion constant, and some relevant parameters relating to the distributions of deterministic drift behavior. This analysis shows that in some cases Gaussian noise is not the dominant process responsible for the angular statistics following a Voigt profile.