What happens when a bacteria tumble? Is it an active or a passive process? I was wondering what actually happens during the tumbling process of a "run and tumble" process. When the bacteria stops to tumble and reorient itself, is that an active process, or is the reorientation due to thermal rotationnal diffusion?
Cheers,
Nico
 A: In all phases of matter there exists energy carried by the individual particles that gives bulk behavior and average quantities like average kinetic energy and temperature which depends on it.
Your question is very general and I will answer for the phase  of matter called gas. There the kinetic energy of the individual gas molecules is random and can be seen in what is called Brownian motion of dust particles.


This is a simulation of the Brownian motion of a big particle (dust particle) that collides with a large set of smaller particles (molecules of a gas) which move with different velocities in different random directions.

A dust particle is inactive, i.e. cannot convert internal energy to motion and affect its course. It follows a random path forever. A bacterium is alive, has internal energy it can convert to motion and could overcome the random bufferings of the air molecules and give a direction to its course or stay at rest with respect to gravity. It is an active choice of the bacterium.
