  The motility of microorganisms in liquid media is an important issue in
active matter and it is not yet fully understood. Previous theoretical
approaches dealing with the microscopic description of microbial movement have
modeled the propelling force exerted by the organism as a Gaussian white noise
term in the equation of motion. We present experimental results for ciliates of
the genus Colpidium, which do not agree with the Gaussian white noise
hypothesis. We propose a new stochastic model that goes beyond such assumption
and displays good agreement with the experimental statistics of motion, such as
velocity distribution and velocity autocorrelation.
